U.S. patent application number 10/012102 was filed with the patent office on 2002-07-04 for method and apparatus for dispensing small volume of liquid, such as with a weting-resistant nozzle.
This patent application is currently assigned to Therics, Inc.. Invention is credited to Materna, Peter A..
Application Number | 20020084290 10/012102 |
Document ID | / |
Family ID | 27400019 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020084290 |
Kind Code |
A1 |
Materna, Peter A. |
July 4, 2002 |
Method and apparatus for dispensing small volume of liquid, such as
with a weting-resistant nozzle
Abstract
A wetting-resistant nozzle for accurately and precisely
dispensing small volumes of liquids. The nozzle comprises an
internal flowpath, and an external surface that recedes from the
discharge point at an angle greater than 90 degrees, and an
exceptionally low surface energy for the external surface. The low
surface energy material may exist as a coating on top of a shaped
substrate. A flat land region may be included and may have sharp
edges, one of which may define the boundary of the low surface
energy region. Another embodiment includes the low surface energy
material as a bulk material through which a hole is drilled. The
internal flowpath inside the nozzle may be smoothly tapered. Liquid
being dispensed tends not to advance past the edge of the low
surface energy region, which may coincide with a geometrically
sharp edge. Such nozzles provide improved dispensing of liquids
that have both low surface tension and low viscosity, such as
organic solvents.
Inventors: |
Materna, Peter A.;
(Metuchen, NJ) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Assignee: |
Therics, Inc.
Princeton
NJ
|
Family ID: |
27400019 |
Appl. No.: |
10/012102 |
Filed: |
November 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60247176 |
Nov 10, 2000 |
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60284783 |
Apr 18, 2001 |
|
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60288025 |
May 1, 2001 |
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Current U.S.
Class: |
222/420 |
Current CPC
Class: |
B05C 5/0212 20130101;
B05B 1/00 20130101; B01L 3/0241 20130101; B01L 2300/166 20130101;
B41J 2/14 20130101; B05B 13/0228 20130101; B05C 13/025
20130101 |
Class at
Publication: |
222/420 |
International
Class: |
B65D 047/18 |
Claims
I claim:
1. A nozzle for dispensing a liquid, comprising: a dispenser
including an internal flow passageway with an inlet and an outlet,
wherein liquid exits the dispenser at the outlet; a land adjoining
and surrounding the outlet, the land is substantially perpendicular
to a longitudinal axis of the internal flow passageway; and a
substantially axisymmetric external surface adjoining and
surrounding the land, the external surface having a surface tangent
angle nearest the land which is greater than 90 degrees, wherein a
surface energy of the external surface is less than about 17
dyne/cm.
2. The nozzle of claim 1, wherein the substantially axisymmetric
external surface is frusto-conical.
3. The nozzle of claim 1, wherein the substantially axisymmetric
external surface is shaped in a concave curve.
4. The nozzle of claim 1, wherein the substantially axisymmetric
external surface is shaped in a convex curve.
5. The nozzle of claim 1, wherein the internal flow passageway, the
land, and the external surface are all substantially axisymmetric
around a common axis.
6. The nozzle of claim 1, wherein the land has an outer edge that
is substantially sharp, and wherein the outer edge is adjacent to
the substantially axisymmetric external surface.
7. The nozzle of claim 1, wherein the land has an inner edge that
is substantially angular, and wherein the inner edge is adjacent to
the internal flow passageway.
8. The nozzle of claim 1, wherein the surface tangent angle nearest
the land is greater than or approximately equal to 135 degrees.
9. The nozzle of claim 8, wherein the surface tangent angle nearest
the land is greater than or approximately equal to 165 degrees.
10. The nozzle of claim 9, wherein the surface tangent angle
nearest the land is greater than or approximately equal to 170
degrees.
11. The nozzle of claim 10, wherein the surface tangent angle
nearest the land is greater than or approximately equal to 175
degrees.
12. The nozzle of claim 1, wherein the land has an outer circular
boundary and an inner circular boundary that is substantially
concentric with each other.
13. The nozzle of claim 1, wherein the land has an outer circular
boundary and an inner circular boundary that is approximately
concentric with each other.
14. The nozzle of claim 1, wherein the land has an outside diameter
and an inside diameter, and the outside diameter divided by the
inside diameter is less than or approximately equal to 5.
15. The nozzle of claim 1, wherein the land has an outside diameter
and an inside diameter, and the outside diameter divided by the
inside diameter is between 1.1 and 1.5.
16. The nozzle of claim 1, wherein the land has a flat land surface
energy, and the land surface energy is greater than or equal to
about 17 dyne/cm.
17. The nozzle of claim 1, wherein the land has a flat land surface
energy, and the land surface energy is less than about 17
dyne/cm.
18. The nozzle of claim 1, wherein the internal flow passageway is
tapered, having a cross-sectional flow area that becomes smaller
upon progressing from the inlet to the outlet.
19. The nozzle of claim 1, wherein the internal flow passageway has
an internal surface having an internal-surface surface-energy, and
the internal-surface surface-energy is greater than 50 dyne/cm.
20. The nozzle of claim 1, wherein, at a distance greater than 0.5
inch away from the outlet, the external surface changes to a shape
different from what it was closer to the outlet, or changes so as
to have a surface energy different from what it had closer to the
outlet.
21. The nozzle of claim 1, wherein the surface energy of the
external surface is less than about 12 dyne/cm.
22. The nozzle of claim 1, wherein the surface energy of the
external surface is less than about 8 dyne/cm.
23. The nozzle of claim 1, wherein the external surface is a
coating on top of a substrate.
24. The nozzle of claim 23, wherein the substrate is made of a
material selected from the group consisting of tungsten carbide,
other ceramics, metals, silicon and polymers.
25. The nozzle of claim 23, wherein the coating is a substance that
hardened from liquid after being applied to the nozzle.
26. The nozzle of claim 23, wherein the coating cures or hardens
from a liquid by heat, by ultraviolet light, by the passage of time
since the mixing of two components, or by evaporation of a
solvent.
27. The nozzle of claim 23, wherein the coating has a thickness of
less than about 50 microns.
28. The nozzle of claim 23, wherein the coating has a lower surface
energy at its surface than it does in its interior.
29. The nozzle of claim 23, wherein the coating comprises a
fluoropolymer.
30. The nozzle of claim 23, wherein the coating comprises a
fluoroepoxy.
31. The nozzle of claim 23, wherein the coating has an exposed
surface and the exposed surface comprises exposed terminal
trifluoromethyl radicals.
32. The nozzle of claim 30, wherein a majority of the exposed
surface is terminal trifluoromethyl radicals.
33. The nozzle of claim 23, wherein the coating comprises exposed
CF.sub.2H radicals.
34. The nozzle of claim 23, wherein the coating is applied by
gaseous deposition or reaction.
35. The nozzle of claim 23, wherein the coating is selected from
the group consisting of fluoropolymers, fluoroepoxies, fluorinated
diamond-like carbon, fluorinated amorphous carbon, a siliconic
polymer, poly-p-xylylene, tantalum, gold, partially fluorinated
alkyl silane, perfluorinated alkane, and perfluorooctyl
methacrylate.
36. The nozzle of claim 23, wherein the coating is a material that
solidifies with a plurality of small cracks in its surface, whereby
it becomes effectively more hydrophobic compared to the same
material in a smooth-surfaced condition.
37. The nozzle of claim 36, wherein the coating is alkyl ketene
dimer.
38. The nozzle of claim 1, wherein the external surface has a
predetermined roughness.
39. The nozzle of claim 38, wherein the external surface has a
dimensional scale of roughness that is between 1 and 50
microns.
40. The nozzle of claim 38, wherein the external surface has a
total surface area and has a projected surface area, and the total
surface area is more than 1.1 times the projected surface area.
41. The nozzle of claim 38, wherein the external surface is a
coating on top of a substrate, and the substrate has roughness.
42. The nozzle of claim 39, wherein the substrate has roughness in
a random pattern.
43. The nozzle of claim 42, wherein the roughness is produced by
spark erosion.
44. The nozzle of claim 42, wherein the substrate comprises a
collection of particles.
45. The nozzle of claim 41, wherein the substrate has roughness in
a prescribed geometric pattern.
46. The nozzle of claim 45, wherein the pattern comprises
circumferential grooves.
47. The nozzle of claim 46, wherein the grooves have a depth of
about 0.001 inch and a width of about 0.001 inch.
48. The nozzle of claim 46, wherein the external surface has a
projected surface area of a grooved region and the grooves in the
grooved region occupy a groove area where material was removed to
make the grooves, and the groove area is more than half of the
projected surface area.
49. The method of claim 45, wherein the pattern comprises
circumferential grooves intersected by slant-height grooves.
50. The nozzle of claim 49, wherein both the circumferential
grooves and the slant-height grooves have a depth of about 0.001
inch and a width of about 0.001 inch.
51. The nozzle of claim 50, wherein the external surface has a
projected surface area of a grooved region and the grooves in the
grooved region occupy a groove area where material was removed to
make the grooves, and the groove area is more than 60% of the
projected surface area.
52. The nozzle of claim 41, wherein the coating is sufficiently
thin that at least some of the substrate roughness appears as the
roughness of the external surface.
53. The nozzle of claim 41, wherein the coating has a thickness of
less than about 50 microns.
54. The nozzle of claim 1 further including a dispensed liquid
comprising a solvent which is selected from the group consisting of
ethanol, methanol, isopropanol, other alcohols, chloroform, other
fluorocarbons, acetone, methylene chloride, and other organic
solvents.
55. The nozzle of claim 1 further including a dispensed liquid
comprising water or an aqueous solution.
56. The nozzle of claim 1, further including a dispensed liquid
comprising dissolved solutes, or insoluble solid particles, or
colloidal particles or micelles suspended in it.
57. The nozzle of claim 1, further including a dispensed liquid
comprising an Active Pharmaceutical Ingredient.
58. The nozzle of claim 1, further including a dispensed liquid
comprising blood or another bodily fluid, or a reagent or
diagnostic substance, or liquid for three-dimensional printing, or
liquid for high throughput screening, or liquid for performing
medical or veterinary tests.
59. The nozzle of claim 1, wherein the dispenser is made by coating
a substrate, which provides the shape of the substantially
axisymmetric external surface, on the external surface with a
coating having a surface energy less than about 17 dyne/cm.
60. A microvalve-based printhead comprising the dispenser of claim
1.
61. A piezoelectrically actuated printhead comprising the dispenser
of claim 1.
62. A bubble-jet printhead comprising the dispenser of claim 1.
63. A continuous-jet printhead comprising the dispenser of claim
1.
64. A nozzle for dispensing a liquid, comprising: a nozzle having
an external surface and an internal passageway, the internal
passageway having a passageway diameter, an inlet and an outlet,
the passageway allowing a fluid to flow therethrough, the external
surface and the internal passageway substantially axisymmetric, the
external surface having a surface tangent angle nearest the outlet
that is greater than 90 degrees; and a transition region adjoining
and surrounding the outlet connecting the external surface and the
internal passageway, the transition region having an outer diameter
and an inner diameter, wherein the outer diameter minus the inner
diameter is less than one-tenth of the passageway diameter.
65. The nozzle of claim 64, further including a surface energy of
the external surface wherein the surface energy is less than about
17 dyne/cm.
66. The nozzle of claim 65, wherein the surface energy of the
external surface is less than about 8 dyne/cm.
67. The nozzle of claim 64, wherein the external surface is
rough.
68. The nozzle of claim 64, wherein the internal flow passageway is
tapered, having a cross-sectional flow area that becomes smaller
upon progressing in the downstream direction.
69. A nozzle for dispensing a liquid, comprising: a nozzle having a
substantially axisymmetrical internal and an external surface, the
internal surface allowing liquid to pass therethrough, the external
surface having a surface energy less than about 17 dyne/cm and a
surface tangent angle from the shared axis that is greater than 90
degrees.
70. The nozzle of claim 69, wherein the internal surface has an
inlet and an outlet to form an internal flow passageway that
conducts the liquid along a principal flow direction to the
outlet.
71. The nozzle of claim 70, wherein the substantially axisymmetric
external surface has a surface tangent angle nearest the outlet
that is greater than 135 degrees and the external surface energy is
less than about 12 dyne/cm.
72. The nozzle of claim 69, wherein the surface energy of the
external surface is less than about 8 dyne/cm.
73. The nozzle of claim 69, wherein the external surface is
curved.
74. The nozzle of claim 69, wherein the external surface is a
portion of a sphere.
75. The nozzle of claim 69, wherein the external surface meets the
flow passageway at an edge that is substantially sharp.
76. The nozzle of claim 68, wherein the internal flow passageway is
gradually tapered, having a cross-sectional flow area that becomes
smaller upon progressing in the downstream direction.
77. The nozzle of claim 69, wherein the external surface is formed
from a drop of resin.
78. The nozzle of claim 69, wherein the nozzle is made by
depositing a drop of a first resin at the end of a hollow tube,
curing or partly curing the first resin to form a partly cured
shape, depositing a drop of a second resin upon the partly cured
shape of the first resin so as to make an outwardly curving
surface, curing both resins, and making a hole through both cured
resins.
79. The nozzle of claim 77, wherein the hole is made by laser
drilling or mechanical drilling or by embedding a leachable
placeholder and then leaching out the leachable placeholder.
80. The nozzle of claim 77, wherein the drop of second resin has a
spherical shape with a spherical radius, and the hole has a hole
radius, and the hole radius divided by the spherical radius is
greater than 0.05.
81. A nozzle for dispensing a liquid, comprising: an internal flow
passageway along an axis that conducts a liquid along a principal
flow direction toward an exit; a fillet adjoining and surrounding
the exit; and a substantially axisymmetric external surface having
a surface tangent angle nearest the fillet which is greater than 90
degrees.
82. The nozzle of claim 81, wherein the fillet has a fillet surface
energy and the fillet surface energy is less than about 17
dyne/cm.
83. The nozzle of claim 81, wherein the fillet has a fillet surface
energy and the fillet surface energy is greater than or equal to
about 17 dyne/cm.
84. The nozzle of claim 81, further comprising a transition region
between the internal flow passageway and the external surface at
the exit that is substantially perpendicular to the principal flow
direction.
85. A method of manufacturing a nozzle for dispensing liquid,
comprising: manufacturing a nozzle having an internal flow
passageway which conducts a liquid along a principal flow direction
to an exit, a flat land which is substantially perpendicular to the
principal flow direction, a substantially axisymmetric external
surface having a surface tangent angle nearest the flat land which
is greater than 90 degrees; coating the external surface; and
curing the coating so that it has a surface energy of less than
about 17 dyne/cm.
86. The method of claim 85 further comprising, coating the external
surface with a liquid coating and directing a jet of gas at the
coating prior to curing to thin the coating.
87. The method of claim 85 further comprising, roughening the
external surface.
88. The method of claim 85 wherein the coating is gaseously
applied.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Provisional
Application No. 60/247,176, filed on Nov. 10, 2000, and Provisional
Application No. 60/284,783, filed Apr.18, 2001, and Provisional
Application No. 60/288,025, filed May 1, 2001, each of which is
incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to dispensing liquids in small volume
droplets or narrow diameter jets, and more particularly, to a
wetting-resistant nozzle capable of dispensing small volume
droplets or narrow diameter jets, wherein the nozzle has a low
surface energy coating and/or a nozzle geometry to provide wetting
resistance.
[0004] 2. Description of the Related Art
[0005] Dispensing liquids is important in various technologies.
Ink-jet printing is a major application involving the precise
dispensing of tiny drops. Another application is three-dimensional
printing (3DP), in which layers of powder are bound together in by
precisely dispensed binder liquid to form three-dimensional
objects. Yet another application is dispensing of pharmaceuticals
and other liquids for manufacturing medical devices and dosage
forms.
[0006] Dispensing small volume droplets to create printed displays,
pharmaceuticals, medical devices and the like, requires precise,
predetermined quantities of liquid delivered in precise,
predetermined locations. One problem currently encountered in
dispensing small volume droplets has been the uncontrolled and
inconsistent degree to which the dispensed liquid wets or does not
wet the exterior surface of the nozzle. Wetting is undesirable for
precision dispensing of liquids because it is a randomizing
influence. Ideally, liquid that passes through the passageway of
the nozzle should be ejected toward the target as soon as it is
dispensed, in a predetermined quantity and directed to a specific
location. However, if wetting occurs, a puddle forms at the nozzle
exit, and when the liquid from the dispenser enters the puddle it
may or may not immediately exit the puddle because the puddle has
variable volume. Some or even all of the liquid may go to changing
the size of the puddle rather than being dispensed. Most or all of
the puddle will eventually be ejected as a very large drop, which
is especially undesirable.
[0007] In addition to introducing a randomizing influence on the
quantitative dispensing of a liquid, wetting or the presence of a
puddle can effect the direction of dispensed liquid. Misdirection
is a change in the angle of the stream, so that the direction of
the stream's travel after leaving the nozzle is different from the
direction of the passageway through the nozzle. Split-streaming
occurs when the liquid dispenses as two distinct streams virtually
simultaneously. These effects are undesirable in drop-on-demand
applications, in which each drop is dispensed by directed action of
the valve or dispenser, as well as other forms of dispensing.
Another undesirable effect that can also occur is called swingback
and is described later.
[0008] Typical ink-jet ink has a surface tension of 33 dyne/cm and
a viscosity of 8 centiPoise (at room temperature), while water has
a surface tension at room temperature of 73 dyne/cm. Achieving
non-wetting dispensing becomes increasingly more difficult when
dispensing ink below the mid-30's for surface tension of the
liquid. Current ink-jet printing technology uses nozzles with
various coatings and designs to print liquids with surface tensions
down to the 30's of dyne/cm.
[0009] With traditional ink printing, the ink has been designed to
meet the limitations of the nozzle. For example, for a given nozzle
design, the composition of the ink will be engineered and modified
with additives to achieve the performance characteristics required
to print with a given nozzle. Thus, ink compositions are engineered
to keep surface tension and viscosity above certain minimum values,
namely, 30's of dyne/cm. Commercial ink-jet developers have thus
avoided designing nozzles for the region of fluid properties that
have surface tensions below 30 dyne/cm by developing aqueous based
inks that are elaborately engineered with combinations of
additives.
[0010] Organic solvents have thus been particularly difficult to
dispense. Organic solvents may have both low surface tension and
low viscosity. The organic solvents of greatest practical interest
have surface tensions in the 20's dyne/cm and viscosity around or
less than 1 cP. Their low surface tension makes the liquid want to
wet or form a puddle at the nozzle exit, and inhibits droplet break
off. The viscosity of the liquid helps to pull liquid off of the
exit region, overcoming surface tension and forming drops. If the
viscosity is low, the fluid stream may not break into droplets, but
instead may stretch and display related problems. This combination
makes organic solvents more likely than water to wet the nozzle
during dispensing. Organic solvent are important in manufacturing
medical products by 3DP because some substances of medical interest
are soluble only in such solvents, not in water.
[0011] One consideration in controlling wetting behavior is the
geometric design of the nozzle. The simplest possible nozzle design
is a simple orifice, for example, a hole through a large flat
surface. Such nozzles are commonly used in applications such as
waterjet cutting and are typically made of sapphire or ruby with a
hole drilled through a flat exit surface. These orifices are only
available with a flat exit surface or with a recessed exit. The
jewel is typically held in the end of a tube of outside diameter
such as 0.050-inch whose edge is typically crimped over the edge of
the jewel. In applications involving dispensing of drops, such
nozzles are prone to wetting because of the flat exit geometry and
the fact that the jewel is not a particularly low-surface-energy
material.
[0012] FIG. 1 illustrates unsatisfactory nozzle performance of a
flat exit surface nozzle 100. As shown, a puddle 120 much larger
than the dispensed drop 130 forms at the flat exit surface 110.
Especially with organic solvents, such an orifice suffers
significantly from wetting with the establishment of an ongoing
puddle that contributes to inconsistent dispensing of the drops.
Thus, such flat exit surface nozzles are not optimal for precision
fluid dispensing, especially of organic solvents.
[0013] Another nozzle currently in use has a sharply tapered cone
having typically 30 degrees total included angle; an internal
passageway with a gradual transition of cross-sectional area inside
the body of the nozzle, being narrowest at the tip of the nozzle;
and a filleted transition region between the internal passageway
and the external surface. These nozzles have been typically made
for use as wire-bonding tools in the microelectronics industry;
thus, the design is not optimal for limiting the spread of the
liquid when printing.
[0014] Some other commercially available nozzles have been made
with a flat end (land), and are intended for use as vacuum pick-up
tools. Materials from which they are commercially manufactured
include tungsten carbide, Delrin.TM. (an acetal polymer), and
alumina (aluminum oxide). In such nozzle geometry, particularly the
vacuum pick-up tools that are flat-ended, the much-reduced size of
tip together with its sharp edges can help limit the size of the
puddle that may form. However, such geometry can exhibit another
problem, namely, swingback. Swingback is illustrated in FIGS.
2A-2C.
[0015] FIGS. 2A-2C are illustrations of dispensed liquid from a
commercially available nozzle illustrated as still frames of video
taken at a capture rate of 30 frames per second. The nozzle
illustrated in FIGS. 2A-2C were made of Delrin.TM. an acetal
polymer. The liquid being dispensed was a solution of 40% ethanol,
60% water, and has surface tension and viscosity closer to the
properties of pure ethanol than to the properties of pure water.
The inside diameter of the nozzle orifice was 0.006 inch (152
microns). The exit geometry was a flat cutoff (sharp-edged) as in a
vacuum pick-up tool, with the outside diameter of the land (flat
region) measuring 0.010 inch (254 microns). After the small flat
land, the nozzle exterior sloped back with a total included angle
of 30 degrees (15 degree half-angle). The direction of dispensing
was vertically downward.
[0016] In FIG. 2A, the stream of liquid 210 is dispensing from the
nozzle 220 and the exterior conical surface 205 of the nozzle 220
is dry. In FIG. 2B, the stream of fluid 210 has shut off and a drop
of liquid 230 has swung up onto the conical exterior 205 of the
nozzle 220. FIG. 2C illustrates that the swung-back drop of liquid
230 pulls a subsequent stream of fluid off-axis.
[0017] If this sequence is repeated, the swung-back drop on the
external conical surface can grow with incorporation of additional
liquid at each shutoff. The swung-back drop is a source of
asymmetry on an otherwise symmetrical nozzle and it interferes with
precise dispensing by causing misdirection and/or split-streaming.
Additionally, the swung-back drop detaches randomly as a dispensed
large drop.
[0018] Dispensed liquid exits the nozzle and moves downward due to
both gravity and momentum from the pressure-driven flow. Liquid has
to move upward in opposition of both the direction of gravity and
the direction of the dispensed momentum in order for swingback to
occur. The dominant physical mechanism causing swingback is the
surface tension of the liquid.
[0019] Similar nozzles made of tungsten carbide with flat ends were
tested and also exhibited results that were unsatisfactory and in
some cases, the results were worse. Similar nozzles of polished
alumina but with fillets also exhibited unsatisfactory performance.
Polished alumina is believed to be slightly better than Delrin.TM.
as far as surface energy, while the geometry involving fillets is
believed to be slightly less favorable than the sharp-edged
geometry. The filleted geometry is manufactured because it is
useful for wire-bond tools and with alumina it is not possible to
manufacture sharp-edged nozzles. Thus, even though all of these
nozzles have a small-tip externally tapered geometry that can be
expected to be more advantageous than the nozzle of FIG. 1, when
used with organic solvents they still suffer from wetting or
swingback.
[0020] Extensive testing has shown that with many of the organic
solvents of interest, the nozzles made from all of these
commercially available materials still suffer wetting of the
outside cone, and in particular suffer swingback of the last little
bit of liquid upon shutoff.
[0021] The literature contains a variety of materials and coatings
that have been developed to attain low surface energy and good
non-wetting characteristics. A convenient reference point is the
well-known material Teflon.TM. (polytetrafluoroethylene), which has
a surface energy variously quoted as 18-22 dyne/cm, most frequently
18 dyne/cm, and has a contact angle with water of 100 degrees. Of
materials that are widely known and available, Teflon is perhaps
the most hydrophobic.
[0022] In Physical Chemistry of Surfaces by Arthur W. Adamson and
Alice P. Gast (John Wiley, New York, 1997) p. 356, referencing E.
G. Shafrin and W. A. Zisman, J. Phys. Chem., 64, 519 (1960), there
is a chart summarizing hydrophobic polymers by chemical family and
by the particular atomic constitution at the surface. Of the
chemical families in that chart, fluoropolymers are in general the
most hydrophobic. The surface constitution --CF.sub.2--, is listed
on that chart with a surface energy of 18 dyne/cm and is described
as representing Teflon.TM. (polytetrafluoroethylene and related
substances). Teflon has a sufficiently low surface energy that
nozzles made of it alone can perform reasonably well with aqueous
solutions. However, even Teflon is not of sufficiently low surface
energy to perform satisfactorily with most organic solvents.
[0023] In the same chart, the chemical radical with the lowest
surface energy is the terminal trifluoromethyl group --CF.sub.3.
Zisman concludes that the best surface constitution for non-wetting
is terminal trifluoromethyl groups (--CF.sub.3). The radical which
is terminal --CF.sub.2H groups is not as hydrophobic as terminal
trifluoromethyl groups but is marginally better than the
--CF.sub.2-- groups which describe Teflon.
[0024] A family of especially low surface energy materials is
available from the Cytonix Corporation, Beltsville, Md. The
materials are described in U.S. Pat. Nos. 5,853,894 and 6,037,168.
These substances are characterized by having a terminal
trifluoromethyl group, and in particular by having the exposed
surface contain a high fraction of these terminal trifluoromethyl
groups. These are believed to be essentially the lowest surface
energy materials known. It is possible to achieve surface energies
as low as 10 or even 6 dyne/cm. The most hydrophobic properties are
achieved when the coating has an exposed surface consisting almost
entirely of trifluoromethyl (--CF.sub.3) groups, that is, 100% of
the area, with no other substituent groups exposed at the
surface.
[0025] Other nonwetting substances are also listed in these
patents. In particular, for these materials, it is found that the
surface energy is lowest on the surface that is exposed to the
atmosphere while curing or drying. At surfaces which were not
created during the curing or drying process but rather were exposed
later such as by a machining or cutting operation, the surface
energy is not so low, i.e., the material is not so hydrophobic.
Under best conditions, the surface energy achievable with these
fluoropolymers at the cured surface is 6 to 12 dyne/cm. Expressed
in terms of contact angle, these substances have reached contact
angles for water as high as 150 degrees. There are various
formulations including highly fluorinated epoxy (fluoroepoxy),
polymer which is heat-curable or curable by exposure to ultraviolet
light with the curing causing polymerization, polymer which is
already polymerized and dissolved in a fluorosolvent, resins of
varying viscosities, etc.
[0026] Other substances have also been investigated in the
literature. While the materials listed below do not have surface
energies as low as that of the Cytonix materials, still they are in
most cases more hydrophobic than Teflon. What is listed here is
sometimes surface energy or critical surface tension and sometimes,
contact angle, whichever is reported in the literature.
[0027] U.S. Pat. Nos. 4,344,993, 4,764,564 and 4,554,325, all
titled "Perfluorocarbon based polymeric coatings having low
critical surface tensions," disclose substances which are
modifications of perfluorocarbon and which have critical surface
tension less than approximately 14 to 15 dyne/cm, which is
described as being lower than that of pure perfluorocarbon.
[0028] U.S. Pat. No. 5,426,458 describes a coating of
poly-p-xylylene, which is available under the trade name Parylene N
and is intended for adhesion resistance and corrosion resistance,
having a contact angle with water of 110 degrees.
[0029] U.S. Pat. No. 5,073,785 comprises applying a coating of
amorphous or diamond-like carbon followed by fluorination of this
layer, having a contact angle with water of about 105.degree.
(100.degree..+-.5 degrees). U.S. Pat. No. 5,900,342 also discloses
diamond-like carbon.
[0030] U.S. Pat. No. 4,120,995 achieves a wetting angle for water
of 105+/-5 degrees that is slightly better than that of Teflon.TM.
(polytetrafluoroethylene). Other patents of interest are U.S. Pat.
Nos. 5,942,317, 4,565,714, and 5,900,342. U.S. Pat. No. 5,736,249
discloses a siliconic polymer having surface energy of 18-21
dyne/cm. The following U.S. Pat. Nos. are for nozzle plates for
ink-jet printing, all in a flat geometry: 5,812,158 (a low surface
energy polymer coating and also a coating of tantalum instead of
gold); 5,350,616 (a Kapton film); 4,643,948 (coatings for ink jet
nozzles, partially fluorinated alkyl silane and a perfluorinated
alkane). Other patents for coatings include U.S. Pat. Nos.
5,608,003; 5,266,222; 4,716,059; 5,942,317; and 4,565,714. Efforts
have also been made to modify the surface properties of silicon for
making nozzles, such as in U.S. Pat. No. 4,623,906 which discloses
a wetting-resistant coating which transitions from silicon to
silicon nitride to aluminum nitride, but the final surface of
aluminum nitride is not adequate to resist wetting by organic
solvents.
[0031] There is yet another factor which influences surface
hydrohobicity, namely surface microgeometry. If a surface is made
of a material that is already hydrophilic, roughness makes it more
hydrophilic. Conversely, if a surface is made of a material that is
already hydrophobic, roughness makes it more hydrophobic. This is
described in Physical Chemistry of Surfaces by Arthur W. Adamson
and Alice P. Gast (John Wiley, New York, 1997) and also in
Principles of Colloid and Surface Chemistry, Third Edition, by Paul
C. Heimenz and Raj Rajagopalan, (Marcel Dekker Inc., 1997).
Roughness of hydrophobic surfaces is cited in U.S. Pat. No.
6,037,168, which describes rough hydrophobic surfaces for use in
making inexpensive laboratory vessels.
[0032] In addition to these various geometry and surface
properties, there is also yet another factor which influences
swingback and may be somewhat related to the particular technique
used for dispensing. It appears that the velocity or momentum of
the departing liquid also has an influence on swingback. In
particular, under any circumstances and for any liquid, extremely
slow flow is unlikely to break away from the nozzle as a small drop
but rather is likely to remain on the nozzle due to lack of
momentum. If dispensing is done through a microvalve and the
shutoff of flow of liquid is not abrupt but instead continues
gradually after nominal shutoff, or if there is any leakage or
outflow of fluid between commanded dispensings, this can aggravate
or cause swingback.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0033] FIG. 1 is an illustration of the operation of a prior art
nozzle with a jewel nozzle opening and flat exit geometry.
[0034] FIG. 2 is an illustration of the operation of a prior art
nozzle with an externally tapered nozzle.
[0035] FIG. 3 illustrates a cross section of one embodiment of a
nozzle, the nozzle incorporating a pre-manufactured externally
tapered end and an exterior coating in accordance with the
principles of the present invention.
[0036] FIG. 4 illustrates the surface tangent angle of the nozzle
of FIG. 3 in accordance with the principles of the present
invention.
[0037] FIG. 5 is a photograph of the dispensing end of the nozzle
of FIG. 3.
[0038] FIGS. 6A-6C show a sequence of operation detailing initial,
steady-state, and shut-off flow of ethanol through the nozzle of
FIG. 3.
[0039] FIG. 7 shows the electrical waveform applied to the valves
in accordance with the principals of the present invention.
[0040] FIG. 8 illustrates a typical appearance of fluid stream with
organic solvents in response to the waveforms of FIG. 7 in
accordance with the principles of the present invention.
[0041] FIGS. 9A-9C are photographs of roughened and coated nozzles
in accordance with the principles of the present invention.
[0042] FIGS. 10A and 10B show nozzles roughened by laser-machining
circumferential grooves in accordance with the principles of the
present invention.
[0043] FIG. 11 shows a nozzle roughened by laser-machining both
circumferential and slant-height grooves in accordance with the
principles of the present invention.
[0044] FIG. 12 illustrates one method of applying a coating to a
nozzle in accordance with the principles of the present
invention.
[0045] FIG. 13 is a schematic view of an apparatus for applying the
coating illustrated in FIG. 12.
[0046] FIGS. 14A-14C illustrate cross sections of various alternate
geometries of nozzles in accordance with the principles of the
present invention.
[0047] FIGS. 15A-15C illustrate a nozzle made of low-surface-energy
resin as a bulk material making up the entire tip of the nozzle in
accordance with the principles of the present invention.
[0048] FIGS. 16A-16C illustrate various geometric calculations
related to FIGS. 15A-15C in accordance with principles of the
present invention.
[0049] FIGS. 17A-17G illustrate alternate manufacturing forms of
FIGS. 15A-15C in accordance with principles of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0050] A wetting-resistant nozzle, and in particular, an apparatus
and corresponding method for manufacturing a wetting-resistant
nozzle for use in dispensing low surface energy fluids wherein the
nozzle includes an exceptionally low surface energy (hydrophobic)
coating and specific nozzle geometry, is described in detail
herein. In the following description, numerous specific details are
provided, such as specific coatings, specific geometric nozzle
configurations, dimensions, specific dispensed fluids, and the
like, to provide a thorough understanding of the embodiments of the
invention. One skilled in the relevant art, however, will recognize
that the invention can be practiced without one or more of the
specific details, or with other coatings, dispensed liquids and the
like. In other instances, well-known structures or operations are
not shown or not described in detail to avoid obscuring aspects of
the invention.
[0051] One embodiment of the present invention includes a
wetting-resistant nozzle for accurately and precisely dispensing
small volumes of liquids. The nozzle comprises an internal
flowpath, and an external surface that recedes from the discharge
point at an angle greater than 90 degrees, and an exceptionally low
surface energy for the external surface. The low surface energy
material may exist as a coating on top of a shaped substrate. A
flat land region may be included and may have sharp edges, one of
which may define the boundary of the low surface energy region.
Another embodiment includes the low surface energy material as a
bulk material through which a hole is drilled. In yet another
embodiment, the internal flowpath inside the nozzle may be smoothly
tapered. Additional aspects such as surface roughening, surface
tangent angles, coatings and geometric configurations are described
herein in more detail with reference to the Figures and Examples.
Nozzles in accordance with the principles of the present invention
provide improved dispensing of liquids that have both low surface
tension and low viscosity, such as organic solvents.
[0052] The nozzle described herein in accordance with aspects of
the present invention may be axisymmetric having cylindrical
symmetry around an axis. The axisymmetric external surface may be
frusto-conical or it may be of other curved shape having
axisymmetry. The axes of symmetry of both the internal flowpath and
the external surface may be identical. The nozzle may have a
surface tangent angle, as defined for particular tip geometry,
which is greater than 90 degrees and perhaps substantially greater
than 90 degrees.
[0053] The nozzle described herein in accordance with aspects of
the present invention may be made of a bulk material which is
suitable for whatever manufacturing operations may be desired to
produce the desired geometry, while its surface attains the desired
surface energy properties in appropriate places by means of a
coating of a material having low surface energy. Alternatively, the
nozzle may use a low surface energy material as a bulk material
that is used to make essentially all of the nozzle tip including
its surface.
[0054] Several embodiments of nozzle geometry are defined herein in
according to the geometric nature of the transition between the
flow passageway and the nozzle exterior. One embodiment of the
nozzle includes a sharp, essentially knife-edge as the transition.
Due to manufacturing limitations it should be considered that any
real tip would have a transition region which is either a finite
(non-zero) radius of curvature or a finite (non-zero) width of flat
region, but in some circumstances the dimension of the radius of
curvature or land width may be very small in comparison to the
diameter of the orifice, and this can be considered a knife edge.
In such a case the detailed shape of the transition region is less
important to the final dispensed fluid. For example, manufacturing
a knife-edge orifice would be possible if the orifice diameter were
of a moderate size rather than extremely small. Appropriate regions
of the nozzle would include surfaces of a low surface energy.
[0055] Alternative embodiments of nozzles have small diameter
orifices such that, given the manufacturing limitations applicable
to manufacturing the tip, it is not possible to attain the limiting
situation of a knife-edge. In such a case, the nozzle may comprise
a small land that provides minimal space for development of a
persistent drop. A land is a flat region essentially perpendicular
to the principal flow direction of the nozzle. A sharp edge at the
outer edge of the land may help to discourage the liquid from
advancing beyond that edge, as does a receding, possibly tapered
(or other-shaped) exterior. Appropriate regions of the nozzles
would have a low surface energy. The nozzles of the first several
examples are for the case where a flat surface is deliberately
manufactured at the tip.
[0056] In accordance with yet another embodiment of the present
invention, there is a nozzle configuration wherein the nozzle
exterior is essentially a portion of a sphere, namely, curved,
having low surface energy, and then a hole is drilled through the
curved material. A fairly sharp corner or edge may be assumed to
exist where the hole meets the curve. Alternatively, nozzles with
fillets are also possible, having low surface energy in appropriate
regions.
[0057] In accordance with another aspect of the invention, another
consideration that influences the wetting of a solid by a liquid is
the surface energy of the nozzle surface material, or, more
exactly, the comparison between the values of the surface tension
of the liquid and the surface energy of the solid. Both of these
properties are expressed in units of dyne/cm or equivalent units.
The surface tension is a property of a liquid, which is determined
largely by chemistry and can be significantly influenced by
additives (e.g., surfactants). The surface energy of a solid is a
material property that is determined in large part by chemistry but
also is influenced to some degree by surface finish or
microstructure, crystal orientation, manufacturing methods,
cleanliness/contaminants, etc. If the solid surface energy is
somewhat greater than the liquid surface tension, by approximately
10 dyne/cm, then wetting typically occurs. If the solid surface
energy is somewhat lower than the liquid surface energy, then
wetting typically does not occur. There is a range of partial
wetting when the two quantities are in the same range of magnitude
as each other.
[0058] In accordance with another aspect of the invention, wetting
is also quantified by the contact angle. The surface contact angle
is the tangent angle where the surface of a liquid drop meets the
solid surface. Non-wetting is characterized by liquid droplets
"beading up" when they are dispensed onto the solid surface.
Contact angles approximately equal to or greater than 90 degrees
are considered non-wetting or minimally wetting. Low contact angles
of several tens of degrees indicate behavior that is almost
wetting. At even lower contact angles or greater excess of the
surface energy over the surface tension, liquids spread rapidly
over the solid surface. Smaller surface energy and larger contact
angle are associated with non-wetting behavior. The relation
between solid surface energy, liquid surface tension and contact
angle is known as Young's equation.
[0059] Under aspect of the present invention, various available
formulations of low surface energy materials are discussed herein.
In general, the surface could be made of any material mentioned
herein or in the incorporated references which has a surface energy
of 17 dyne/cm or smaller (less than that of the commonly available
Teflon). A particular material which may be used here is a highly
fluorinated epoxy (fluoroepoxy). A material that may be used is a
material having a large fraction of terminal trifluoromethyl groups
at its surface such as is available from the Cytonix Corp. Many of
these are curable materials, typically heat-curable or curable by
application of ultraviolet light. Another version is already
polymerized and is dissolved in a fluorosolvent for purposes of
application, with the fluorosolvent then disappearing by
evaporation (possibly at elevated temperature).
[0060] In particular, for these materials, it is found that the
surface energy is lowest on the surface that is exposed to the
atmosphere while curing or drying, which is associated with the
preferred orientation of the terminal trifluoromethyl group. At
surfaces which are exposed after curing or drying, as a result of a
machining or cutting operation, the surface energy is not so low,
i.e., the material is not so hydrophobic, which is less favorable
for resisting wetting. At surfaces that are cut or otherwise
exposed after curing, the surface energy is approximately in the
range of the surface energy of conventional Teflon. This means that
design and processing techniques may be arranged so that the final
surface in the desired parts of the nozzle is an as-cured surface.
One way of achieving this is to apply the material as a coating on
a substrate that is already in the desired final shape.
[0061] Under best conditions, the surface energy achievable with
these fluoropolymers at the cured surface is 6 to 12 dyne/cm.
Expressed alternatively in terms of contact angle, these substances
have reached contact angles for water as high as 150 degrees. In
addition, the fluoroepoxy materials are suitably hard and able to
be drilled if necessary. The fluoropolymers can also be put onto a
surface in a thin layer and cured to form a coating. They are also
immune to organic solvents of interest including alcohols and
chloroform.
[0062] Except where stated otherwise, the data presented herein is
for nozzles which have been coated with a Cytonix fluoropolymer
FluoroPel.TM. PFC1604A, involving an oligomer with a carbon chain
length 5 to 18, which was applied as a liquid resin having a low
viscosity similar to that of water. This substance is already
polymerized and is dissolved in a fluorosolvent. In such a case
heat is used to evaporate the solvent but heat is not needed for
curing (i.e., polymerization). Another available formulation used
for one example is designated GH000 (heat curable) with the curing
causing polymerization. Ultraviolet curing formulations are also
available.
[0063] In the data reported here, liquid was dispensed through
miniature solenoid-operated valves. Fluid was caused to flow
through the valve under action of a steadily maintained pressure
from a fluid reservoir. When electric power to the valve is on, the
plunger of the valve lifts up from the seat all the way or at least
partway. When electrical power is off, the plunger returns and
closes due to the action of a spring and also fluid pressure. Two
sizes or designs of valves were used during this work. One type of
microvalve had valve seats made of Viton-GF so as to be
chloroform-compatible and it was used in experiments involving
chloroform. The other type of valve used in the present work has
the smallest seat size of any standard design currently available.
In this case the seat was made of EPDM (ethylene propylene diene
rubber). These latter valves were used with ethanol and
ethanol-water mixtures. All data reported here were taken with the
microvalve being driven by a pulse width modulated driving
waveform, with a microvalve actuation frequency of 800 Hz, pulse
width approximately 200 microseconds, and a voltage magnitude of
pulse of 40 Volts.
[0064] The invention is further described, but is in no way
limited, by the following examples.
EXAMPLE 1
Smooth, Tapered, Coated Nozzle
[0065] In one embodiment of the present invention, the nozzle
design includes several features that together result in an
improved wetting resistant nozzle performance. The features of the
present embodiment are summarized in FIG. 3. The overall bulk shape
or body of the nozzle is made of a material which is selected in
part for its ability to be manufactured in the desired shape. The
desired surface properties may be obtained by applying a coating to
selected places on the nozzle.
[0066] FIG. 3 illustrates one nozzle configuration in accordance
with the principles of the present invention. The nozzle 300 has an
external surface 310, an internal passageway or flowpath 320, an
inlet 305 and an outlet 315. The internal flowpath 320 in the
present embodiment is smoothly contoured and tapered to allow
laminar flow of fluid. The external surface 310 includes a sharply
angled exterior surface 330, a coating 340 applied to the external
surface 310 between the angled exterior surface 330 and the outlet
315, and a land region 350 at the outlet 315. The exterior surface
330 may optionally be surface roughened before applying the coating
340. The external surface 310 may be axisymmetric. The axisymmetric
external surface 310 may be frusto-conical as shown in the present
embodiment.
[0067] In this example, the transition region between the internal
passageway 320 and an external axisymmetric surface 310 is a small
flat region that is essentially perpendicular to the axis of the
internal flowpath 320. This flat region may be referred to as the
land 350. This geometry is easier to manufacture than a knife-edge
or even a rounded knife-edge, partly because it provides a chance
for dimensional errors to be absorbed simply as irregularity of the
dimension of the land. For example, if the external surface is
highly tapered, minor dimensional errors in the coaxiality of
alignment of the internal passageway with the external surface
would cause a knife-edge to wander seriously out of plane or
circularity, whereas with the use of a flat land such errors would
merely cause the annular land to have a minor amount of
eccentricity while the land would remain in a well defined plane.
Keeping the exit plane perpendicular to the flow direction has more
impact on flow than keeping the interior edge and the exterior edge
of the annulus perfectly concentric with each other. Eccentricity
of the land has a much less serious impact on dispensing geometry
than would be the impact of a knife-edge wandering out of
plane.
[0068] A flat land is defined by where it meets or adjoins two
other surfaces: the internal passageway 320 and the external
axisymmetric surface 310. The angle at which it intersects either
of those other two surfaces is defined as being 90 degrees at the
interior, and at the exterior it is less than 90 degrees but can be
close to 90 degrees. It is known that sharp corners or edges, such
as where the land meets the exterior, can arrest the spread of a
liquid on a surface. Because of the use of a design with a flat
land, the edges where the land adjoins those other two features may
each be designed to be substantially sharp. The term sharp is used
with the realization that, at some size scale, any real edge has
some detail that may approximate a rounded edge having an edge
radius. For the present embodiment, a substantially sharp edge
means that the edge radius may be small compared to the lateral
dimension (outer radius minus inner radius) of the land, such as
edge radius being less than one-fifth of the lateral dimension of
the land.
[0069] The land 350 is made as small as possible compared to the
diameter of the orifice or outlet 315 in order to minimize space
available for a puddle to form. The land is typically a result of a
particular manufacturing process and cannot realistically be of
zero width, but for purposes of the present embodiment, is
minimized. Therefore, for some sets of nozzle dimensions, the land
dimension may be determined by being the smallest dimension that is
practical for manufacturing. An approximate manufacturing limit is
believed to be the outside diameter minus the inside diameter being
no smaller than approximately 0.001 inch. The nozzles for which
experimental results are reported in this example include a small
flat land.
[0070] FIG. 3 further illustrates an interior flowpath of the
nozzle 300 wherein the flowpath includes an angle 380 and a taper
385. The taper 385 transitions the internal flowpath from an inlet
diameter ID down to the outlet diameter D of the nozzle at the
outlet 315 for dispensing. A gradually tapering internal flowpath
320 helps to achieve a good quality dispensed flow because it
minimizes flow disturbances that might be created in the flow just
before it exits from the orifice. Nozzles have been tested which
had a more abrupt entrance to the final narrow hole, and these
other nozzles had a greater tendency to exhibit split-streaming.
With the nozzles as shown here, the internal flowpath 320 is
tapered at an angle typically less than or approximately equal to
10 degrees total included angle. The internal flowpath 320 may have
a constant-diameter flowpath for a small distance immediately back
from the nozzle exit, such as several diameters of the exit hole,
and, preceding that, a gradual taper of the described angle.
[0071] In yet another embodiment of the present invention, the
surface of the internal flowpath or passageway 320 has a relatively
large surface energy, namely, a wettable or hydrophilic surface.
The hydrophilic surface encourages the liquid to occupy an
acceptable place in the nozzle, in contrast to the unacceptable
presence of liquid on the external surface of the nozzle.
[0072] FIG. 4 illustrates surface tangent angle SA for a nozzle
400. In specifying a nozzle design, one geometric specification is
the surface tangent angle SA. One ray of this angle is the flow
vector that is typically the direction of discharge of flow along
the axis of symmetry 405 of the flow passageway 410 in the nozzle
400. The other ray is a ray, coplanar with the first ray, which
goes through the corner where the land 415 meets the nozzle
external surface 425, and is tangent to the nozzle external surface
425 at that point. This surface tangent angle SA is greater than 90
degrees, and may be substantially greater than 90 degrees. For
example, the surface tangent angle SA may be as large as 165
degrees or even 170 degrees. In the nozzle 400 shown in FIG. 4, the
conical exterior has a total included angle 420 of 30 degrees
(half-angle 430 being 15 degrees), which means that the external
surface angle 410 with respect to the flow direction is 165
degrees.
[0073] There is a certain distance back from the tip of the nozzle
at which the features of the present invention, such as surface
tangent angle and surface energy, have an effect. Beyond that
distance it is no longer important to maintain those features of
the present invention and it is possible to depart from those
features with no adverse impact. For example, beyond such distance
it is possible to depart from the frusto-conical shape, and it
would also be possible to change to a surface composition or other
surface property so as to be not as hydrophobic as in the region
near the tip. For example, such a distance would be related to the
largest distance that liquid is capable of swinging back, or the
largest drop that forms if swingback does occur. Such a distance
away from the exit may be estimated as 0.5 inch for typical
dimensions of small nozzles for 3DP printing applications.
[0074] In the nozzles of the present embodiment, a sharp corner is
where the surface properties change from the moderately-wetting or
highly-wetting surface energy of the land to the lower surface
energy of the hydrophobic external surface. This abrupt physical
edge and materials change provides two disincentives to the further
spread of fluid, namely, the sharp change of angle and the change
to a much more hydrophobic surface.
[0075] Therefore, the coating procedure is conducted so as to
attempt to cause the resin to stop specifically at the sharp comer
or edge where the conical exterior meets the land, but not to coat
the land. If the land is small, it is acceptable for the land to be
of a relatively high surface energy. In fact, as described, the
presence of a small land of relatively high surface energy may even
be helpful in controlling swingback of drops. The land may provide
a place for excess fluid to collect that might otherwise swing back
onto the exterior cone. The use of a sharp edge where the land
meets the external surface may serve the purpose of arresting the
spread of fluid on a solid; namely, to discourage liquid on the
land from spreading onto the conical exterior.
[0076] The coating material applied to the nozzles of this example
was Cytonix FluoroPel PFC 1604A, which is a relatively low
viscosity formulation, with heating in accordance with the
manufacturer's instructions
[0077] The overall bulk shape or body of the nozzle may be made of
a material that is selected in part for its ability to be
manufactured in the desired shape. Examples of suitable bulk
materials include tungsten carbide and also other ceramics, metals,
silicon and polymers. Manufacturing processes appropriate to
various of these materials for purposes of making the desired
nozzle shape include machining, drilling, various forms of EDM,
etching, molding/casting, etc. The desired surface properties may
be obtained by applying a coating to selected places on the
nozzle.
[0078] The nozzles of this embodiment were made of tungsten
carbide. Tungsten carbide can be manufactured into varied and
complex geometries by sintering powder together. Tungsten carbide
can be ground to provide finer resolution and is electrically
conductive. The electric conductivity means the tungsten carbide
can be cut by Electrical Discharge Machining so as to make cuts
that are accurately dimensioned, sharp-edged and burr-free.
Tungsten carbide has a relatively large surface energy, which is
not a useful property for the vast majority of the external surface
when fluid is being dispensed. Therefore, the nozzles of the
present embodiment were coated, such as with the Cytonix material,
to provide low surface energy on appropriate surfaces. Having the
uncoated regions such as the small exposed land and the interior of
the flow passageway be of relatively large surface energy
(hydrophilic or wetting) may be of some use in encouraging the last
segment of liquid to remain in those places rather than swinging up
onto the exterior cone surface. For example, this surface may have
a surface energy of 50 dyne/cm or greater.
[0079] FIG. 5 illustrates a nozzle outlet in accordance with the
above-described embodiment. The nozzle is made out of tungsten
carbide, and had a coating of fluoropolymer resin applied to its
external conical surface. The nozzle has a land outside diameter OD
of 0.012 inch (305 microns), an outlet hole inside diameter ID of
0.0107 inch (272 microns), exterior coated with fluoroepoxy.
Alternative dimensions include, for example, combinations of
orifice diameter and land outside diameter, respectfully, of: 0.004
inch, 0.005 inch; 0.003 inch, 0.0045 inch; 0.002 inch, 0.003 inch;
and 0.001 inch, 0.002 inch.
[0080] FIGS. 6A-6C are frames of videos of various flow regimes
through the nozzle of FIG. 5. The photos in FIGS. 6A-6C are a
sequence of successive frames from a video, taken at around the
time of shutoff of the programmed sequence of valve pulses. FIG. 6A
is a photo of a nozzle dispensing a fully-established non-wetting
flow of pure ethanol through the nozzle. FIGS. 6B and 6C are
successive frames of video at a speed of 30 frames per second
depicting the shutoff sequence of the nozzle dispensing the pure
ethanol stream. In FIG. 6A, flow is still fully established. In
FIG. 6B, a wisp of fluid appears some distance downstream of the
nozzle, but there is no fluid immediately at the nozzle. In FIG.
6C, fluid has stopped flowing out of the nozzle, and whatever fluid
already left the nozzle has fully detached from the nozzle and has
left no swingback or drop on the nozzle.
[0081] Videos of flow through the nozzle show that there is a very
small probability of swingback happening at the time of turning on
a stream, and that the most likely time for a drop to swing up and
attach to the exterior cone is at the time of shutoff. The present
embodiment described herein uses microvalves pulsed at 800 Hz.
Programmed operation of the microvalve is intermittent, with
approximately 50 consecutive valve actuations at intervals of 1/800
sec., followed by a somewhat longer quiet period.
[0082] FIG. 7 illustrates a graph of a typical electrical waveform
applied to a microvalve shown as a Pulse as a function of Time. In
the current embodiment, a pulse 710 is sent for a time T.sub.1 to
open the valve and begin flow. A period of no pulse or voltage 720
for a period of time T.sub.2 follows in which the valve is off.
[0083] FIG. 8 illustrates flow from a nozzle pulsed by the waveform
of FIG. 7. During the time that a pulse was being dispensed, the
stream appeared to issue from the nozzle 800 as a connected string
of bulges 810, even though the microvalve was being discretely
pulsed, namely, turning on and off repeatedly, being on for a time
T.sub.1 of typically 200 microseconds followed by off for a time
T.sub.2 of typically 1050 microseconds. Such discharge is
characteristic of fluids of certain combinations of properties as
defined by the Ohnesorge number.
[0084] Experiments in accordance with the present embodiment
produced good coherence of streams issuing from the nozzle. Usually
the stream not only was coherent when it issued from the nozzle but
also remained essentially just as coherent one inch or even several
inches below the nozzle. This was due in part to the smooth and
very gradually tapering internal flowpath inside the nozzle.
EXAMPLE 2
Roughness Made by EDM Roughening or Other Random-Roughness
Methods
[0085] Yet another embodiment of the present invention continues to
use the overall geometric features and the hydrophobic materials of
the nozzle design of the previous embodiment, while incorporating
surface roughness, described earlier in very general and brief
terms. In general, this alternative embodiment and those described
in this application are substantially similar to previously
described embodiments, and the same reference numbers identifies
common elements and steps. Only significant differences in
construction or operation are described in detail.
[0086] The nozzle of the present embodiment takes advantage of the
theoretically predicted increase in apparent hydrophobicity due to
surface roughness by making a surface that is both rough and of low
surface energy. In the present embodiment, roughening a bulk piece
of hydrophobic material has not done, although it could be done for
applications dispensing relatively easy fluids not requiring the
extreme lowest surface energies. The reason the bulk piece is not
roughened is because the leading candidate resins are not as
hydrophobic as would be desired when below-surface material is
exposed. Therefore, the nozzles of this embodiment were produced by
making a nozzle body or substrate which is rough in desired places,
and then coating those places with the hydrophobic material of
interest in such a way that the final surface of the coating also
had at least some of the surface roughness features of the
substrate. In this way the final rough surface was an as-cured
surface having the preferred orientation of molecules at the
surface. In this example the coating material was the fluoropolymer
but it could also be any of the other related substances that are
discussed herein.
[0087] This nozzle of this embodiment was manufactured by
manufacturing the nozzle with a smooth surface and then roughening
the surface in a random pattern. This example used a form of EDM or
spark erosion to roughen the surface of the nozzle. This is
possible because the tungsten carbide, of which the nozzle was
made, was electrically conductive. In it, the electrode was a
closely fitting mating shape which was rotated with respect to the
nozzle, and as current flowed, sparks at the interface between the
electrode and the nozzle caused erosion of the nozzle. The
dimensions of the surface roughness or texture produced by this
process appear to be on the order of single digits of microns.
[0088] After such a rough substrate has been created, the surface
coating for this embodiment should be such that at least some of
the substrate roughness is apparent on the coating surface. For
example, the coating thickness dimension must be small enough so as
not to fill up or smooth out the depressions in the patterned or
roughened surface. Accordingly, a resin of relatively low viscosity
is used. The resin used was FluoroPel(.TM.) PFC 1604A from Cytonix
Corp. After a coating of this liquid resin was applied, a jet of
compressed gas was directed against the coating in a direction away
from the tip of the nozzle so as to thin out the coating layer
while it is still liquid by pushing some of the liquid to a place
where the coating thickness is not important or excess liquid can
be removed. After this was done, heating to 60-80 C. was performed
according to the manufacturer's instructions to evaporate the
solvent or cure the resin.
[0089] FIG. 9A shows a nozzle 900 of the present embodiment as
originally manufactured with a smooth exterior surface 910. In this
photograph, the diameter D of the orifice is 0.005 inch (127
microns). FIG. 9B shows the nozzle with a roughened exterior
conical surface 920. The roughening is accomplished by electrical
discharge from a rotating closely fitting mating surface
(electrode) EDM. The estimated characteristic dimension of this
roughness, average feature-to feature distance, or average depth of
roughness, is in the single digit microns. FIG. 9C shows the nozzle
of FIG. 9B coated with the described hydrophobic coating 930. As
illustrated, the coating has a sufficiently low viscosity such that
the resin does not completely fill up or smooth over the
roughness.
[0090] Alternatively, a similar random roughness on the surface by
abrasive mechanical means could be created by sandblasting,
abrasive water jet, rubbing with sandpaper or abrasive (which may
be selected so as to be harder than the material of the nozzle),
and the like. It would also be possible to prepare a rough surface
by adding material onto an originally manufactured smooth surface,
such as by adhering grains of particulate matter of a suitable size
using a suitable adhesive. In any of these cases, the coating would
then be applied as previously described.
[0091] Additionally, instead of forming a coating by application of
liquid, it would be possible to use any of the coatings which are
known as being formed by a process amenable to gaseous deposition,
such as formation of diamond-like carbon followed by fluorinating
by reaction with a fluorine-containing gas, or the formation of the
poly-p-xylylene coating known as Parylene. Such processes,
involving only gaseous substances, are adapted to coating minute
surface features and very closely following the shape of the
surface features. These coating options apply as well to the
laser-machined examples that follow and in general to any nozzles
of the present invention.
EXAMPLE 3
Roughness Made by Laser-Machining, Having Predetermined Geometry,
Circumferential Grooves
[0092] Yet another embodiment of the present invention creates
surface roughness, in specific predetermined geometries. Removing
portions of the tungsten carbide material, making grooves or
similar relieved features by laser machining, provide the
roughening in this embodiment. The method of laser machining used
is photoablation/decomposition, which is said to be superior to
melting/evaporation in terms of cutting quality.
[0093] In accordance with aspects of this embodiment, the positions
and dimensions of material that is either removed or left
undisturbed can be predetermined and controlled quite accurately by
the programmed position of the laser beam during the laser
machining process. Feature sizes such as 0.001 inch are possible.
It is believed that for structural integrity an appropriate value
of the groove-to-groove thickness of remaining material (i.e.,
distance from the interior machined wall surface of one groove to
the corresponding surface of the adjacent groove) is at least
approximately 0.0005 inch, for structures manufactured by this
method. An appropriate depth of groove is 0.001 inch (25
microns).
[0094] A laser-machined roughness pattern which has been found to
work reasonably well for purposes of enhancing wetting resistance
is a depth of groove of 0.001 inch (25 microns), a width of groove
of 0.001 inch (25 microns), and a remaining wall thickness between
grooves of 0.0005 inch (13 microns). This results in a situation
where only a relatively small fraction of the original surface
(such as one-third) survives undisturbed to form high spots, while
the rest of the original surface is recessed, which is believed to
be helpful, Dimensions such as these have been used with good
results.
[0095] It can be appreciated that if a roughness pattern is going
to be machined into the conical surface by removing material from
that surface, it is necessary to start with a greater thickness of
the land and the nozzle wall than would be used in the non-rough
strategy. Sufficient land and wall thickness must be provided to
avoid having the laser-machined grooves puncture into the internal
flow passageway thereby creating leaks. For a groove whose depth is
nominally 0.001 inch, the bare minimum requirement is that the land
outside diameter be greater than the orifice diameter by 0.002 inch
(51 microns), in which case the bottom of the laser-machined groove
would just start to break through into the interior passageway,
assuming there were no dimensional inaccuracy in the groove depth
and no eccentricity of the internal flowpath with respect to the
nozzle external surface. Practically speaking, some additional
amount of material remaining between the bottom of the
laser-machined groove and the internal flow passageway will remain.
Additionally, this material allows for possible dimensional
inaccuracy and eccentricity, in order to avoid puncturing into the
internal flowpath.
[0096] When a groove is laser-machined to a nominal depth of 0.001
inch (25 microns), it is satisfactory to start with a land outside
diameter which is 0.005 inch greater than orifice inside diameter,
leaving a nominal wall thickness of 0.0015 inch between the base of
the groove and the internal passageway. This leaves enough of a
margin against cutting too deep and puncturing. Slightly smaller
margins might also be satisfactory. Of course, for typical nozzle
designs, as one goes farther away from the tip, the internal
flowpath widens less steeply than the exterior of the cone, and so
the wall thickness improves as one goes further away from the
tip.
[0097] After the laser-machining operation, the nozzles may be
coated with Cytonix PFC 1604A, which is a low viscosity
formulation. In order to thin out the deposited resin and retain as
much as possible of the initially rough topography when the coating
process is completed, a jet of clean dusting gas is directed at
nozzles just after they are coated with liquid resin, in a
direction away from the tip. Curing of the resin is then performed
according to the manufacturer's instructions.
[0098] FIGS. 10A and 10B illustrate laser machined nozzles. FIG.
10A is a photograph of a nozzle that has been laser machined in
accordance with the above description. FIG. 10B illustrates the
same laser machined nozzle coated with a low viscosity coating.
[0099] In the present embodiment, laser machining has been used
here to remove material in predetermined patterns having small
feature dimension. However, alternate methods of achieving the same
structure including photolithography and similar techniques known
from the microelectronics fabrication industry, or even by
programmed wire EDM machining using fine wire. The method of
material removal may be somewhat dependent on the selection of the
material into which grooves or roughness are being cut. It would
also be possible to create surface roughness of predetermined
geometry by addition of material (such as by adhesion of particles
or by vapor deposition or plating) onto an initially smooth surface
rather than by removal of material.
[0100] One parameter for roughness-enhanced hydrophobicity is the
ratio of the actual final surface area including all the recesses
and protrusions, compared to the original or projected or flat
surface area. The projected surface area is the area that exists,
without the surface irregularities, when viewed looking normal to
the overall surface. The larger the ratio of actual surface area to
projected surface area, the greater the improvement in
hydrophobicity compared to a flat unmodified surface. There are
formulas in the literature of surface science where this ratio
appears as a parameter. Another way of describing the same
phenomena is to imagine that perhaps droplets rest only on peaks,
and if the area of peaks is small compared to the area of the
unmodified surface, then the improvement is substantial. Either way
of looking at the phenomenon provides the same insight as to how to
maximize its effect, namely by making the surface geometry more
extreme and irregular.
EXAMPLE 4
Crossed Laser-Machined Grooves
[0101] The preceding example suggested that roughness-enhanced
hydrophobicity benefits from a more extreme, irregular surface such
as by having only a relatively small fraction of the original
surface survive. The previous example accomplished this in a
one-dimensional sense with parallel grooves being created in only
one direction, namely, circumferential. There is probably some
limitation as to how far this trend can be carried in a
one-dimensional sense, because at some point the actual width of
the groove might have to become impractically large, such as
comparable to the natural dimension of a drop of dispensed
liquid.
[0102] Accordingly, yet another embodiment of the present invention
is to make this groove effect more pronounced by making
intersecting cuts in two approximately orthogonal directions, so
that the ridges of the preceding example become essentially
interrupted ridges or, as a limiting case, spikes or bristles.
[0103] Accordingly, nozzles were made with a crossed pattern of
laser-machined grooves machined in two mutually orthogonal
directions rather than just circumferential grooves. The
circumferential grooves were as described in the preceding example,
with a groove depth of 0.001 inch and a groove width of 0.001 inch
and a groove-to-groove spacing or remaining wall thickness of
0.0005 inch. The second group of grooves may be termed slant-height
grooves because they exist on the slant height of the cone. They
were also 0.001 inch deep and 0.001 inch wide. This resulted in a
pattern in which the most-elevated remaining features of the
original conical external surface were islands or interrupted
ridges rather than continuous ridges. The crossed grooves removed a
greater fraction of the original surface by removing material from
two directions instead of just one direction.
[0104] It can be appreciated that if a certain number of
slant-height grooves, having constant groove width, are designed so
as to remove a certain fraction of the circumference of a ridge at
the tip of the nozzle, then as the cone becomes wider further from
the tip, the fraction of the ridge which is removed will not be as
large and the circumferential dimension of those remaining ridge
segments will increase. Eventually the length of ridge segments
will increase to the extent that the fraction of a circumference
removed by the slant-height grooves may no longer be significant.
At this point it may be worthwhile to introduce an additional set
of slant-height grooves (such as one groove between each of the
already-described slant-height grooves), that start some distance
away from the tip of the nozzle where there is sufficient room. If
necessary, as one goes even further from the tip, yet another set
of slant-height grooves can be introduced, and so on.
[0105] FIG. 11 is a photograph of a nozzle having laser-machined
grooves in both directions. One set of slant-height grooves begins
immediately at the tip of the nozzle, and a second set of
slant-height grooves begins some distance away from the nozzle tip
where there is sufficient room for an additional groove. This
nozzle in FIG. 11 is not yet coated. The nozzles were then coated
with Cytonix resin as described in preceding examples. The
slant-height grooves are shown as being straight lines but could
also be curved lines if desired.
EXAMPLE 5
Experimental Results Concerning Swingback
[0106] In accordance with yet another embodiment of the present
invention, swingback of the nozzle is minimized or eliminated. One
way of comparing the performance of various different nozzles is to
list conditions for which each type of nozzle does or does not
experience swingback. As already described, swingback is deposition
of some liquid on the tapered exterior surface of a nozzle.
Although swingback has occasionally been observed to a very minor
extent at startup of flow, it is primarily a phenomenon associated
with the shutoff of flow.
[0107] Instead of all the dispensed liquid continuing to proceed
from the nozzle toward the target at the time of shutoff, sometimes
the last little bit of liquid to pass through the nozzle does not
detach from the nozzle, and does not travel toward the target, and
instead ends up on the outside of the nozzle as a result of a
rather large change in the direction of its motion. This is
undesirable because it results in a sustained drop on a surface of
the nozzle (such as the conical exterior) that is usually
asymmetrical and is near enough to the intended exit path of liquid
that it can interact with subsequent dispensed liquid. A sustained
drop in that location can pull later exiting liquid away from its
intended path, making for inaccurate direction of travel of liquid
and hence inaccurate position of printing. Also a large swung-back
drop can itself detach from the nozzle occasionally at
unpredictable times and fall onto the surface being printed, which
might ruin a printed part.
[0108] Sometimes one shutoff event is enough to produce a large
sustained drop on an initially dry nozzle exterior. In other
circumstances, sometimes a sustained drop grows very slowly with
each shutoff event and only becomes a problem after many shutoffs.
Unless otherwise noted in the table, a notation of swingback in
this example indicates that one shutoff event is sufficient to
produce a swingback of a significant size.
[0109] In Table 1, the vertical axis represents (from top to
bottom) a sequence of increasing degree of difficulty for liquid to
dispense and break off cleanly at the end of a commanded flow or
series of drops. The ordering of degree of difficulty can perhaps
be best understood by thinking in terms of how much momentum the
last piece of liquid has coming out of the nozzle at the time of
shutoff, on the thought that such momentum helps to pull or carry
the liquid away from the nozzle in opposition to surface tension
forces which tend to make it swing up and back. The first case in
this axis of the table, which is easiest case for avoiding
swingback, is the case of continuous flow dispensing, also known as
line-segment printing. In this case the valve remains open
continuously for a substantial length of time and so steady-state
continuous pressure-driven flow exists. In line-segment printing
the flowrate and consequently the liquid average velocity is as
large as it can possibly be under given fluid reservoir
conditions.
[0110] The next, slightly more difficult case is a pulse train
whose length is a very large number of pulses. In this mode of
operation the microvalve is energized by an electrical waveform
having a pulse width that is about 20% of the duration of one
cycle, and then for the remainder of the cycle it is not energized,
and this pattern is repeated for many consecutive pulses. This is
an attempt to produce a fluid stream that is a succession of
discrete drops. However, for the organic solvent liquids used here,
the appearance of the dispensed fluid stream, at any distance from
the nozzle short enough to be useful in three dimensional printing,
is believed to be connected bulges rather than discrete individual
drops as already described in FIG. 8.
[0111] When the pulse train contains a very large number of
consecutive pulses, this means that the situation regarding flow
and form of fluid structures has reached quasi-steady-state and
there is no influence of any possible startup transient which might
last for some number of cycles or pulses. The fluid stream contains
an average fluid exit velocity that is some fraction of what it was
in the case of line-segment printing, somewhat reflecting the fact
that the valve is open for only a fraction of the overall time. In
the situation that exists after a substantial number of consecutive
actuations, this (average) exiting velocity is as large as it can
possibly be for pulsed operation.
[0112] Cases of further increasing difficulty are when the number
of pulses in each pulse train is finite and becomes smaller and
smaller. In between the described pulse trains are time intervals
of sufficient length that there is no carry-over effect from one
pulse train to another. It has been observed in calibrations of
flowrate for pulsed operation of microvalves that there is a
correction factor describing the fact that the flowrate (during the
time that flow is on), or volume per drop, becomes smaller as the
pulse train becomes shorter and shorter. The correction is in the
range of 10% to 15% for the shortest pulse trains compared to very
long pulse trains. It is believed that this correction illustrates
the existence of a startup transient at the beginning of a pulse
train, implying that there is less momentum contained in a short
pulse train than in a pulse train that is long enough to have
reached quasi-steady-state. This decreased momentum for shorter
pulse trains probably also makes for more difficulty as far as
clean breakoff of fluid from the nozzle. Of course, for a nozzle to
be able to print finely detailed features such as in 3DP, it is
desired that flow be able to shut off cleanly without swingback
even for rather short numbers of pulses in a pulse train.
[0113] The other axis of the table is a progression of degree of
roughness of the external conical surface of the nozzle, which is
further elaborated by listing results for two different fluids to
also illustrate a sort of progression in terms of difficulty of the
fluid for non-wetting purposes. The progression in this axis of the
table is based on the belief that rougher is better, at least for
dispensing pure solvents. The first three columns in the table form
a progression from smooth to slightly rough to rougher. The
progression of roughness is from a smooth surface to an EDM
roughened surface to a surface with laser-machined grooves in only
the circumferential direction. In the last column of the table, the
progression further advances to a surface with laser-machined
grooves in two mutually orthogonal directions. All of the surfaces
are coated with a low surface energy coating.
[0114] In this table, the first column is for a nozzle whose
exterior was smooth as purchased, and was coated with Cytonix. The
slightly rougher situation for a nozzle that was EDM-roughened and
then coated with Cytonix. This surface was somewhat rougher but was
not an extreme amount of roughening. The next column was for a
still rougher nozzle that was laser-machined with circumferential
grooves and was then coated with Cytonix. It was a nozzle of
0.003-inch inside diameter, 0.011-inch land outside diameter,
laser-roughened with groove width and ridge width around 0.001 inch
depth of groove half to 1 thousandth of an inch, coated with
Cytonix PFC 1604A (low-viscosity). The crossed groove case in a
later column had similar size grooves in the other direction as
well. The crossed-groove design had circumferential grooves
dimensioned as 0.001-inch deep and 0.001-inch wide with 0.0005-inch
thickness of wall or remaining material between grooves. The
slant-height grooves were similar with a spacing which permitted
the ridge segments to be slightly longer than wide, near the tip of
the nozzle, with further increases in segment length further away
from the tip. All laser-machined nozzles had orifice diameter
0.003-inch (76 microns). For data in this table, all nozzles had a
total included angle of 30 degrees.
[0115] The generally observed ranking of the fluids is that ethanol
is an easier fluid and chloroform is a more difficult fluid as far
as avoiding wetting. Because of this pattern, one of the better
nozzles for ethanol is repeated for the more difficult chloroform,
and after that is included the still rougher design of nozzle
having crossed laser-machined grooves.
[0116] The following table contains the experimental observations
concerning swingback, reported as observations of conditions that
do or do not result in swingback after repeated shutoff of pulse
trains. This observation is of usage which is very closely related
to the intended use of the nozzles, which is for 3D printing
articles such as dosage forms which often include fine printed
features having a dimension which is thin along the fast axis of
motion. The table shows how short a pulse train can be delivered
while remaining free of swingback, which implies how thin a 3D
printed wall or feature would be practical by printing with such a
nozzle.
[0117] The first column or nozzle design (smooth +coated, with
ethanol) provided swingback-free shutoff only for line-segment and
continuously pulsed operation. The second column (EDM-roughened
+coated, with ethanol) provided swingback-free shutoff for
line-segment, for continuously pulsed and for pulse trains as short
as approximately 24 consecutive pulses. The third column
(laser-machined with circumferential grooves +coated, with ethanol)
provided swingback-free shutoff for line-segment, for continuously
pulsed, and for pulse trains as short as approximately 4
consecutive pulses, which is just about short enough to be useful
for building walls of oral dosage forms. Thus, for these three
ethanol cases, the rougher the surface the better is the
performance in resisting swingback.
[0118] The fourth column shows the same nozzle as column 3 but used
with chloroform. Compared to column 3, there was degradation of
performance, in that swingback-free shutoff with chloroform could
only be achieved for line segment, for continuously pulsed and for
pulse trains longer than approximately 300 consecutive pulses. This
is worse than the ethanol results, in which swingback-free
operation was achieved for pulse trains as short as 4 pulses. With
chloroform and these nozzles, pulse trains shorter than 300 pulses
resulted in formation of a drop on the outside of the nozzle. Thus,
the further roughening feature of crossed grooves was tested with
chloroform, and performance improved to being able to shut off
chloroform swingback-free at a pulse train as short as 50 pulses.
Operation of these nozzles with chloroform at somewhat less than 50
consecutive pulses did not result in immediate swingback, but after
many shutoffs a swung-back drop did develop on the conical surface.
In this table, the notation clean means clean breakoff of drops
upon shutoff with no swingback, and is the desired state.
1TABLE 1 Performance of nozzles with pure ethanol and pure
chloroform Short description of Slightly Moderately Moderately
Greatest surface Smooth rough rough rough roughness Liquid Ethanol
Ethanol Ethanol Chloroform Chloroform Smooth + EDM Laser rough
Laser rough Laser rough Cytonix rough + Circumferential +
Circumferential + Crossed Cytonix Cytonix Cytonix grooves + Cytonix
Line segment clean clean clean clean clean Continuously clean clean
clean clean clean pulsed Pulse train forms drop clean clean clean
clean 300 pulses Pulse train forms drop clean clean swings back
clean 200 pulses Pulse train 50 forms drop clean clean swings back
clean pulses Pulse train 24 forms drop clean clean swings back
swings back pulses Pulse train 6 forms drop slow drop clean swings
back swings back pulses Pulse train 4 forms drop slow drop clean
swings back swings back pulses Pulse train 2 forms drop forms drop
swings back swings back swings back pulses
[0119] When all of the entries in this table are taken together,
they define regions of parameter space in which wetting-free
operation can be expected or should not be expected.
EXAMPLE 6:
Nozzle With Complicated Fluid Containing Solute (Smooth,
Sharply-Angled Nozzle)
[0120] In accordance with yet another embodiment of the present
invention, a nozzle for dispensing complicated a fluid-containing
solute is described. The binder liquids that were used in the
preceding examples (ethanol and chloroform) were simple pure
solvents that might be called prototypical of binder liquids that
would be dispensed for purposes such as manufacturing medical
articles. The fact that they are pure solvents means that if any
drops or splashes occur and the solvent evaporates, nothing is left
behind on the surfaces that received the drops or splashes.
[0121] Binder liquids actually dispensed in the fabrication of
medical products by 3DP are likely to have additives dissolved in
them that result in somewhat different fluid properties. Such
binder liquids are likely to require some amount of additional
characterization work. One example of a more complicated binder
liquid that has been tried is a solution containing 64% ethanol,
21% water, and 15% triethyl citrate (a plasticizer), having a
surface tension of 26 dyne/cm and a viscosity of 1 to 2
centiPoise.
[0122] It has been found that for this liquid, roughness of the
coated exterior of the nozzle did not enhance hydrophobicity as it
did for pure solvents. For this particular fluid, it has been found
that the fluid wet the roughened coated surfaces rather easily,
which is a contrast to the results for pure solvents. For this
particular fluid it was found to be preferable to use a
smooth-surfaced, coated nozzle such as was described in Example 1.
In particular, it was found that under those circumstances a taper
of 20 degrees total included angle for the conical external surface
worked better than a taper of 30 degrees. It is believed that even
smaller total included angles may be even better. It is also
believed that, at least for this family of substances, more dilute
solutions are easier than more concentrated solutions as far as
dispensing without wetting or swingback. For complicated fluids
such as these, optimum wetting-resistant nozzle design may depend
on the constituents of the fluid and their concentration.
EXAMPLE 7
Nozzle Coating Apparatus
[0123] In accordance with yet another embodiment of the present
invention, a method and apparatus for coating the nozzles
previously describe herein is shown and described. For certain
sizes of nozzles, it is possible to apply the coating to the nozzle
external surface using an applicator by hand, possibly while
working under magnification. However, improved control of coating
placement can be achieved if some sort of positioning apparatus is
used. Such apparatus may be constructed of commercially available
optical breadboarding and positioning apparatus. It may include a
rotary table for mounting and rotating the nozzle, since the nozzle
is axisymmetric, and apparatus such as a multidimensional precision
motion stage for positioning an applicator, and may further include
a magnifying visual system.
[0124] In applying liquid coatings at dimensions as small as those
of interest here, an important influence is the behavior of the
surface of a drop of coating resin or liquid in contact with a
solid, with the shape and position and motion of the liquid drop
showing the influence of the surface tension of the liquid. The
nozzle coating apparatus and technique take advantage of the
surface-tension-dominated behavior of liquids involving the advance
and recession of small drops on solid surfaces.
[0125] When a liquid such as a drop contacts a solid surface, there
is a contact angle that is determined principally by the relative
values of the liquid surface tension and the solid surface energy
and is described by Young's Equation. This behavior includes the
existence of an advancing contact angle, for which a drop is on the
verge of advancing, and a receding contact angle, for which a drop
is on the verge of receding. Between these two limiting angles is a
range of angles such that the contact angle of the liquid on the
solid can have any value between these two limiting angles without
the position of the liquid edge advancing or receding or changing
its position at all.
[0126] This description so far describes the behavior of a drop of
liquid on a flat smooth solid surface. For a geometry which
includes a sharp convex edge, when a liquid drop reaches the sharp
convex comer or edge, it hesitates at the sharp convex corner or
edge and the position of its edge or the extent of its advance is
defined by the sharp convex comer or edge. This behavior has
already been described in regard to control of wetting at the
nozzle tip, but it is also relevant and useful for the positioning
of the edge of resin as part of the nozzle coating process. When
the edge of a liquid puddle or drop, in this case made of resin, is
at a sharp convex comer or edge, the difference between contact
angle for advancing past the comer or edge and the contact angle
for receding from the comer or edge can be substantially larger
than simply the difference between the advancing and receding
contact angles on a flat surface. This makes it easy to define the
edge of an applied liquid coating by a sharp convex edge. Such
hesitation behavior would not be apparent, or would be much less
apparent, if an edge were a gently curved surface, as opposed to
sharply cornered.
[0127] Thus, the geometry of the nozzle itself can be used in
defining the edge of the region upon which the coating is applied,
just as the sharpness of the edge together with the change of
surface energy (created by the present method) later helps to
define the edge of the possible puddle during dispensing. If the
desired position of the edge of the coating coincides with a sharp
edge, then the position of the coating edge can be largely defined
by the as-manufactured sharp edge, with the result that the
position of the coating edge becomes far less dependent on
positional accuracy of the applicator, or even the skill of the
operator, than would be the case if the desired coating edge were
at a more ordinary place. This fact allows precision in coating
placement and achievement of the desired design of spatial pattern
of surface energy, which can help to direct dispensed liquid to
remain in certain regions and avoid other regions.
[0128] FIG. 12 illustrates one embodiment of a technique suitable
for applying the low-surface-energy resin to the exterior conical
surface of a nozzle. Steps 1-6 of FIG. 12 can be viewed as steps
that are performed at one angular location at a time, or they can
be viewed as steps that are performed as the nozzle surface
undergoes rotation around its axis of symmetry.
[0129] Step 1 shows, in cross-section, a bare nozzle 1200 before
any coating is applied. The conical nozzle is shown pointing
vertically upward so that gravity will pull the resin away from the
tip.
[0130] Step 2 shows a drop of resin 1210 as it is brought into
contact with the external conical surface 1220 using a small sharp
applicator 1230 such as a pin. The resin 1210 is brought into
contact with the external conical surface 1220 a slight distance
below the nozzle tip 1240.
[0131] Step 3 continues after step 2 and shows that the drop of
resin 1210 may then be nudged upward by the upward motion of the
applicator 1230 in a controlled manner. When the drop of resin 1210
reaches the sharp external edge 1240, which is the meeting place of
the land 1250 and the external surface 1220, it hesitates and forms
a slight bulge, as typically occurs due to surface tension when any
liquid meets a sharp edge. The applicator 1230 can be moved a
slight distance higher than the edge 1240, which insures that the
resin 1210 goes all the way to the edge. However, as long as the
extra distance is modest, the resin will not progress past the
sharp edge. This behavior of the resin puddle as it is being nudged
illustrates that it is quite useful and convenient to have a sharp
external edge, because the hesitation which the edge causes in the
spreading of liquid resin provides a sharply-defined edge of the
resin-coated region and it is possible to know with quite a degree
of certainty that the resin has reached all the way to the edge and
no farther.
[0132] Step 4 shows that once contact of resin 1210 all the way to
the sharp corner 1240 has been achieved, the applicator 12320 may
be brought back down below the comer, which allows the resin drop
1210 to drift back downward under the influence of gravity. This
illustration is similar to the illustration for Step 2, except that
now the resin remains in contact with the conical exterior 1220 of
the nozzle 1200 all the way to the sharp comer 1240. Shortly after
this step, the applicator 1230 may be removed from contact with the
resin 1210.
[0133] Step 5 shows the situation after the applicator 1230 is
removed. A layer of resin 1230 hangs downward under the influence
of gravity. For low viscosity resins this layer would be rather
thin, but for high viscosity resins this layer may be thicker. The
layer starts at the edge where the conical exterior meets the land,
and is thicker at lower elevations. In FIG. 12, the thickness of
this layer is exaggerated for purposes of illustration.
[0134] Step 6 illustrates that if the resin is a relatively viscous
heat-curing resin, as the resin becomes warm before actual curing,
its viscosity decreases. This may cause the resin to creep or drip
lower on the nozzle under the influence of gravity, and with the
result that the layer becomes thinner especially near the tip of
the nozzle. Nevertheless, the layer never completely disappears
from the surfaces that have been wetted with resin, even those
surfaces closest to the tip of the nozzle. Eventually, with a
combination of time and temperature, the resin cures and remains in
a permanent place and shape. If the resin is such that the exposed
as-cured surface has a lower surface energy than the interior of
the resin layer, then that will be achieved in this process.
[0135] With any coating liquid and any curing mechanism, while the
coating is still liquid, it is also possible to direct a jet of
clean gas at the applied liquid, blowing in a direction away from
the nozzle tip, to thin the liquid layer by pushing liquid away
from the nozzle tip to a place where its thickness does not matter
or where it can be removed from the nozzle. It is estimated that
the thickness of the cured coating at the tip of the nozzle is less
than several thousandths of an inch even with the more viscous
resin, and well under that dimension for the low-viscosity
resin.
[0136] FIG. 13 illustrates one embodiment of apparatus used to
perform the coating operation. The apparatus provides a positioning
system for an applicator that is substantially precise, stiff and
free of looseness or backlash. The apparatus may be mounted on a
base plate 1310 and includes a rotary table 1330, which holds the
nozzle 1320 being coated. The nozzle 1320 may be a small nozzle
with a conical exterior, having an axis of symmetry 1322, and
includes a central hole or orifice 1324 which must be kept free of
resin. The axis of rotation 1332 of rotary table 1330 may coincide
with the axis of symmetry 1322 of nozzle 1320 and its orifice 1324.
As shown, the axis of rotation of rotary table 1330 may be vertical
and the orientation of the nozzle 1320 may be vertical such that
gravity pulls the resin away from the orifice 1324, which helps to
prevent the orifice 1324 from accidentally becoming filled or
blocked with resin. The rotary table 1330 can be rotated by hand or
upon command as needed, or it can be continuously rotated such as
by a motor (not shown), at a suitably slow speed.
[0137] Also mounted onto base plate 1310 is a motion stage 1340,
preferably providing three axes of motion, which moves an
applicator 1350 relative to nozzle 1320. Stage 1340 may comprise
three screw micrometers 1342, 1344 and 1346 in mutually orthogonal
directions. Actuators or positioners other than micrometers could
also be used, as known in the fields of motion control and
optics.
[0138] As illustrated in FIG. 13, the three directions of motion of
the stage 1340 may be vertical and two mutually perpendicular
horizontal directions. However, it also would be possible for one
direction of motion to be approximately parallel to or tangent to
the external surface of the axisymmetric surface being coated
(i.e., when moving in the direction parallel to the slant height of
a conical nozzle, the distance of the applicator from the cone
surface does not change), and another direction to be perpendicular
to that direction (i.e., purely toward or away from the conical
surface).
[0139] The applicator 1350 that is moved by the stage 1340 may be a
solid slender and sharp-pointed such as a pin, which is capable of
moving an attached drop of resin around on the nozzle 1320. In some
circumstances it may be desirable that the applicator be a hollow
tube having an interior passageway, which may be pointed or beveled
like a hypodermic needle, such that liquid or resin can be
delivered through its interior passageway onto the nozzle 1320.
[0140] In addition to the already described apparatus, the
apparatus may include a visual observation system that offers
visual magnification to aid in working on small parts. This system
may be a purely optical system such as a conventional microscope.
The system may include an adjustable magnification (zoom) lens
1360, an electronic camera (not visible), focusing means 1370 which
may be along a vertical axis looking down at the coating apparatus,
and a display monitor 1362. Image processing software including
edge recognition or contrast enhancement, as is known in the art,
may be used to process an electronically acquired image, possibly
in real time, to enhance visualization of the position of edges of
the drop of liquid or resin, such as through contrast enhancement
and edge detection.
[0141] It has been found that this apparatus can control the
placement or position of the actual edge of a puddle of liquid or
resin on a small axisymmetric work piece, sufficient to routinely
and successfully coat the external conical surfaces of nozzles
whose orifice diameter is at least as small as 25 microns (0.001
inch).
EXAMPLE 8
Various Other Nozzle Shapes
[0142] In accordance with yet another embodiment of the present
invention, alternative nozzle configurations are described and
illustrated. The nozzle shapes described so far, using a
pre-manufactured shape that has then been coated with a low surface
energy coating, have generally been frusto-conical. That is not the
only possible shape, even for pre-manufactured and coated
shapes.
[0143] First of all, the category of knife-edge orifices has
already been mentioned briefly. In a knife-edge orifice, there is a
transition region between the flow passageway and the extended
surface, but the features at the transition region are so small
compared to the orifice diameter, that the details are unimportant
(such as whether the transition is flat or rounded). This
embodiment may be defined as land outside diameter minus land
inside diameter being less than one-tenth of the orifice diameter.
Although small-diameter nozzles such as 0.003 inch diameter
orifices may not afford that luxury, there could be some nozzles
manufactured according to the present invention on a sufficiently
large size scale, or with an appropriate manufacturing method, such
that it would be possible to manufacture the orifice edge as
essentially a knife-edge, such as land dimension or radius less
than 1/10 orifice diameter. In such an event, a low surface energy
coating may be applied on the external surface up until the
knife-edge. The external surface may be conical, curved in either a
concave or convex sense, or of other shape. If the external surface
is other than frusto-conical, the surface tangent angle is as
defined in the next paragraph.
[0144] It is possible that, for nozzles having a flat land, as
described in Example 1, within the region where swingback is
possible and nozzle design features are important, the nozzle
external shape could have axisymmetric shapes other than
frusto-conical, such as curved in either the concave or the convex
sense. Curvature of the nozzle external surface, as one moves along
the slant height, would still fall within the scope of the present
invention.
[0145] FIGS. 14A-14C illustrate various nozzle shape embodiments
along with reference of the relevant surface tangent angle SA on
each embodiment. FIG. 14A is a nozzle with an outwardly curving
exterior surface 1410 and a flat land 1415. The exterior surface
tangent angle SA is minimized. FIG. 14B illustrates a nozzle with
an inwardly curving exterior surface 1420 and a flat land 1425. The
exterior surface tangent angle SA is greater than in FIG. 14A. FIG.
14C is a nozzle with a relatively flat exterior surface area 1440
and a rounded land 1430, outwardly extending from the internal
passageway 1450 of the nozzle. The land 1430 in FIG. 14C is in the
shape of a fillet. Each illustrated embodiment has a surface
tangent angle greater than 90 degrees where the external shape
meets the land. FIG. 14A and 14B illustrate the surface tangent
angle for a nozzle that has a land of finite dimension and that
also has an external surface which is curved in either a concave or
convex sense, as opposed to being a simple frusto-conical
shape.
[0146] It can also be realized that departures from the previously
described nozzle designs are possible at a sufficiently great
distance from the tip of the nozzle. To the extent that the
axisymmetric low surface energy exterior is advantageous, it only
exerts its advantage within a certain distance of the exit. There
can be locations that are so far removed from the exit that no drop
would ever be able swing up that far. Accordingly, at such places
it is no longer necessary to maintain the combination of
frusto-conical or other axisymmetric shape and low surface energy.
It would be permissible to violate either or both of those
criteria. The distance beyond which swingback could never reach,
and beyond which the stated nozzle design need not be maintained,
is not precisely known, but may be estimated as 0.5 inch for
typical orifice and nozzle dimensions of interest for 3DP
printing.
[0147] In the descriptions of the shape in previous embodiments,
the nozzle has been described as having axisymmetry, which implies
that the land is an annulus. While the land may be designed to be
an annulus having its inner and outer circular edges being
concentric with each other, it should be realized that
manufacturing imperfections resulting in relative eccentricity of
the two circles are permissible.
[0148] With any of these geometric alternatives, roughness could be
incorporated such as is described in Examples 2,3 and 4.
[0149] In the examples so far, the low surface energy property has
only been provided at the external surface such as frusto-conical
surface. The land has not been coated or required to have any
particular surface energy. It is possible that the land be left
uncoated and have relatively high surface energy (such as greater
than 50 dyne/cm) as has already been described in the earliest
Examples, displaying the surface energy of the material it is made
from, which may be a high surface energy exhibiting hydrophilic
behavior.
[0150] Alternatively, it is also possible that the land could be
manufactured to have a small surface energy similar to that of the
external surface, such as by coating. For example, it is possible
to coat the land with a low surface energy coated just as the
external surface has been coated. The coating apparatus described
in the preceding example could be used, and the sharp edge where
the internal passageway meets the land could similarly be used to
arrest and define the spread of the coating liquid. The usefulness
depends on individual circumstances such as particular fluid being
dispensed.
[0151] Alternatively, nozzles may include fillets. In some
instances, the filleted ends seem to be less effective than sharp
edges in arresting the spread of liquid or limiting the size of a
possible puddle of dispensed liquid, or in defining the edge of a
coating. However, there could be cases in which such a filleted
nozzle could be useful if coated with a low surface energy coating
in appropriate places. Such a nozzle could also include a flat land
region on either side of the fillet, i.e., the fillet could be
closer to the fluid passageway or closer to the external surface,
as shown in FIG. 14C.
EXAMPLE 9
Alkyl Ketene Dimer
[0152] In accordance with yet another embodiment of the present
invention, a coating is applied to the nozzle to increase the
wetting-resistant properties of the nozzle. A type of hydrophobic
surface which is hydrophobic as a result of producing a
microscopically rough surface as it solidifies, is described in
"Super-Water-Repellent Fractal Surfaces," by T. Onda, S. Shibuichi,
N. Satoh, and K. Tsujii, in Langmuir the ACS Journal of Surfaces
and Colloids, Vol. 12 no. 9, May, 1996, pages 2125-2127. The
material described in this reference is alkylketene dimer (AKD) and
is a naturally hydrophobic waxy substance that produces a pattern
of crinkles or cracks as it solidifies from a melted state.
[0153] AKD undergoes fractal growth when it solidifies, although
the mechanism has not been clarified yet. The paper compares a
surface containing this fractal geometry with a surface of the same
material prepared, by cutting, so as to produce an ordinary flat
(non-fractal) surface. The conventional (cut) surface had a
moderately good contact angle with water of 109 degrees, but, when
this material was prepared so as to have a fractal surface, that
surface had an extraordinarily large contact angle with water of
174 degrees. This represents extreme hydrophobicity and is far
better than the contact angle for any material not having this
microgeometry.
[0154] AKD could be used as a coating for nozzles instead of the
fluoropolymer resin described in the previous embodiments. The
hydrophobicity of this material is dependent on the presence of
surface cracks resembling fractals as it solidifies from liquid.
The preparation of the fractal surface in the cited article
included heating the AKD material to 90 degrees C. in dry nitrogen
and then letting it cool and solidify. The technique for using this
material as a nozzle coating material could include depositing a
thin film of this as liquid on the substrate to be coated or
treated (the nozzle exterior), at a temperature of around 90 C.,
and then letting it cool at room temperature in the presence of dry
nitrogen gas.
[0155] AKD could be applied to the exterior conical surface of a
nozzle by essentially the same method and apparatus described in
Example 7 for applying resin, provided that the application of the
AKD is carried out at a temperature, such as 90 C., which is
suitable for the melting of AKD. For example, the apparatus that
holds the nozzle during application of the coating can be heated so
as to melt the AKD during times when it is desired to be melted.
Heat could also be applied from an external source. Solid AKD could
be touched to the heated nozzle. The applicator could be heated or
it does not have to be. When the nozzle is completely coated with
liquid AKD in the desired places, heat could be turned off or down
allowing the AKD to solidify in the desired manner.
EXAMPLE 10
Hole Drilled Through Bulk Hardened Fluoropolymer
[0156] Example 10 provides yet another embodiment of the present
invention. The previous Examples provided an as-cured hydrophobic
surface at the surface of a well-defined external geometry. This
was achieved by manufacturing a base shape having sharp
well-defined geometric features and then applying the
low-surface-energy resin as a thin coating over the
pre-manufactured shape. A different approach, which also provides
an effective nozzle for some purposes, is to make the entire
discharge region of the nozzle out of a low-surface-energy resin as
a bulk material. In such a case, because a drop of resin naturally
assumes a gently curved shape, it is not likely that one could
achieve such sharply tapered and sharp-edged geometries as in the
earlier embodiments, especially given the preference for having the
final surface be an as-cured surface. Nevertheless, even assuming
that most surfaces will be gently curved, it is still possible to
achieve geometries that are useful for certain fluids and certain
purposes.
[0157] This example depends on having a drop of resin fill certain
small openings or bridge certain small gaps, therefore, the
extremely low viscosity formulation PFC 1 604A from Cytonix has not
been used. Instead, the more viscous heat-curable fluoroepoxy resin
GH000 from Cytonix has been used. As in previous discussion, this
example pertains especially to a curable resin that has its lowest
surface energy at its exposed as-cured surface.
[0158] FIGS. 15A-1SC illustrate the dispensing sequence of one such
nozzle. FIGS. 15 illustrate the sequence of manufacturing a nozzle
for dispensing a liquid through a tube 1520 having a small body of
a cured resin 1510, 1530 at its end, with a hole 1540 through the
cured resin.
[0159] The tube may be a metal tube of an inside diameter such as
0.030 inch (0.75 mm), which is such that the surface tension of the
resin will be an important factor in the placement and shape of the
drop of resin at the end of the tube. When liquid is placed across
the end of such a small diameter tube, the liquid will wick into
the tube and will have an inward meniscus at the end. Presumably at
the other, hidden surface of the resin inside the tube, the resin
will also have a similar meniscus that also wicks onto the wall
with a concave curvature.
[0160] One way of manufacturing the desired final geometry is to
allow a first drop 1510 of resin to assume a natural
inwardly-curving shape at the end of the tube, cure it at least
partially, as shown in FIG. 15A. Then, deposit a second drop 1530
of resin in the depression formed by the meniscus, as shown in FIG.
15B. The first drop 1510, being at least partially cured, will
retain its shape, and provides a resting place for the second drop
1530, ensuring that the second drop bulges outward as desired. FIG.
15B illustrates the sequence after the second drop 1530 has been
deposited. The second drop 1530, assuming an appropriate volume of
resin is deposited, will bulge outward by an amount depending on
the volume of resin deposited, and will cure in that shape, which
is a portion of a sphere. This outwardly bulging shape is what is
desired and is what is desired to be of low surface energy
material. FIG. 15C illustrates a dispensing hole 1540 in the resin
1510, 1530.
[0161] The first material is not actually required to be low
surface energy, and it could be any material that retains its shape
after partial or full curing because its principal function is to
retain its shape to prevent the second externally bulging drop,
which is made of low-surface-energy material, from wicking in to
the tube before it cures. If the first material is identical to the
second material, it may be desirable to only partially cure the
material in the first drop just enough so that the material in the
first drop becomes highly viscous but still retains some ability to
bond with the next drop, whereas if it were fully cured the low
surface energy of its exposed surface could make it difficult for
anything else, even the next layer of the same material, to stick
to the first layer.
[0162] Thus, making the first layer out of a higher surface energy
resin material might be useful simply to promote adhesion with both
the tube wall and the second drop. In order to enhance adhesion of
any resin to the tube, it may be advantageous to provide a
geometric attachment feature such that the resin enters the
attachment feature, cures and as a result the entire resin plug is
mechanically trapped in its desired location at the end of the
tube. The attachment feature could for example, be an internal
groove or other indentation on the interior of the tube near the
end where the resin plug will exist, or small holes through the
tube wall, roughness on the interior surface of the tube, etc. The
extent of curing of heat-curable low-surface-energy fluoroepoxy
resin GH000, from Cytonix, is typically observable by color, with
the resin turning brown or dark brown when cured.
[0163] As shown in FIG. 15C, a hole is created through the cured
resin plug. The hole may be drilled so as to be concentric and
coaxial with the tube. The hole may be made by conventional
mechanical drilling, laser drilling, or any other appropriate
method. The diameter of the drilled hole may typically range from
0.007 inch (177 microns) down to 0.002 inch (51 microns) or even
smaller. This drilling may be performed with precautions so as to
avoid disturbing the exposed as-cured surface of the resin adjacent
to the hole. Laser drilling may include the use of lasers whose
wavelength is in the vacuum ultraviolet range, which is believed to
be especially well suited for cutting fluoropolymers.
[0164] FIGS. 16A-16C illustrate some geometric relationships which
pertain to the tangency angle at the edge of the hole of FIG. 15C.
The shape of the drop as it is curing is a portion of a sphere
having a radius of curvature. How much of a sphere and what is the
radius of curvature are determined by how much resin is placed
there, on the surface tension of the liquid resin, and on other
details. It is assumed that there is symmetry around the
longitudinal axis of the cylindrical tube and the cylindrical hole,
i.e., the center of the sphere is on the axis of the cylindrical
hole.
[0165] The angle of interest that effects the droplet breakoff is
the local tangent angle of the surface SA at the edge of the nozzle
exit 1610 surface, referenced to the direction of travel of the
jet. This angle may be termed the surface tangent angle SA. For an
ordinary hole drilled perpendicularly through a flat surface, the
surface tangent angle is 90 degrees. Surface tangent angles greater
than 90 degrees may be attained by the present invention. The
principal variables, labeled in FIGS. 16, are the radius of the
hole r.sub.h, and the radius of the spherical surface r.sub.s. In
all cases illustrated in FIGS. 16, the surface tangent angle SA is
at least slightly greater than 90 degrees.
[0166] In FIG. 16A, the radius of the hole r.sub.h is much smaller
than the radius of curvature of the spherical drop surface r.sub.S.
In this case, the surface tangent angle is only slightly greater
than 90 degrees, perhaps only a few degrees greater than 90
degrees. In FIG. 16B, the hole radius r.sub.h is about one-quarter
of the spherical radius r.sub.s, (i.e., r.sub.h/r.sub.s=0.25), and
the surface tangent angle is 104 degrees, i.e., about 14 degrees
beyond 90 degrees. In FIG. 16C, the hole radius r.sub.h is about
one-half of the spherical radius r.sub.s, (i.e.,
r.sub.h/r.sub.s=0.5), and so the surface tangent angle is 120
degrees. The actual relation between the two radii and the angle is
given by
surface tangent angle=90+arcsin (r.sub.h/r.sub.s)
[0167] It can be observed that in FIGS. 16 the flow geometry for
flow entering the resin plug with the hole in it includes an abrupt
contraction from the inside diameter of the tube to the diameter of
the hole. Such an abrupt contraction may be undesirable for at
least some fluid flow purposes because it introduces into the fluid
flow disturbances that may show up as irregular flow beyond the
nozzle.
[0168] FIGS. 17A-17G illustrate various embodiments that provide a
smoothly-tapering interior flowpath 1705 even for this non-coating
based approach, and also achieves a surface tangent angle SA
significantly greater than 90 degrees just as in the previous FIGS.
16A-16C. It involves creating, such as by machining, a nozzle body
1700 (essentially a tube) having a smoothly-tapering internal
passageway 1705. Then a drop of resin 1720 is applied to the end. A
place for application of the resin, such as a recess or pocket
1710, may be provided for this purpose. Either one-step or two-step
application of resin, as before, could be used. The resin could be
cured or dried to form a low-surface-energy surface. Finally, a
hole 1730 is made through the resin 1720. In FIGS. 17C, 17F and
17G, the diameter of the hole is shown as being essentially equal
to the diameter of the lower end of the tapering flowpath in the
nozzle body. This results in a smooth internal flowpath without any
abrupt change in cross-sectional area. As before, the surface
tangent angle at the edge of the hole is determined by the ratio of
the hole radius to the radius of curvature of the spherical surface
of the resin drop.
[0169] FIGS. 17A-17F shows a tube whose interior is tapered near
the discharge end just before the resin plug. In the case
especially of small diameter orifices, such a tapering avoids the
large pressure drop associated with a long length of small bore,
while still not introducing major flow disturbance. It also keeps
the L/D of the drilled hole within reasonable range. Most hole
manufacturing methods have a limitation on length to diameter ratio
of the hole, and smaller overall size of the resin drop will help
to keep the L/D of the drilled hole within a reasonable range. The
exterior of the pre-manufactured nozzle body in FIGS. 17A-17G can
be either straight-sided as shown in FIGS. 17A-17C, 17G or tapered
as shown in FIGS. 17D-17F.
[0170] If the diameter of the drilled hole is to be particularly
small, such as 0.002 inch, or if it is desired that the surface
tangent angle be particularly acute, it may be desirable for the
resin region to be of particularly small diameter. Smaller size for
the resin region will accentuate the curvature of the resin drop
bulging outward.
[0171] Yet another way of making such a nozzle with a hole in it
would be to cast a placeholder such as a wire into the resin and
then, after curing, remove the placeholder such as by etching the
wire out. The placeholder would have to be of a suitable material
such as a metal wire such that it could be etched away by an acid
that does not damage the cured resin.
[0172] For example, copper wire can be etched away by a solution of
hot nitric acid without damaging the fluoroepoxy. When the wire is
etched away, the hole that remains is of the diameter of the wire.
Where the wire enters the drop of resin, there can be expected to
be a meniscus by which the resin tries to rise up onto the surface
of the wire. The dimensions of this meniscus will be of the same
order of magnitude as the diameter of the wire. The shape of the
meniscus may be influenced by whether the wire was simply inserted
into the resin drop or whether the wire was inserted and then
withdrawn slightly, because the resin follows the motion of the
wire. When the wire is etched away, the meniscus will remain, and
this provides a natural way of making a nozzle having a desirable
kind of curvature leading to a sharp edge right at the discharge.
This nozzle shape is shown in FIG. 17G.
[0173] Experiments have been conducted involving discharge of fluid
from nozzles made out of fluoroepoxy, made as illustrated in FIGS.
15A-15C and drilled with mechanical drills, in both drop-on-demand
and continuous mode. The flow characteristics with nozzles of the
present embodiment have been significantly better than the flow
characteristics of earlier nozzles made of more conventional
materials and designs. The nozzle of this embodiment, when
operating with a solution of propylene glycol and water in an 80:20
proportion, remained dry in almost all operating conditions and
produced good quality drops.
[0174] Further Discussion
[0175] The nozzles of the present invention could be used to
dispense almost any form of fluid discharge. They could, for
example, be used in dispensing a continuous jet. By virtue of their
physical properties, some liquids, when dispensed intermittently,
tend to immediately produce discrete drops, while other liquids
dispense as a series of bulges connected by narrower strings of
liquid. The nozzles of the present invention could be used in
either case.
[0176] With microvalves, the nozzles of the present invention could
be used with both drop-on-demand and line-segment modes of
operation. Although the examples have been tested with
solenoid-operated microvalves, the nozzles of the present invention
could also be used equally well at the discharge of other types of
valves and other types of dispensers. The nozzles could be used at
the discharge of a piezoelectric based drop-on-demand dispenser or
fluid ejection system or also with still other types of dispensers,
including boiling (bubble-jet), continuous jet with deflection, and
in general, any type of liquid dispenser. Among the expected
benefits of wetting-resistant nozzles would be improved accuracy of
drop placement and hence print quality.
[0177] For a piezoelectric dispenser, it may be arranged that when
fluid is not being dispensed, the liquid forms a meniscus at the
nozzle exit such that the meniscus bulges inward, i.e., toward the
direction from which the fluid is supplied to the nozzle. This may
be achieved by supplying the fluid from a fluid source that is
maintained at a fluid source pressure, while the dispenser operates
in a surrounding gas which is at atmospheric pressure, wherein the
fluid source pressure is at lower than atmospheric pressure. This
could for example be attained if the fluid source reservoir is open
to atmosphere and the liquid surface level in the reservoir is at a
lower elevation than the elevation of the exit of the nozzle. This
will tend to draw stagnant fluid at the nozzle exit back into the
flowpath until the negative pressure of the fluid supply system
corresponds to the negative pressure associated with an inwardly
bulging meniscus. This will encourage the last little bit of fluid
at the nozzle exit, at the time of shutoff, to be drawn back into
the fluid supply system, thereby making that last little bit of
fluid less available for swingback. Immediately after shutoff of a
dispensing or pulse train, the negative pressure and the tendency
toward an inward meniscus would help remove fluid from the
immediate region of the exit and this would cooperate with the
nozzle design features already described, which discourage any
fluid which does exist at the exit from swinging back onto the
nozzle exterior, to produce a situation which is even more
resistant to wetting and swingback.
[0178] In general, the present invention is defined by the use of a
surface that is more non-wetting (lower surface energy) than the
commonly known and used Teflon. The radical describing Teflon is
--CF.sub.2--. On the Zisman chart there are two listings with
smaller surface energies, namely --CF.sub.2H and --CF.sub.3.
Accordingly, a usable coating with the present invention could be
any coating in which the atomic constitution at the surface is a
monolayer ending in --CF.sub.3, or in --CF.sub.2H or a material
with a high proportion of such chemical entities at the surface.
Any such material is an example of a material that could be used in
the current invention. Materials described in cited patents, all of
which are incorporated by reference, are examples of materials that
could be used.
[0179] Another example of a coating material which could be used,
having a surface energy lower than that of Teflon, is the substance
FC721 (also FC732) made by the 3M Company, Minneapolis, Minn.
(cited in Adamson and in Contact Angles on Hydrophobic Solid
Surfaces and Their Interpretation, by D. Li and A. W. Newmann,
Journal of Colloid and Interface Science, vol. 148 No. 1 January
1992 p. 190-200), which is 99% perfluorooctyl methacrylate, with 1%
acrylic acid.
[0180] The liquids that could be dispensed through a nozzle of the
present invention include a very wide variety; essentially any
liquid that has a low enough viscosity to flow through the nozzle
sufficiently quickly. Specifically it includes the category of
organic solvents, which includes without limitation alcohols
(ethanol, methanol, isopropanol, propanol, and others), chloroform,
dichloromethane, other halocarbons (including chlorocarbons,
fluorocarbons, chlorofluorocarbons, hydrofluorocarbons and other
halocarbons), acetone, methylene chloride, ethers (methyl tertiary
butyl ether), ethyl acetate, toluene, benzene, dimethyl sulfoxide,
N-methyl-2--pyrrolidone, formamide, dimethyl sulfoxide (DMSO),
dioxane, acetonitrile, gamma-butyrolactone, propylene carbonate,
etc. A list of possible solvents of interest is given in the CRC
Handbook under "Solvents for Liquid Chromatography."
[0181] The liquid may also include mixtures or solutions of these
fluids. It can also include liquids which are any organic solvent
including the above named solvents with any additive or additives
dissolved in it. The additive may be either liquid or solid.
Examples of additives include soluble polymers, polycaprolactone,
poly-lactic acid, poly-lactic-co-glycolic acid, propylene glycol,
triethyl citrate, etc. The additive could also be any Active
Pharmaceutical Ingredient. The liquid may further comprise solid
particles, or colloidal particles or micelles suspended in it.
Nozzles of the present invention could also be used advantageously
with water and with aqueous solutions including polyacrylic acid
(PAA), propylene glycol, etc.
[0182] Nozzles of the present invention could be used other than
for three-dimensional printing, such as for dispensing for high
throughput screening of pharmacological substances. Similarly, such
nozzles could be used for dispensing of liquids for biological
testing, assays, etc., for medical or veterinary or other general
laboratory purposes. The dispensed liquid could be blood, other
bodily fluids, or reagents or diagnostic substances that are part
of the testing. For example, DNA testing for biological
identification, genetic research etc. involves dispensing of minute
quantities of expensive substances.
[0183] Advantages of wetting-resistant nozzles for such
applications include the possibility of using reduced quantities of
expensive chemical or biological substances, and reduced likelihood
of cross-contamination. Applications would also exist throughout
the processes of manufacturing pharmaceuticals, beyond simply
dispensing the completed pharmaceutical substances into dosage
forms.
[0184] A filter may be mounted on each fluid line immediately
before the dispenser or printhead, so as to catch particles or
debris originating in any part of the fluid supply system upstream
of the filter location. Such a filter may be mounted directly on
the printhead.
[0185] The above description of various illustrated embodiments of
the invention is not intended to be exhaustive or to limit the
invention to the precise form disclosed. While specific embodiments
of, and examples for, the invention are described herein for
illustrative purposes, various equivalent modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize. The teachings provided herein of the
invention can be applied to other purposes, other than the examples
described above.
[0186] The various embodiments described above can be combined to
provide further embodiments. Aspects of the invention can be
modified, if necessary, to employ the process, apparatuses and
concepts of the various patents, applications and publications
described above to provide yet further embodiments of the
invention. All patents, patent applications and publications cited
herein are incorporated by reference in their entirety.
[0187] These and other changes can be made to the invention in
light of the above detailed description. In general, in the
following claims, the terms used should not be construed to limit
the invention to the specific embodiments disclosed in the
specification and the claims, but should be construed to include
all devices that operate under the claims to provide a method for
dispensing a liquid. Accordingly, the invention is not limited by
the disclosure, but instead the scope of the invention is to be
determined entirely by the following claims.
* * * * *