U.S. patent application number 13/219758 was filed with the patent office on 2011-12-22 for fluid management system and method for fluid dispensing and coating.
Invention is credited to Rajib Ahmed, Thomas B. Jones, Thomas N. Tombs.
Application Number | 20110308952 13/219758 |
Document ID | / |
Family ID | 39515763 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308952 |
Kind Code |
A1 |
Jones; Thomas B. ; et
al. |
December 22, 2011 |
FLUID MANAGEMENT SYSTEM AND METHOD FOR FLUID DISPENSING AND
COATING
Abstract
A system and method are provided including a coating method and
apparatus using a dielectrophoretic fluid movement system to coat
with a non-conducting fluid along a surface that includes a
non-conducting surface to receive the non-conducting fluid and a
first and second array of one or more substantially parallel
microelectrodes positioned on the surface, said first array having
microelectrode(s) positioned between, and alternating with, the
microelectrode(s) of the second array, forming an interleaved
pattern as well as an electric power source in communication with
the first array and second array so that the first array and second
array interact to create a non-uniform electric field such that the
non-conducting fluid moves parallel to the microelectrodes in
response to the applied non-uniform electric field.
Inventors: |
Jones; Thomas B.;
(Rochester, NY) ; Ahmed; Rajib; (Anderson, IN)
; Tombs; Thomas N.; (Rochester, NY) |
Family ID: |
39515763 |
Appl. No.: |
13/219758 |
Filed: |
August 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11565027 |
Nov 30, 2006 |
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13219758 |
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Current U.S.
Class: |
204/477 |
Current CPC
Class: |
G03G 15/2025
20130101 |
Class at
Publication: |
204/477 |
International
Class: |
C25D 13/00 20060101
C25D013/00 |
Claims
1. A coating method for moving non-conducting fluid along a
surface, the method comprising the steps of: a. applying a
non-conducting fluid to a non-conducting surface including a first
and second array of one or more substantially parallel
microelectrodes positioned on said surface, said first array having
microelectrode(s) positioned between, and alternating with, the
microelectrode(s) of the second array, forming an interleaved
pattern; and b. applying electric power to the first array and
second array so that the first array and second array interact to
create a non-uniform electric field such that the non-conducting
fluid moves parallel to the microelectrodes in response to the
applied non-uniform electric field.
2. The method of claim 1, wherein the power to the electrodes,
comprising electrodes is controlled to stop and start fluid
movement
3. The method of claim 1, wherein the power to electrodes is
controlled so that fluid moves in some regions of the array and the
fluid is prevented from movement in other areas by the
electrodes.
4. The method of claim 3, wherein the ratio between the electrode
spacing and an electrode width is between 2:1 and 3:1.
5. The method of claim 1, wherein the microelectrodes have a
non-conducting coating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of application Ser. No. 11/565,027,
Filed Nov. 30, 2006.
FIELD OF THE INVENTION
[0002] The invention relates generally to the fields of coating and
printing, and more particularly to processes and apparatus for
enhancing digital color reproduction systems.
BACKGROUND OF THE INVENTION
[0003] It is desirable to be able to coat discrete areas of a
flexible support in a continuous roll-to-roll manner, to enable the
fabrication of flexible electronics, micro lens arrays or display
devices, etc. There are a variety of existing techniques based on
printing technology, such as flexography, offset and screen
printing, currently available to meet this desire, although
generally the coating generated by such techniques is not
controllable in a way that allows spatial or temporal changes in
coating thickness that can be continuously modified.
[0004] The use of differential wettability to pattern the support
prior to overcoating with the target liquid in a continuous
manner--termed continuous discrete coating (CDC)--has been
demonstrated in PCT/GB2004/002591. The CDC method allows the use of
existing coating hardware to pattern layers but this method relies
on a predetermined surface pattern to control the coating thickness
and cannot affect coating thickness in a variable way (the coating
is either present or absent) nor does the process allow the coating
thickness and placement to be continuously controllable.
PCT/GB2004/002591 discloses the CDC technique. U.S. Pat. No.
6,368,696 describes a method of depositing multiple layers and
subsequently patterning the dried multilayer pack with an
additional step, for the manufacture of plasma display panels.
JP10337524A discloses a method to manufacture dielectric/electrode
panels.
[0005] Also desirable is a method to electrically control the
movement of small quantities of liquid across a surface. Existing
methods employ barriers, airflow, or gravity.
[0006] In an electrophotographic modular printing machine of known
type, for example, the Eastman Kodak NexPress 2100 printer
manufactured by Eastman Kodak, Inc., of Rochester, N.Y., color
toner images are made sequentially in a plurality of color imaging
modules arranged in tandem, and the toner images are successively
electrostatically transferred to a receiver member adhered to a
transport web moving through the modules. Commercial machines of
this type typically employ intermediate transfer members for the
transfer to the receiver member of individual color separation
toner images.
[0007] Sometimes electrophotographic copiers and printers use a
release agent to prevent paper sheets from sticking to the fuser
roll after transferred images have been heat fused. Dispensing this
oil, typically silicone oil, onto the fuser roller using a blade,
roller, or other mechanical means in a controllable manner is
complicated by the highly wetting nature of the oil. Oil is only
required in image areas (areas containing toner) to affect release
of the toner from the heated fuser roller. However oil is typically
applied across the entire surface of the fuser roller because there
is no means to readily control the application of the oil. Broad
application of oil in this manner often causes image artifacts
because the oil tends to contaminate sensitive components when the
printed media is sent back through the imaging unit to receive an
image on the media's rear surface. A means to precisely control the
application of highly wetting liquids such as silicone oil is
needed. Especially needed is continuous control, both temporally
and spatially, of the quantity (or thickness) of such liquids.
SUMMARY OF THE INVENTION
[0008] In accordance with an object of the invention, a system and
a method are provided for coating surfaces wherein real-time,
temporal and spatial control of a coating material is achieved. The
present invention overcomes shortcomings noted above by using
voltage-controlled microfluidic structures and hydrophobic surface
treatments to controllably dispense a fluid across a surface.
[0009] More specifically, the invention relates to a coating method
and apparatus using a dielectrophoretic fluid management system
that dispenses non-conducting fluid from a non-conducting substrate
patterned with a first and second array of one or more
substantially parallel microelectrodes, said first array having
microelectrode(s) positioned between, and alternating with, the
microelectrode(s) of the second array and forming an interleaved
pattern. The system uses an electric power source in communication
with the first array and second array so that the first array and
second array interact to create a non-uniform electric field such
that the non-conducting fluid moves parallel to the microelectrodes
in response to the applied non-uniform electric field. In one
embodiment of this method the surface and microelectrodes are
coated with a material such that the contact angle of the
non-conducting liquid is greater than 10 degrees and the voltage to
the electrodes is controlled to stop and start fluid movement.
[0010] A second object of the invention is a system and a method
for improving the image quality and reliability of printing
systems, and specifically the efficiency and accuracy of the
application of fluid needed in the electrostatographic process. The
invention is in the field of color reproduction printing systems,
which include digital front-end processors, color printers and
post-finishing systems such as UV coater, glosser, laminator, and
etc.
[0011] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed the invention will be better
understood from the following detailed description when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a better understanding of the characteristics of this
invention, the invention will now be described in detail with
reference to the accompanying drawings, wherein:
[0013] FIG. 1a is a schematic illustration of a portion of a
printer system according to the present invention for use in
conjunction with an image control system and method.
[0014] FIG. 1b is a schematic illustration of a fluid movement
system according to the present invention for use in conjunction
with a print engine or printer apparatus.
[0015] FIG. 1c is a cross-section of the microelectrode.
[0016] FIGS. 2a, 2b, and 2c show the phenomenology of liquid
dielectrophoresis. FIG. 2a is a schematic illustration of the
dielectrophoretic force on a liquid in an electric field created by
applying voltage to electrodes. FIG. 2b illustrates how the liquid
conforms to the electric field lines.
[0017] FIGS. 3a, 3b, and 3c are schematic illustrations of
representative portions of FIG. 1a showing additional details.
[0018] FIGS. 4-6 show the results of using the fluid movement
system according to one aspect of the invention.
[0019] FIG. 7 relates to one embodiment of the invention.
[0020] FIGS. 8a, 8b, and 8c show the results of using the fluid
movement system according to one aspect of the invention.
[0021] FIG. 9 shows the results of using the fluid movement system
according to one aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present description will be directed in particular to
elements forming part of, or cooperating more directly with,
apparatus and methods in accordance with the present invention. It
is to be understood that elements not specifically shown or
described may take various forms well known to those skilled in the
art.
[0023] FIG. 1a shows schematically a portion of a printing system
10, such as an electrophotographic printer or other printing
devices, hereafter referred to as simply printers but not limited
to the traditional printer but also including plate production
devices, and copiers that can print on a receiver 11, such as
paper, metal, press sheets, cloth, ceramics and substrates that are
printable. Electrophotographic printers are well known in the art,
and are preferred in many applications; alternatively, other known
types of printing systems may be used. Plural writer interfaces and
development stations may be provided for developing images in
plural colors, or from marking particles of different physical
characteristics. Full process color electrophotographic printing is
accomplished by utilizing this process for each of four, five or
more marking particle colors (e.g., black, cyan, magenta, yellow,
and clear).
[0024] The portion of a printer 10 shown includes a pressure roller
12 and a fuser roller 14, as well as a controller or logic and
control unit (LCU) 16, preferably a digital computer or
microprocessor operating according to a stored program for overall
control of the printer and its various subsystems. The Logic and
Control Unit (LCU) 16 is preferably a digital computer or
microprocessor operating according to a stored program for
sequentially actuating the workstations within the printer,
affecting overall control of the printer and its various
subsystems. Aspects of process control are described in U.S. Pat.
No. 6,121,986 incorporated herein by this reference.
[0025] The LCU 16 includes a microprocessor and suitable tables and
control software which is executable by the LCU 16. The control
software is preferably stored in memory associated with the LCU 16.
Sensors associated with the fusing and glossing assemblies, as well
as other image quality features, provide appropriate signals to the
LCU 16. In any event, the LCU 16 issues command and control signals
that adjust all aspects of the image that affect image quality,
such as the heat and/or pressure within fusing nip (not shown) so
as to reduce image artifacts which are attributable to and/or are
the result of release fluid disposed upon and/or impregnating a
receiver member. Additional elements provided for control will be
described below and include a power supply as well as liquid
control and release mechanisms.
[0026] Printing systems, such as the NexPress 2100 family of
high-speed digital production color presses made by Eastman Kodak,
Inc. of Rochester, use a very thin layer of oil applied to the
heated fuser roller to detack individual sheets after the toner
images have been fixed on the paper. The amount of oil consumed is
typically less than 10 microliters per sheet for a machine printing
70 copies per minute. The silicone-based oil is applied to the
fuser roller continuously using a roller. A problem with the system
is that the oil has a very low contact angle with most surfaces and
tends to wet virtually everything so that, over time, oil
contaminates other components in the machine. The contamination
results from the need to print on both sides of paper. The fuser
oil on the paper from the first pass gets transferred to the
sensitive components in the printing engine when printing the
second side of the page. This oil contamination increases
maintenance costs while reducing image quality and the life of
machine components. The coating system 20 described below is an
effective means to control fuser oil dispensing, This control
lowers the oil consumption and minimizes the application of oil in
unwanted areas thus reducing contamination problems.
[0027] The coating systems 20, shown in FIGS. 1a, 1b, and 1c for an
electrophotographic printer is a voltage-actuated microfluidic
structure 22 exploiting the liquid dielectrophoretic (DEP) force
exerted on dielectric liquids 24 by a non-uniform electric field
25. The new system offers a means to control liquid flow to the
fuser roller 14 in real time. The control of the power source by
the LCU 16 offers the option of using feedback and/or feed forward
to achieve optimized dispensing of the fluid, which is preferably
an oil in this embodiment, such as silicon oil. A preferred
embodiment of a geometrically simple and easy to fabricate
electrode structure that can be used is shown in FIG. 1b and may be
amenable to implementation in a retrofit kit designed for
installation in existing machines. A portion of the coating system
20 for moving non-conducting fluid along a surface is shown in FIG.
1b and includes a non-conducting surface 30, which may be planar or
curved, to receive the non-conducting fluid 24 and also includes
the voltage-actuated microfluidic structure 22. The
voltage-actuated microfluidic structure 22 includes a first array
32 and second array 34 of substantially parallel microelectrodes
35, shown here as positioned essentially flat with the top of a
surface, said first array 32 having microelectrode(s) positioned
between, and alternating with, the microelectrode(s) of the second
array 34, forming an interleaved pattern. An electric power source
36 is in communication with the first array 32 and second array 34
so that the first array and second array interact to create a
non-uniform electric field 25 such that the non-conducting fluid 24
moves parallel to the microelectrodes 35 in response to the applied
non-uniform electric field 25. The microelectrodes 35 are spaced at
a distance less than 0.1 mm and preferably have a non-conducting
coating 38.
[0028] FIG. 1c shows one embodiment where the micro electrode has a
thickness of 0.1 micron, a width of 30 micron and is coated with 5
micron of insulating material.
[0029] In a preferred embodiment the coating system 20 moves the
non-conducting fluid 25 along the non-conducting surface 30 using
the microfluidic structure 22, where the microelectrodes are less
than 1 mm in width and are spaced less than 0.1 mm apart and more
preferably between 60 and 90 micrometers apart. The microelectrodes
of the first array 32 are positioned between, and alternating with,
the microelectrode(s) of the second arrays 34 to form an
interleaved pattern as shown in FIG. 1b. The microelectrodes and
the surface of this embodiment are covered with a non-conducting
coating that ensures a contact angle between the surface and the
non-conducting liquid of greater than 10 degrees. In this
embodiment the electric power source 36 is in communication with
the first array 32 and second array 34 such that the first array
and second array interact to create a non-uniform electric field
such that the non-conducting fluid moves parallel to the
microelectrodes as will be discussed in more detail below. In one
preferred embodiment of the coating system 20, the ratio between
the electrode spacing and an electrode width is between 2:1 and 3:1
and the dielectric breakdown strength of the non-conducting coating
38 is greater then 50 Volts/micron. The non-conducting fluid can be
a polymer that is at an elevated temperature and hardens when
cooled such as a thermoplastic. The non-conducting fluid can also
be a polymer dissolved in a solvent that hardens when the solvent
is removed. Additionally the non-conducting fluid may include dye
or particles. The non-conducting liquid preferably has a volume
resistivity greater than 1.times.10.sup.13 ohm-cm.
[0030] The described coating system 20 can be broadly used for the
dispensing of other insulating liquids in many industrial processes
ranging from roll and web coating to application of adhesives and
possibly to critical microfabrication operations where thin layers
must be laid down on large-area substrates.
[0031] The controlled flow of dielectric liquids can be achieved by
a non-uniform electric field produced by properly designed
electrodes. Early experiments with structures having dimensions of
.about.1 millimeter required voltages in excess of 20 kV that
necessitated a high-pressure nitrogen gas environment to avoid
electrical breakdown [T. B. Jones, M. P. Perry, and J. R. Melcher,
"Dielectric siphons", Science, vol. 174, pp. 1232-1233, Dec. 17,
1971; T. B. Jones and J. R. Melcher, "Dynamics of electromechanical
flow structures", Physics of Fluids, vol. 16, pp. 393-400, March
1973]. It has been found that reducing electrode dimensions to less
than 0.1 millimeters invokes favorable scaling relations that
drastically reduce the voltage requirement, avoid air breakdown,
and create the opportunity for electric-field-coupled
microfluidics.
Dielectrophoretic Liquid Control
[0032] Dielectrophoresis (DEP) is an example of the classical
ponderomotive effect, that is, the force exerted on dipoles by a
non-uniform electric field. The dipoles--individual molecules in
the case of a liquid--tend to collect in regions of higher electric
field intensity as shown in FIGS. 2a, 2b and 2c. FIG. 2a shows the
critical phenomenology of liquid dielectrophoresis (DEP) where a
liquid of dielectric constant .kappa..sub.1>.kappa..sub.2 is
drawn into a region of strong electric field. In FIG. 2b the
dielectric liquid surface conforms to the electric field lines
where E=F/q or qE=F. FIG. 2c shows the isofoltage potential lines
in one example. This same force repels gas or vapor bubbles within
this liquid from strong field region. Ordinarily, the preferred
equilibrium of the collected liquid fixes the liquid surface to be
parallel with the electric field lines, as depicted in FIG. 2b.
[0033] Liquid DEP differs from other electrohydrodynamic (EHD)
phenomena used in microfluidics in that it does not act as a
conventional pumping mechanism where a pressure differential
initiates the flow. Instead, the non-uniform electric field created
by the electrodes establishes a new hydrostatic equilibrium to
which the liquid responds when voltage is applied. Once the
equilibrium is reached, the flow ceases unless fluid is
continuously removed from the structure by some means. This
hydrostatic equilibrium is best exemplified by Pellat's classic
experiment, consisting of two plane, parallel electrodes at spacing
D, oriented vertically and partially immersed in a pool of
dielectric liquid of mass density .rho., and dielectric constant,
K. Gas of negligible density and polarizability approximately equal
to free space, .di-elect cons..sub.0=8.854.times.10.sup.-12 F/m,
covers the liquid. For an applied voltage V, the liquid rises
between the electrodes to a static height
h DEP = 0 ( .kappa. - 1 ) E 2 2 .rho. g ( 1 ) ##EQU00001##
where E.apprxeq.V/D estimates the uniform electric field between
the electrode plates, and g=9.81 m/s.sup.2 is the terrestrial
acceleration due to gravity but ignoring any contribution due to
fringe fields. Also note that h.sub.DEP is proportional to the
product of the difference in dielectric constants of the liquid and
the gas, that is, (.kappa.-1) and the square of the electric field,
E.
[0034] Liquid dielectrophoresis can be implemented to initiate bulk
electromechanical flow of insulating liquids. Such a method of
liquid transportation has potential applications in controlling
both spatial and temporal flow with high precision. The flow of
liquid becomes a critical factor in various applications where
volume flow control is required. Such a method can be instrumental
in thin film coating on various substrates that require conformal
and uniform coverage.
[0035] One realization of a liquid DEP flow structure is the simple
coplanar scheme shown in FIG. 3a for a dielectric flow structure
for dielectric liquids with a liquid rivulet and a dielectric
coating on the coplanar structure.
[0036] This geometry has been used to dispense droplets of
conductive, aqueous liquids ranging from .about.10 picoliters to
.about.100 nanoliters see R. Ahmed and T. B. Jones, "Dispensing
picoliter droplets on substrates using dielectrophoresis," Journal
of Electrostatics, vol. 64, pp. 543-549, 2006. When voltage is
turned on, the electric field causes a liquid finger (rivulet) to
emerge from the parent droplet and move rapidly along the
electrodes. The rivulet flows to the end of the structure where it
stops. FIG. 3b shows the rivulet in cross-section, for fuser oil
dispensing to rolls and/or webs. To provide the required mechanical
flexibility for a wiper structure. The substrate is a flexible
material such as a polyimide which is maintained by the electric
field in a roughly semi-circular profile. DEP liquid actuation is
not a true pumping mechanism; rather, it is analogous to
capillarity; however, when the voltage is on, the DEP force easily
overwhelms both capillarity and gravity. If voltage is then
removed, the well-known capillary instability ensues, rapidly
breaking up the static liquid rivulet into droplets.
[0037] The electrodes, as shown in of FIG. 3c, in a schematic
representation of the co-planar electrode structure, can be used to
control the flow of insulating liquids. The long horizontal lines
are the electrodes and the solid blocks represent connection pads
for voltage application. The gray region depicts the dielectric
layer coating to control the oliophobic nature of the electrode
surface. Because the dielectric constant of most such liquids is
much lower than water, required voltages are higher. At the same
time, the purely capacitive current requirement remains quite low,
power consumption is minimal, and low-frequency square wave
excitation can be used to minimize the risk of electrical
breakdown. Experiments used a 1-10 .mu.l droplet of silicon oil
with viscosity from 350-3 cSt. At the T-junction. FIGS. 4, 5, and 6
demonstrate the operational principles of flow control. FIG. 4
shows selected video frames of the flow in an isolated, coplanar
electrode structure consisting of two parallel electrode strips
patterned in evaporated aluminum metal and coated with a few
microns of Cytop.TM., a commercially available hydrophobic coating
material. This was performed using 625 V-rms and 50 cSt fuser
oil.
[0038] Due to wetting behavior, initial actuation is slow; however,
the natural behavior of the liquid finger can be exploited to
achieve rapid turn-on and turn-off flow control once the flow
structure has been initially primed. Refer to FIG. 5 showing the
behavior of the rivulet when voltage is removed. This was performed
using 625 V-rms and 50 cSt fuser oil. FIG. 5 shows rivulet breakup
into regularly spaced droplets at various times after the voltage
has been removed. The camera paned from left to right starting in
frame `e`. Within seconds, the capillary instability breaks up the
rivulet into droplets, thus severing liquid communication and
cutting off the flow. With proper design of the electrodes to
promote initiation of the capillary instability, the response time
of the structure for stopping the flow of oil is adequate for fuser
oil dispensing. If droplets already have been formed along the
structure, reapplying voltage rapidly reestablishes the rivulet and
the flow. The video frames in FIG. 6 show rapid re-establishment of
liquid communication starting from an array of sessile droplets
along the length of the structure after the voltage is reapplied
and when the voltage is increased the response time rapidly
decreases. By properly combining electrode geometries with
hydrophobic and hydrophilic patterning of the surface to create
distributed liquid reservoirs, excellent on/off control is
achieved.
[0039] This microfluidic system can be used to control and dispense
fuser oils and other fluids based on the interplay between
electrical and capillary forces. DEP actuation is
voltage-controlled, but both proper design of the electrodes and
choice of materials having appropriate wetting properties are
critical for effective control of flow rate and response time.
Voltage can be used to control the viscous-limited volumetric flow
rate because the cross-section of the electric-field-mediated
rivulet, dependent on the voltage, determines the effective
hydraulic diameter.
[0040] FIG. 7 shows a coating method used for moving non-conducting
fluid 24 along a flexible curved surface 30, the method comprising
the steps of applying a non-conducting fluid to the non-conducting
surface 30 including the first and second array of one or more
substantially parallel microelectrodes 25 positioned on said
surface, said first array having microelectrode(s) positioned
between, and alternating with, the microelectrode(s) of the second
array, forming an interleaved pattern so that the applied electric
power to the first array and second array is such that the first
array and second array interact to create a non-uniform electric
field that moves the non-conducting fluid parallel to the
microelectrodes 35 in response to the applied non-uniform electric
field.
[0041] In the fuser oil application envisioned, the electrode
structure will consist of hundreds or thousands of parallel
electrode pairs at least 1 cm long. The coplanar electrode
structures, in any of several designs, create a 2D electrostatic
field when excited by sufficient voltage. The design shown above is
only one possible design intended merely to exemplify the
invention. The substrate on which the electrodes are patterned is
preferably a flexible insulating material such as polyimide
(Kapton.TM.) but could also be a rigid material such as glass.
[0042] In one preferred embodiment, the electrodes are coated with
a moderately oleophobic (low surface energy) material, such as
DuPont Teflon-AF.TM. having surface tension of 18 dynes/cm or
Cytop.TM., made by Asahi Glass and having surface tension of 19
dynes/cm. Probably the most effective group of such coatings is the
fluoropolymers, more specifically amorphous Perfluoropolymers. Most
fluoropolymers, including PTFE, FEP, PFA, PVDF, and/or PTFE, have
suitably low surface energy and make good coating materials because
they have the desired electrical properties, namely, high
dielectric breakdown strength (>50 MV/m) and high volume
resistively (>1e14 ohm-cm).
[0043] The volumetric flow rate calculated per electrode pair shows
the wide range attainable, from 1 pL to 10 mL per second, as a
function of voltage.
[0044] This liquid DEP has been used in conjunction with the fluid
flow system and method. One preferred embodiment uses "co-planar"
aluminum electrodes that are essentially flat to the surface and
that are patterned using conventional photolithography on glass
substrates for this microactuation scheme. The electrode width is
90 um and the gap is 30 um. For an individual experiment, a 1-10
.mu.L droplet of insulating oil is dispensed at the T-junction of
the electrode pair as depicted in FIG. 3c.
[0045] The electrodes are coated with a low surface energy,
non-conducting material 44 (shown in FIG. 3b). The insulating oil
has very low surface energy that causes it to spread over glass in
an uncontrolled manner. An appropriate oleophobic surface coating
is required with low surface energy that will make the oil drop
bead up, thereby minimizing droplet spreading. Proper surface
coatings that promote the pinning of the oil finger along its edges
once it emerges from the parent droplet is critical to maintaining
flow control. Good results are attained with Teflon-AF and
Cytop.TM.. These materials have comparable dielectric constants
(.about.2.1) but the breakdown strength of Cytop.TM. is five times
higher than Teflon-AF.
Factors Influencing the Flow
[0046] There are three important parameters that control the flow
of the oil. First and foremost, the liquid viscosity determines
actuation speed and maximum flow rates. A high viscosity silicone
oil, for example 350 centistoke (manufactured by Dow Corning),
requires very high voltage (>1.5 kV) and exhibits very sluggish
flow. On the other hand, for lower viscosity oils, for example, 50
centistoke, higher flow rates can be achieved at lower voltages.
Second, the applied voltage controls actuation speed. The higher
the magnitude of voltage is, the faster the liquid finger moves.
Voltage also controls the cross sectional profile of the liquid
finger. A lower voltage will confine the finger between the inner
edges of the electrodes. When the voltage is increased, the
cross-section expands laterally to cover the entire width of the
electrode structure, thereby, increasing the flow of the liquid.
Third, the electrode geometry influences the flow. From
experimental tests, it is found that an electrode width to gap
ratio of 3:1 is optimal.
[0047] The flow control scheme has two regimes: (i) voltage on,
with the oil controlled by the DEP force and (ii) voltage off, with
the oil controlled by capillarity. FIGS. 4, 5, and 6 show a
collection of frames depicting the actuation scheme. In FIG. 4,
frame (1) shows a parent drop of 50 centistoke silicone oil
residing at the T-junction of the electrodes before applied
voltage. When sufficient voltage is applied, at time, t=.about.1.2
sec, a liquid finger emerges from the droplet and starts to travel
along the length of the structure as shown in frame (2). Frame (3)
and (4) show the subsequent forward progress of the finger as time
is increased to .about.4.7 sec. At t=22 sec, the liquid finger
reached the end of the structure.
[0048] FIG. 5 is a sequence of video frames that show the rupture
of the rivulet when the voltage is removed. In frame (1), the
intact rivulet extends to the end of the electrode structure, held
in a stable configuration by the non-uniform electric field lines.
When the voltage is turned off, the finger starts to retract due to
capillary instability (frame 2), but at the same time multiple
droplets form due to capillary instability (frames 3-7). Due to the
limited field of view of the microscope, the camera has been panned
left to right to capture the phenomenon. Frame (7) shows multiple
droplets equidistant from each other. Once the structure has been
primed by droplets, in an initial time frame typically between 0.1
to 10 seconds (depending on viscosity and applied voltage),
electric-field and capillarity assisted on/off control mechanism
can be utilized at a much faster rate. The response time of the
droplets to reform the finger is much shorter than the initial time
to create the droplets. FIG. 6 is an illustration of this
valve-like operation. FIG. 6(a) shows the initial droplets sitting
atop the electrode structure. When the voltage is reapplied, the
droplets recombine to form a rivuletas shown in FIG. 6(b).
[0049] The transit time for initial priming of the liquid finger
was recorded as a function of voltage for three viscosity grades of
silicone oil: 3, 50, and 350 centistokes. FIGS. 8a, b, and c plot
these transit time data. Each viscosity grade has a threshold
voltage below which the finger will not travel the entire distance
of 5 mm. This threshold value, V.sub.threshold can be determined
experimentally. For 50 cst oil, V.sub.threshold is 575 V-rms. At
this voltage, it takes .about.61 seconds for the finger to emerge
from the droplet and reach the end of the structure. Increasing the
voltage from 575 to 600 decreases the time from 61 sec to 55 sec.
If the voltage is increased from 600 to 650, there is a drastic
reduction in the transit time to .about.22 seconds. When the
voltage is increased to 1000 V-rms, the transit time drops to
.about.2 seconds.
Cross-Sectional Profile as a Function of Voltage
[0050] Once the finger reaches the end of the structure, the flow
ceases unless the oil is removed, as will be the case in the
preferred embodiment discussed above. Here, oil is being
continuously applied to the fuser roll of a printer. In order to
determine the volumetric flow rate in the DEP flow structure under
these conditions, a paper blotter is weighed and then mounted at
the end of the structure. Voltage is applied to the electrodes and
liquid flows to the blotter until the initial liquid droplet as
been entirely absorbed, at which time the voltage is removed. The
volumetric flow rate is then determined by weighing the blotter
again, subtracting the tare weight, and dividing this mass by the
product of the lapsed time and the mass density of the oil. Data
are provided in Table A below.
TABLE-US-00001 TABLE A Steady-state volumetric flow rate as a
function of voltage results: per electrode per cm of per page
Voltage pair electrode array (8.5'' .times. 11'') 675 V 29 pl/s 2.4
nl/s 60 nl/page 800 V 1 nl/s 83 nl/s 1.5 .mu.l/page 900 V 3 nl/s
250 nl/s 4 .mu.l/page These rates are average rates over the 6
mm.
[0051] The steady state volumetric flow rate is a strong function
of the applied voltage. As mentioned previously, increasing the
voltage increases the cross-sectional profile of the finger and
therefore the hydraulic diameter. FIG. 9 shows how changing the
voltage affects the cross-sectional shape of the finger.
[0052] The first frame of FIG. 9 shows that at 675 V-rms, which is
slightly above the threshold voltage value, the finger is confined
within the inner edges of the electrodes. Once the voltage is
increased to 800 V-rms (second frame), the liquid finger has a much
fuller profile evidenced by the partial overlap on both electrodes
as shown. Finally, when the applied voltage is 900 V-rms (third
frame), the finger spans across the outer edges of both electrodes.
The volumetric flow rate at 675 V-rms is approximately 29
picoliters/second. When the voltage is increased to 800 V-rms, the
flow rate increases to 1 nanoliter/second. Finally, at 900 V-rms,
the flow rate is about 3 nanoliters/second. These flow rates are
based on a single pair of electrodes. These results indicate that
simply increasing the voltage by a factor of approximately two
makes it possible to control the volumetric flow rate by 3 orders
of magnitude. This wide range of flow rates will be very useful in
printers, where it is desirable to control the amount of oil
dispensed to the fuser roll in time and space, depending on the
size of the page and the amount and location of solid color area on
each page of a document.
Preferred Power Supply
[0053] The electric power source 16 is preferably an alternating
current (AC) with a frequency greater than 5 Hz but it could range
from 50 Hz-100 KHz and could be a DC power source. The waveform of
the AC power source can be a sinusoid, square, saw tooth or any
other shape, but is preferably a square wave. The duty cycle of the
waveform is not restricted but 50% is preferred.
[0054] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
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