U.S. patent application number 10/403159 was filed with the patent office on 2004-09-30 for method and apparatus for controlling the movement of a liquid on a nanostructured or microstructured surface.
Invention is credited to Kornblit, Avinoam, Kroupenkine, Timofei Nikita, Mandich, Mary Louise, Schneider, Tobias Manual, Taylor, Joseph Ashley, Yang, Shu.
Application Number | 20040191127 10/403159 |
Document ID | / |
Family ID | 32989866 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040191127 |
Kind Code |
A1 |
Kornblit, Avinoam ; et
al. |
September 30, 2004 |
Method and apparatus for controlling the movement of a liquid on a
nanostructured or microstructured surface
Abstract
A method and apparatus is disclosed wherein the movement of a
droplet disposed on a nanostructured or microstructured surface is
determined by at least one characteristic of the nanostructure
feature pattern or at least one characteristic of the droplet. In
one embodiment, the movement of the droplet is laterally determined
by at least one characteristic of the nanostructure feature pattern
such that the droplet moves in a desired direction along a
nanostructured feature pattern. In another embodiment, the movement
of the droplet is determined by either at least one characteristic
of the nanostructure feature pattern or at least one characteristic
of the droplet in a way such that the droplet penetrates the
feature pattern at a desired area and becomes substantially
immobile.
Inventors: |
Kornblit, Avinoam; (Highland
Park, NJ) ; Kroupenkine, Timofei Nikita; (Warren,
NJ) ; Mandich, Mary Louise; (Martinsville, NJ)
; Schneider, Tobias Manual; (Marburg, DE) ;
Taylor, Joseph Ashley; (Orlando, FL) ; Yang, Shu;
(New Providence, NJ) |
Correspondence
Address: |
Docket Administrator (Room 3J-219)
Lucent Technologies Inc.
101 Crawfords Corner Road
Holmdel
NJ
07733-3030
US
|
Family ID: |
32989866 |
Appl. No.: |
10/403159 |
Filed: |
March 31, 2003 |
Current U.S.
Class: |
422/400 |
Current CPC
Class: |
B01L 2300/166 20130101;
B01L 2300/168 20130101; B01L 2400/088 20130101; G01N 21/552
20130101; B82Y 30/00 20130101; B01L 3/50273 20130101; B01J
2219/00853 20130101; B01L 2400/0406 20130101; B01L 3/502746
20130101; B01L 2300/0896 20130101; B01F 25/431971 20220101; B01L
2400/0415 20130101; B01L 2300/089 20130101; B01J 19/0093 20130101;
B01J 2219/00833 20130101; B01L 2400/082 20130101; B81B 1/00
20130101; B01F 33/3031 20220101; G01N 21/4788 20130101; B01J
2219/00783 20130101; B01L 3/5088 20130101; B01F 25/4317 20220101;
B41J 2002/14395 20130101; B81B 2203/0361 20130101; B01L 2400/0688
20130101; B01F 25/43161 20220101 |
Class at
Publication: |
422/100 |
International
Class: |
B01L 003/02 |
Claims
What is claimed is:
1. An apparatus comprising a surface having a pattern of features,
said surface comprising: a plurality of nanostructures or
microstructures having intra-pattern characteristics, at least one
of said intra-pattern characteristics adapted to produce controlled
motion in a predetermined direction of a droplet of liquid disposed
on said surface.
2. The apparatus of claim 1 wherein said at least one of said
intra- pattern characteristics is a voltage of at least one of said
nanostructures or microstructures relative to said droplet.
3. The apparatus of claim 1 wherein said at least one said
intra-pattern characteristics is a distance between each of at
least a portion of said plurality of nanostructures or
microstructures.
4. The apparatus of claim 1 wherein said at least one of said
intra- pattern characteristics is a shape of at least a portion of
said plurality of nanostructures or microstructures.
5. An apparatus comprising: a surface having a pattern of
nanostructures or microstructures, the spatial density of at least
a portion of said pattern of nanostructures or microstructures
being varied; and a droplet of liquid disposed on said surface;
wherein the spatial density of said pattern is adapted to produce
controlled motion of said droplet in a predetermined direction.
6. An apparatus comprising: a surface having a pattern of
nanostructures or microstructures, the spatial density of at least
a portion of said pattern of nanostructures or microstructures
being varied; and a droplet of liquid disposed on said surface;
wherein the area of contact between said surface and said droplet
is adapted to produce controlled motion of said droplet in a
predetermined direction.
7. An apparatus comprising: a surface having a pattern of
nanostructures or microstructures; a droplet of liquid disposed on
said surface; and means for causing said droplet to penetrate said
surface.
8. The apparatus of claim 7 wherein said means for causing
comprises means for applying a voltage to said droplet.
9. The apparatus of claim 7 wherein said means for causing
comprises means for reducing a surface tension of said droplet.
10. The apparatus of claim 9 wherein said means for reducing
comprises means for introducing at least a chemical or biological
element into at least a portion of said droplet.
11. The apparatus of claim 7 wherein at least a portion of a beam
of light incident upon said surface passes through said surface,
said portion depending upon whether said droplet is penetrated into
said surface.
12. The apparatus of claim 7 wherein at least a portion of a beam
of light incident upon said surface is reflected off of said
surface, said portion depending upon whether said droplet is
penetrated into said surface.
13. The apparatus of claim 7 wherein a first wavelength of a beam
of light is reflected off of said surface in a first predetermined
direction.
14. The apparatus of claim 13 wherein, upon said droplet of liquid
penetrating said pattern, a second wavelength of light is reflected
off of said surface in a second predetermined direction.
15. The apparatus of claim 7 wherein said means for causing
comprises means for increasing the temperature of said droplet of
liquid.
16. The apparatus of claim 7 wherein said means for causing
comprises means for increasing the temperature of said surface.
17. An apparatus comprising a surface, said surface adapted to
produce controlled motion of a droplet of liquid in a predetermined
direction, said droplet disposed on said surface, said surface
comprising a plurality of nanostructures or microstructures,
wherein at least a first portion of said plurality has a different
spatial density from at least a second portion of said
plurality.
18. An apparatus comprising a surface, said surface adapted to
produce controlled motion of a droplet of liquid in a predetermined
direction, said droplet disposed on said surface, said surface
comprising a plurality of nanostructures or microstructures,
wherein each of the nanostructures or microstructures in at least a
first portion of said plurality has a predetermined asymmetric
shape.
19. The apparatus of claim 18 wherein each of the nanostructures or
microstructures in at least a second portion of said plurality has
a varied spatial density.
20. A method for detecting the presence of a chemical or biological
element with a detector, said detector having a surface, said
surface comprising a pattern of nanostructures or microstructures
and a droplet of a liquid disposed on said surface, said method
comprising the step of exposing said detector to a medium, wherein
said liquid has been selected in a way such that said droplet
penetrates said surface in response to a chemical or biological
element in said medium being introduced into said droplet.
21. The method of claim 20 further comprising generating an
indication in response to said droplet penetrating said
surface.
22. The method of claim 21 wherein said indication comprises a
change in the color of at least a portion of said surface.
23. The method of claim 21 wherein said indication comprises an
electrical signal.
24. A microfluidic mixer comprising: a first channel adapted to
accommodate a first liquid; a second channel adapted to accommodate
a second liquid; a third channel adapted to accommodate a third
liquid, wherein said third liquid comprises said first liquid and
said second liquid; means for merging said first channel and said
second channel into said third channel; and means for disturbing a
flow in said third channel by causing at least one of said first
liquid and said second liquid to penetrate a surface of said third
channel.
25. A tunable heat dissipation device comprising: a channel adapted
to accommodate a liquid wherein a heat generating apparatus is
disposed on a surface of said channel; and means for disturbing a
flow in said channel by causing said liquid to penetrate a surface
of said channel.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the motion of
liquids disposed on a surface with extremely small, predetermined
surface features and, more particularly, to controlling the
movement of a liquid on a surface with predetermined nanostructure
or microstructure features
BACKGROUND OF THE INVENTION
[0002] Many beneficial devices or structures in myriad applications
are characterized at least in part by having a liquid that is in
contact with at least one solid surface. For example, liquid
droplets disposed on surfaces or within channels are the hallmarks
of many microfluidic devices, biological/chemical sensors, chemical
reactors, optical components, heat dissipation devices, and
patterning applications. Many of these devices and applications are
characterized in that liquid moves or is caused to be moved while
in contact with a surface. Since the characteristics of both the
liquid and the surface determine the interaction between the liquid
and surface, it is often desirable to understand and control those
characteristics to achieve control of the interaction of the liquid
with those surfaces. This is especially so when the application in
question involves relatively small quantities of liquid.
[0003] FIG. 1 shows one illustrative prior art embodiment of small
liquid droplet 102 disposed on a surface in a way such that it
forms a liquid microlens 101. Such a liquid microlens is the
subject of copending U.S. patent applications Ser. No. 09/884,605,
filed Jun. 19, 2001, entitled "Tunable Liquid Microlens" and Ser.
No. 09/951,637, filed Sep. 13, 2001, entitled "Tunable Liquid
Microlens With Lubrication Assisted Electrowetting." Both of these
copending Patent Applications are hereby incorporated by reference
herein in their entirety. The microlens embodiment of FIG. 1 is
useful to demonstrate the interaction between any droplet of liquid
and the surface on which it is disposed, whether or not the droplet
and surface are part of a microlens or another application. In FIG.
1, droplet 102 is a droplet of a transparent liquid, such as water,
typically (but not necessarily) with a diameter from several
micrometers to several millimeters. The droplet is disposed on a
transparent substrate 103 which is typically hydrophobic or
includes a hydrophobic coating. The contact angle .theta. between
the droplet and the substrate is determined by interfacial surface
tensions (also known as interfacial energy) ".gamma.", generally
measured in milli-Newtons per meter (mN/m). As used herein,
.gamma..sub.S-V is the interfacial tension between the substrate
103 and the air, gas or other liquid that surround is the
interfacial tension between the droplet 102 and the air, gas or
other liquid that surrounds the droplet, and .gamma..sub.Y-L is the
interfacial tension between the substrate 103 and the droplet 102.
The contact angle .theta. may be determined from equation (1):
cos .theta.=(.gamma..sub.S-V-.gamma..sub.S-L)/.gamma..sub.L-V
Equation (1)
[0004] Equation (1) applies to any instance where a droplet of
liquid is disposed on a surface, whether or not the droplet is used
as a microlens.
[0005] In the microlens embodiment of FIG. 1 and in other instances
where a liquid is disposed on a surface, it is often desirable to
be able to change the shape of the droplet. FIG. 2 shows a prior
art microlens 201, similar to the microlens of FIG. 1, whereby the
phenomenon of electrowetting is used to change the shape of the
droplet by reversibly changing the contact angle .theta. between
droplet 202 of a conducting liquid and a dielectric insulating
layer 203 having a thickness "d" and a dielectric constant
.epsilon..sub.r. An electrode, such as metal electrode 204, is
positioned below the dielectric layer 203 and is insulated from the
droplet 202 by that layer. The droplet 202 may be, for example, a
water droplet, and the dielectric insulating layer 203 may be, for
example, a Teflon/Parylene surface.
[0006] When no voltage difference is present between the droplet
202 and the electrode 204, the droplet 202 maintains its shape
defined by the volume of the droplet and contact angle
.theta..sub.1, where .theta..sub.1, is determined by the
interfacial tensions .gamma. as explained above. When a voltage V
is applied to the electrode 204, the voltage difference between the
electrode 204 and the droplet 202 causes the droplet to spread. The
dashed line 205 illustrates that the droplet 202 spreads equally
across the layer 203 from its central position relative to the
electrode 204. Specifically, the contact angle .theta. decreases
from .theta..sub.1 to .theta..sub.2 when the voltage is applied
between the electrode 204 and the droplet 202. By using separate
electrodes under different parts of the droplet, and varying the
voltage to those individual electrodes, spreading of the droplet
can be achieved such that the droplet moves from its centered
position to another desired position. Such a movement is described
in the aforementioned copending '605 and '637 patent applications.
The voltage V necessary to achieve this spreading, whether to
change the shape of the droplet or its position, may range from
several volts to several hundred volts. The amount of spreading,
i.e., as determined by the difference between .theta..sub.1 and
.theta..sub.2, is a function of the applied voltage V. The contact
angle .theta..sub.2 can be determined from equation (4):
cos .theta. (V)=cos .theta.(V=0)+V.sup.2(.epsilon..sub.0
.epsilon..sub.r)/(2d.gamma..sub.L-V) Equation (4)
[0007] where cos .theta. (V=0) is the contact angle between the
insulating layer 203 and the droplet 202 when no voltage is applied
between the droplet 202 and electrode 204; .gamma..sub.L-V is the
droplet interfacial tension described above; .epsilon..sub.r is the
dielectric constant of the insulating layer 203; and
.epsilon..sub.0 is 8.85.times.10.sup.-12 F/M--the permittivity of a
vacuum.
[0008] In implementations such as the liquid microlens described
above, while the surface upon which the droplet is disposed is
hydrophobic, the characteristics of that surface are such that the
droplet flattens significantly at the area where it comes into
contact with the surface. Thus, due to the resulting large contact
area between the surface and the droplet, a significant amount of
flow resistance is present between the surface and the droplet.
This is desirable in the above microlens because, if there were too
little flow resistance present, the droplet would freely move and
it would become impossible to maintain the droplet in its desired
stationary position or shape in the absence of other means for
controlling the droplet. However, in many instances, it is often
desirable to reduce the flow resistance experienced by a liquid on
a surface.
[0009] Therefore, recent applications relying on liquids disposed
on such surfaces have centered on attempts to reduce the
aforementioned flow resistance exerted on the liquid. Many devices,
such as those referred to above, can benefit from such a reduced
flow resistance because of the resulting significant reduction in
the operational power consumption of the devices. One such
application is described in "Nanostructured Surfaces for Dramatic
Reduction of Flow Resistance in Droplet-based Microfluidics", J.
Kim and C. J. Kim, IEEE Conf. MEMS, Las Vegas, Nev., January 2002,
pp. 479482, which is hereby incorporated by reference herein in its
entirety. That reference generally describes how, by using surfaces
with predetermined nanostructure features, the flow resistance to
the liquid in contact with the surface can be greatly reduced.
[0010] The Kim reference teaches that, by finely patterning the
surface in contact with the liquid, and using the aforementioned
principle of liquid surface tension, it is possible to greatly
reduce the area of contact between the surface and the liquid. It
follows that the flow resistance to the liquid on the surface is
correspondingly reduced.
[0011] FIGS. 3A-3F show how different, extremely fine-featured
microstructure and nanostructure surface patterns result in
different contact angles between the resulting surface and a
droplet of liquid. FIGS. 3A and 3B show a microline surface and a
micropost surface, respectively. Each of the lines 301 in FIG. 3A
is approximately 3-5 micrometers in width and each of the
microposts 302 in FIG. 3B is approximately 3-5 micrometers in
diameter at its widest point. Comparing the microline pattern to
the micropost pattern, for a given size droplet disposed on each of
the surfaces, the contact area of the droplet with the microline
pattern will be greater than the contact area of the droplet with
the micropost pattern. FIGS. 3D and 3E show the contact angle of a
droplet relative to the microline surface of FIG. 3A and the
micropost surface of FIG. 3B, respectively. The contact angle 303
of the droplet 305 on the microline pattern is smaller (.about.145
degrees) than the contact angle 304 of the droplet 306 with the
micropost pattern (.about.160 degrees). As described above, it
directly follows that the flow resistance exerted on the droplet by
the microline pattern will be higher than that exerted by the
micropost pattern.
[0012] FIG. 3C shows an even finer pattern than that of the
microline and micropost pattern. Specifically, FIG. 3C shows a
nanopost pattern with each nanopost 309 having a diameter of less
than 1 micrometer. While FIG. 3C shows nanoposts 309 formed in a
somewhat conical shape, other shapes and sizes are also achievable.
In fact, cylindrical nanopost arrays have been produced with each
nanopost having a diameter of less than 10 nm. Specifically, FIGS.
4A-4E show different illustrative arrangements of nanoposts
produced using various methods and further show that such various
diameter nanoposts can be fashioned with different degrees of
regularity. Moreover, these figures show that it is possible to
produce nanoposts having various diameters separated by various
distances. An illustrative method of producing nanoposts, found in
U.S. Pat. No. 6,185,961, titled "Nanopost arrays and process for
making same," issued Feb. 13, 2001 to Tonucci, et al, is hereby
incorporated by reference herein in its entirety. Nanoposts have
been manufactured by various methods, such as by using a template
to form the posts, by various means of lithography, and by various
methods of etching.
[0013] Referring to FIG. 3F, a droplet 307 disposed on the nanopost
surface of FIG. 3C, is nearly spherical with a contact angle 308
between the surface and the droplet equal to between 175 degrees
and 180 degrees. The droplet 307 disposed on this surface
experiences nearly zero flow resistance. As a result, as is noted
by the Kim reference, prior attempts at placing a droplet on such a
surface were problematic, as this extremely low flow resistance
made it almost impossible to keep the water droplets stationary on
the nanostructured surface. As shown in FIG. 5, the reason for this
low flow resistance is that the surface tension of droplet 501 of
an appropriate liquid (depending upon the surface structure) will
enable the droplet 501 to be suspended on the tops of the nanoposts
with no contact between the droplet and the underlying solid
surface. This results in an extremely low area of contact between
the droplet and the surface (i.e., the droplet only is in contact
with the top of each post 502) and, hence low flow resistance.
[0014] Thus, as exemplarily taught by the Kim reference, prior
attempts to reduce flow resistance of liquids through the use of
nanostructures have been limited to disposing the droplets in a
narrow channel, tube or other enclosure to control the freedom
movement of the droplet to within a prescribed area.
SUMMARY OF THE INVENTION
[0015] While prior attempts to advantageously dispose a liquid
droplet on a nanostructure or microstructure feature pattern have
been limited to disposing that droplet in a confining channel, we
have realized that it would be extremely advantageous to be able to
variably control the movement of a droplet disposed on a
nanostructured or microstructured surface without having to place
the liquid droplet in a channel. We have also realized that it
would be especially advantageous to be able to control the
properties of the interface of a liquid droplet with a
nanostructured or microstructured surface, such as the amount of
flow resistance exerted on the liquid as a result of the contact
area between a nanostructured or microstructured surface.
Additionally we have realized that in many applications it would be
highly advantageous to be able to control the degree of penetration
of the droplet inside the nanostructured or microstructured
surface.
[0016] We have invented a method and apparatus wherein the movement
of a liquid droplet disposed on a nanostructured or microstructured
surface is determined by at least one intra-pattern characteristic,
defined herein below, of the nanostructure or microstructure
feature pattern on that surface or at least one characteristic of
the droplet. In one embodiment, the lateral movement of the droplet
is determined by at least one characteristic of the nanostructure
or microstructure feature pattern such that the droplet moves in a
desired direction along the feature pattern. To achieve this
movement, illustratively, the size, shape, density, or electrical
properties of the nanostructure or microstructure are designed such
that the contact angle of the leading edge of a droplet is made to
be lower than the contact angle of the trailing edge of the droplet
to achieve a desired movement.
[0017] In another embodiment, the movement of the droplet is
determined by either at least one intra-pattern characteristic of
the feature pattern or at least one characteristic of the droplet
such that the droplet penetrates the feature pattern at a desired
area and becomes immobile. This characteristic can be, for example,
the surface tension of the droplet, the temperature of either the
droplet or the pattern or the voltage differential between the
droplet and the feature pattern.
[0018] One or both of these embodiments of the present invention
are useful in a variety of applications, such as, illustratively, a
biological or micro-chemical detector, a chemical reactor, a
patterning application, a tunable diffraction grating, a total
internal reflection mirror, a microfluidic mixer, a microfluidic
pump and a heat dissipation device.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1 shows a prior art microlens device that illustrates
the interaction of a liquid disposed on a substrate;
[0020] FIG. 2 shows how prior art electrowetting principles used
with the microlens of FIG. 1 can be used to move the droplet in a
predetermined direction across a substrate;
[0021] FIG. 3A shows a prior art microline surface;
[0022] FIG. 3B shows a prior art micropost surface;
[0023] FIG. 3C shows a prior art nanopost surface;
[0024] FIG. 3D shows a droplet of liquid disposed on the prior art
surface of FIG. 3A and the corresponding contact angle that results
between the droplet and that surface;
[0025] FIG. 3E shows a droplet of liquid disposed on the prior art
surface of FIG. 3B and the corresponding contact angle that results
between the droplet and that surface;
[0026] FIG. 3F shows a droplet of liquid disposed on the prior art
surface of FIG. 3C and the corresponding contact angle that results
between the droplet and that surface;
[0027] FIGS. 4A, 4B, 4C, 4D and 4E show various prior art
nanostructure feature patterns of predefined nanostructures that
are suitable for use in the present invention;
[0028] FIG. 5 shows an illustrative prior art device wherein a
liquid droplet is disposed on a nanostructured feature pattern
[0029] FIG. 6 shows a more detailed view of the prior art
nanostructure feature pattern of FIG. 4C;
[0030] FIGS. 7A, 7B, 7C and 7D show droplets of different liquid
having different surface tensions disposed on the nanostructure
feature pattern of FIG. 6;
[0031] FIG. 8A shows a cross section of the droplet and
nanostructure feature pattern of FIG. 7A;
[0032] FIG. 8B shows a cross section of the droplet and
nanostructure feature pattern of FIG. 7C;
[0033] FIGS. 9A and 9B show a device in accordance with the
principles of the present invention whereby the electrowetting
principles of FIG. 2 are used to cause a liquid droplet to
penetrate a nanostructure feature pattern;
[0034] FIG. 10 shows the detail of an illustrative nanopost of the
nanostructure feature pattern of FIGS. 9A and 9B;
[0035] FIG. 11 shows how, by placing a droplet on a nanostructure
feature pattern with having a varied density of nanoposts, the
droplet will move toward the area of highest density of
nanoposts;
[0036] FIG. 12 shows how, by placing a droplet on a nanostructure
feature pattern with nanoposts arranged in a saw-toothed pattern,
the droplet will move in a known direction relative to that
pattern;
[0037] FIGS. 13A and 13B show a chemical or biological detector in
accordance with the principles of the present invention;
[0038] FIG. 14 shows how the detector of FIGS. 13A and 13B can be
arranged in an array in able to detect multiple elements or
compounds;
[0039] FIG. 15 shows how a pattern can be made in accordance with
the principles of the present invention;
[0040] FIGS. 16A and 16B show a diffractive grating in accordance
with the principles of the present invention;
[0041] FIGS. 17A and 17B show a total internal reflection (TIR)
mirror in accordance with the principles of the present
invention;
[0042] FIGS. 18A, 18B and 18C show a microfluidic mixer in
accordance with the principles of the present invention; and
[0043] FIG. 19 shows a heat dissipation device in accordance with
the principles of the present invention.
DETAILED DESCRIPTION
[0044] FIG. 6 shows an illustrative known surface 601 with a
nanostructure feature pattern of nanoposts 602 disposed on the
surface. Throughout the description herein, one skilled in the art
will recognize that the same principles applied to the use of
nanoposts or nanostructures can be equally applied to microposts or
other larger features in a feature pattern. The surface 601 and the
nanoposts 602 of FIG. 6 are, illustratively, made from silicon. The
nanoposts 602 of FIG. 6 are illustratively approximately 350 nm in
diameter, approximately 6 .mu.m high and are spaced approximately 4
.mu.m apart, center to center. It will be obvious to one skilled in
the art that such arrays may be produced with regular spacing or,
alternatively, with irregular spacing.
[0045] FIGS. 7A, 7B, 7C and 7D show how different liquids behave
when disposed on the illustrative surface 601 of FIG. 6. FIG. 7A
shows that, when a water droplet 701 with a surface tension
(.gamma.) of 72 mN/m is disposed on the surface 601, the droplet
701 retains a nearly spherical shape for the aforementioned
reasons. FIGS. 7B, 7C and 7D show how for liquid droplets 702, 703
and 704, respectively, with decreasing surface tension
(ethyleneglycol [.gamma.=47 mN/m], cyclopentanol [.gamma.=33 mN/m]
and octanol [.gamma.=27 mN/m], respectively) the droplets spread in
increasing amounts over a greater area, with the droplet having the
lowest surface tension (droplet 704) spreading to the greatest
extent.
[0046] As used herein, unless otherwise specified, a
"nanostructure" is a predefined structure having at least one
dimension of less than one micrometer and a "microstructure" is a
predefined structure having at least one dimension of less than one
millimeter. The term "feature pattern" refers to either a pattern
of microstructures or a pattern of nanostructures. Further, the
terms "liquid," "droplet," and "liquid droplet" are used herein
interchangeably. Each of those terms refers to a liquid or a
portion of liquid, whether in droplet form or not. Additionally,
medium, as used herein, is a gas or liquid in which a biological or
chemical element may be present, as discussed herein below.
Finally, intra-pattern characteristics, as used herein, are defined
as a) characteristics of the individual feature pattern elements
relative to other elements (as opposed to inter-pattern
characteristics, which are macro characteristics of the feature
pattern, such as orientation of the entire pattern), or b) certain
characteristics of individual feature pattern elements such as
shape, size, height and electrical characteristics.
[0047] FIGS. 8A and 8B show a cross-section illustration of the
interactions between the nanostructured surface 601 of FIG. 6 and
droplets of different liquids. FIG. 8A represents, for example, the
droplet of water 701 of FIG. 7A. Due to the relatively high surface
tension of the water, along with the intra-pattern characteristics
of the nanostructures, droplet 701 is suspended on the tops of the
nanoposts 602 (shown in greater detail in FIG. 6) and, as
previously discussed, has a very high angle of contact with the
nanostructured surface 601. As a result, droplet 701 experiences
very low flow resistance. FIG. 8B represents, illustratively, the
droplet 703 of cyclopentanol of FIG. 7C. Compared to the droplet
701of water of FIG. 8A, the droplet 703 of cyclopentanol is not
suspended on the tops of the nanoposts 602. Instead, because of the
relatively low surface tension of the liquid, the droplet 703
completely penetrates the surface 601, thereby coming into contact
with the solid surface underlying the nanoposts 602. The droplet
has a low angle of contact, relative to the droplet 701 of FIG. 8A
and, due to the complete penetration of the nanostructured surface
601, experiences a relatively high flow resistance.
[0048] The present inventors have recognized that it is desirable
to be able to control the penetration of a given liquid into a
given nanostructured or microstructured surface and, thus, control
the flow resistance exerted on that liquid as well as the wetting
properties of the solid surface. FIGS. 9A and 9B show one
embodiment in accordance with the principles of the present
invention where electrowetting, similar to that used in the
illustrative microlens of FIG. 2, is used to control the
penetration of a liquid into a nanostructured surface.
[0049] Referring to FIG. 9A, a droplet 901 of conducting liquid is
disposed on nanostructure feature pattern of conical nanoposts 902,
as described above, such that the surface tension of the droplet
901 results in the droplet being suspended on the upper portion of
the nanoposts 902. In this arrangement, the droplet only covers
surface area f.sub.1, of each nanopost. The nanoposts 902 are
supported by the surface of a conducting substrate 903. Droplet 901
is illustratively electrically connected to substrate 903 via lead
904 having voltage source 905. An illustrative nanopost is shown in
greater detail in FIG. 10. In that figure, nanopost 902 is
electrically insulated from the liquid (901 in FIG. 9A) by material
1001, such as an insulating layer of dielectric material. The
nanopost is further separated from the liquid by a low surface
energy material 1002, such as a well-known fluoro-polymer. Such a
low surface energy material allows one to obtain an appropriate
initial contact angle between the liquid and the surface of the
nanopost. It will be obvious to one skilled in the art that,
instead of using two separate layers of different material, a
single layer of material that possesses sufficiently low surface
energy and sufficiently high insulating properties could be
used.
[0050] FIG. 9B shows that, by applying a low voltage (e.g., 10-20
volts) to the conducting droplet of liquid 901, a voltage
difference results between the liquid 901 and the nanoposts 902.
The contact angle between the liquid and the surface of the
nanopost decreases and, at a sufficiently low contact angle, the
droplet 901 moves down in the y-direction along the surface of the
nanoposts 902 and penetrates the nanostructure feature pattern
until it complete surrounds each of the nanoposts 902 and comes
into contact with the upper surface of substrate 903. In this
configuration, the droplet covers surface area f.sub.2 of each
nanopost. Since f.sub.2>>f.sub.1, the overall contact area
between the droplet 901 and the nanoposts 902 is relatively high
and, accordingly, the flow resistance experienced by the droplet
901 is greater than in the embodiment of FIG. 9A. Thus, as shown in
FIG. 9B, the droplet 901 effectively becomes stationary relative to
the nanostructure feature pattern in the absence of another force
sufficient to dislodge the droplet 901 from the feature
pattern.
[0051] FIG. 11 shows an illustrative device in accordance with the
principles of the present invention whereby, instead of moving the
droplet in the y-direction to penetrate the nanostructure feature
pattern, the nanostructures (nanoposts 1102 in this illustrative
embodiment) are arranged such that the droplet 1101 moves laterally
in the x-direction 1104. Specifically, the nanoposts 1102 are
arranged so that the density of nanoposts 1102 increases in the
x-direction 1104. This increased density will lead to a lower
contact angle at the leading edge 1105 of the droplet relative to
the contact angle at the trailing edge 1106 of the droplet. The
lower contact angle at edge 1105 leads to a lower force in the
x-direction applied to the droplet 1101 than does the relatively
higher contact angle at edge 1106. Thus, the droplet 1101 will
"drift" in the x-direction 1104 toward the area of higher density
of nanoposts 1102 as the liquid droplet 1101 attempts to achieve
equilibrium. Thus, by placing the highest density of nanoposts at
that location at which it is desired to have the liquid disposed on
the surface, a liquid droplet can be initially disposed at another
location on the surface and it will autonomously move toward that
area of highest density.
[0052] While the movement achieved by the illustrative embodiment
of FIG. 11 allows for a droplet to move to a final equilibrium
location at the area of highest nanostructure density, it may also
be desirable to reverse this movement away from the area of highest
density. FIG. 12 shows one embodiment in accordance with the
principles of the present invention that enables such a reversible
movement. Specifically, FIG. 12 shows a droplet 1201 disposed on a
surface having nanostructures 1202 arranged in sawtooth
configuration. At equilibrium, the droplet will remain stationary
in position B, just as it would with a nanopost feature pattern
such as that exemplified by FIG. 8A. However, when a time-periodic
excitation is applied to the droplet 1201, it will begin to drift
in direction 1204. Such a periodic excitation may be generated,
illustratively, by an alternating voltage applied to specific
nanostructures 1202 or, alternatively, by ultrasound of an
amplitude and frequency that disturbs the equilibrium of the
droplet 1201. One skilled in the art will recognize that many
different voltages, frequencies and amplitudes of sound waves may
be used to generate the force necessary to disturb the equilibrium
of the droplet 1201. Additionally, it will also be obvious to one
skilled in the art that this excitation may be generated by many
different methods.
[0053] The movement of droplet 1201 begins because, when disturbed,
the droplet will periodically change the size of its contact spot
and, thus, the edges 1205 and 1206 of the droplet 1201 will move
back and forth over the nanostructures 1207 in the feature pattern.
However, due to the asymmetric shape of the nanostructures 1207,
the contact angle hysteresis at edge 1206 of the droplet relative
to the surface will be lower than the contact angle hysteresis at
edge 1205. In other words, in moving back and forth over the
surface, it is much more difficult for the droplet 1201 to travel
up the vertical face 1211 of, for example, feature 1208 than it is
for the droplet 1201 to travel up face 1212 of feature 1208.
Accordingly, once the droplet has crossed over a particular
nanostructure in direction 1204, it will tend not to move back in
the opposite direction and will establish a new equilibrium
position, such as position D. If the time-periodic excitation (such
as ultrasound) continues, the droplet will continue to travel back
and forth across the surface until it crosses over the next
nanostructure1213 in direction 1204. Once again, the droplet will
tend not to move back after crossing nanostructure 1213 and will
attain yet another new equilibrium position in direction 1204. As a
result, by continuing the periodic excitation, the droplet will
stochastically move in direction 1204.
[0054] By combining the embodiment of FIG. 12 with areas of higher
density of nanostructures, such as shown in the embodiment of FIG.
11, reversible lateral movement of the droplet 1201 may be
achieved. For example, referring once again to FIG. 12, if an area
of higher density of nanostructures 1202 is located at location A
on the surface, the droplet 1201 will, as described above, tend to
move toward location A absent any counteracting force. However, if
the illustrative sawtooth pattern of FIG. 12 is used coupled with,
illustratively, a disturbing force generated by ultrasound to
disturb the equilibrium of the droplet 1201, the force tending to
move the droplet toward location A (the higher density of
nanostructures) will be overcome and the droplet 1201 will move in
direction 1204 toward location C on the surface. If the force is
removed (i.e., the ultrasound source is turned off), the droplet
1201 will once again tend to move toward the highest density of
nanostructures at location C. Thus, reversible lateral motion is
achieved.
[0055] Cumulatively the illustrative embodiments of FIGS. 9A, 9B,
10, 11 and 12 show that it is possible, by using the principles of
the present invention, to desirably move a droplet of liquid
laterally along a surface with almost no flow resistance and to
also move the droplet vertically such that the droplet penetrates
that surface at a predetermined location and becomes practically
immobile. Many applications can be found for such movement
possibilities. For example, FIGS. 13A and 13B show an embodiment of
a biological or chemical detector that uses the principles of the
present invention. Referring to FIG. 13A, droplet 1301 is disposed
on nanostructures 1302 similar to that shown in FIG. 9A. Detectors
1306, which are able to detect the desired biological or chemical
compound 1303 are disposed on surface 1304. The liquid for droplet
1301 and the nanostructures 1302 are chosen such that, when the
desired compound 1303 enters the liquid in a desired amount, the
surface tension of the liquid drops and, as shown in FIG. 13B, the
liquid 1301 penetrates the nanostructure pattern and comes into
contact with the detectors 1306. When the compound 1303 comes into
contact with the detectors 1306, an indication of such contact can
be generate by well-known methods, such as via the generating of an
electrical signal or the changing of the color of the detector.
[0056] It will be apparent to one skilled in the art that, in
addition to being used as a detector, the embodiment of FIGS. 13A
and 13B may also be used as a method of achieving a desired
chemical reaction. For example, once again referring to FIG. 13A,
it is possible to select a liquid for droplet 1301 such that the
liquid already contains a chemical compound 1303. Detectors 1306 in
this embodiment are fashioned out of a desired reactant compound
that will achieve a desired reaction when in contact with element
or compound 1303. These detectors/reactants 1306 are disposed
between the nanostructures such that, when the liquid droplet
penetrates the nanostructure feature pattern as shown in FIG. 13B,
the two chemicals come into contact with each other and the desired
reaction occurs. As previously described (e.g., in the discussion
associated with FIGS. 9A and 9B, above), the droplet can be made to
penetrate the feature pattern by either applying a voltage to the
droplet or, alternatively, by using some method for lowering the
surface tension of the liquid droplet 1301 (and, thus, the contact
angle it forms with the surfaces of the nanostructures) such as,
for example, increasing the temperature of the liquid droplet
1301.
[0057] FIG. 14 shows a possible arrangement of the illustrative
embodiments of FIGS. 13A and 13B, whether used as a
chemical/biological detector or used in a chemical reaction
application. Specifically, a liquid can be made to flow in
direction 1401 across the surface of array 1402, which has a
predetermined arrangement of nanostructures patterned on its
surface. Each of areas 1403 may, for example, have
detectors/reactants (such as 1306 in FIGS. 13A and 13B) disposed
between the nanostructures that are suited, for example, for
detecting or reacting with a different chemical/biological
compound. Thus, if used as a detector, the array 1402 of FIG. 14
could be used to detect multiple different compounds. If used as a
chemical reactor, each of the areas could be designed so as to
react with only a certain compound to achieve the desired
reactions.
[0058] Another use for the principles of the present invention is
illustratively shown in FIG. 15. Specifically, FIG. 15 shows how
selective patterning of a desired pattern onto a surface can be
achieved by utilizing the principles of the present invention. In
this figure, a desired pattern (in this case starshaped patterns
1502, 1503, 1504 and 1505) is defined on a substrate 1506 which is
characterized by nanostructure features. It is a desirable goal in
such a patterning application to cause the liquid to move within
the patterns 1502-1505 and to the remain in those patterns. One
method of accomplishing this goal is to use the well-known voltage
differential electrowetting, described above, between a liquid
flowing in direction 1501 across the substrate and the
nanostructures within star-shaped patterns 1503 and 1505. When the
liquid passes across the surface, the liquid only penetrates
between the nanostructures within those two patterns, thus becoming
practically immobile. Due to this resulting immobility, when the
liquid flow is removed, liquid remains only within patterns 1503
and 1505.
[0059] An alternative method of causing a liquid to move to the
star-shaped patterns in a patterning application is to use a
varying density pattern, such as that illustrated in FIG. 11, to
move the liquid to a relatively dense pattern, such as pattern
1502, and then held in place by such electrowetting. Electrowetting
could also be used in a patterning (as well as other applications)
to more fully wet the star-shaped patterns of FIG. 15.
Specifically, it may be difficult, using the aforementioned varying
density pattern to move the droplet to a complex pattern, such as
star-shaped pattern 1502, to cause the liquid to move entirely to
the tips 1507 of the pattern. However, by applying sufficient
electrowetting voltage, total wetting may be obtained.
[0060] Thus, it is possible to pattern a liquid within specific and
complex areas on a substrate such as substrate 1506. By selecting a
well-known liquid susceptible to polymerization (such as, for
example, an acrylic-based monomeric liquid including, but not
limited to, the NA72 optical adhesive manufactured by Norland,
Inc.), and applying, for example, ultraviolet light to that liquid,
a polymerized, hardened material can be achieved that conforms to
patterns 1503 and 1505. It will be obvious to one skilled in the
art that this polymerization process can be used with any of the
illustrative embodiments herein in order to move a liquid to a
desired location, cause the liquid to penetrate the nanostructured
feature pattern, and then fix the droplet in a polymerized state in
that location.
[0061] FIGS. 16A and 16B show another useful application of the
principles of the present invention. Specifically, in FIG. 16A, an
optical diffractive grating is shown wherein a droplet 1601 of
liquid which is transparent to at least some wavelengths of light
is disposed on nanostructures 1602. Nanostructures 1602 are, in
turn, disposed on surface 1603 which is, for example, a silicon
substrate, as previously described. When light beam 1604 is
incident upon droplet 1601, at least some wavelengths pass through
droplet 1601 and are reflected off of surface 1603 in such a way
that the light travels along path 1606 back through the droplet of
liquid. By passing through the liquid droplet 1601, then through
area 1605 (having dielectric constant .epsilon..sub.1), and
reflecting off of the underlying substrate1603, various frequencies
of light are filtered out (due to the difference in refractive
index between the liquid and area 1605) and only wavelength
.lambda..sub.1 emerges to propagate in the predetermined direction.
FIG. 16B demonstrates that, by causing the liquid droplet 1601 to
penetrate the nanostructures 1602 (through the use of one of the
methods described above), the dielectric constant of area 1605
changes to .epsilon..sub.2, thus changing the refractive index of
the medium through which the light travels and, therefore, only
.lambda..sub.2 will emerge to propagate in the predetermined
direction. Thus, one skilled in the art will recognize that a
tunable diffractive grating is created that, when the liquid 1601
penetrates the nanostructure feature pattern, allows a different
wavelength of light to pass through the grating, compared to when
the liquid 1601 is not penetrated into the feature pattern.
[0062] FIGS. 17A and 17B show another illustrative optical use of
the principles of the present invention, specifically as a total
internal reflection (TIR) mirror. Illustratively, referring to FIG.
17A, substrate 1701 (for example a glass substrate), which is
transparent to at least one wavelength of light, supports a feature
pattern of nanostructures 1702. Droplet 1703 is suspended on the
nanostructures 1702 as described herein above. The substrate 1701
is positioned such that, when light beam traveling in direction
1704 passes through the substrate at a particular angle of
incidence (that is a well-known angle depending on the wavelength
of light) the light beam is reflected when it encounters the
boundary of the upper surface 1705 and the gas 1706. This
reflection is achieved because the gas (e.g., air) has a dielectric
constant .epsilon..sub.1 that results in the refractive index of
the gas 1706 to be lower than that of the substrate 1701.
[0063] FIG. 17B shows a droplet 1703 that has penetrated the
nanostructure feature pattern 1702 (once again, via the methods
described above). The penetrated area of the nanostructure feature
pattern has a dielectric constant .epsilon..sub.2 that results in a
refractive index higher than that of the substrate 1701. As a
result, the light passes through the droplet 1703 and is not
reflected. One skilled in the art will recognize the specific
angles of incidence of a light beam of specific wavelengths that,
when combined with an appropriate substrate material, gas and
droplet liquid, will achieve the tunable reflective properties
described herein and shown in FIGS. 17A and 17B.
[0064] FIGS. 18A, 18B and 18C show another embodiment in accordance
with the principles of the present invention whereby a
nanostructure feature pattern is disposed on the interior wall of a
channel, such as a microfluidic tube 1802. FIG. 18A shows that, if
no nanostructures are used on the interior surface of the tube, as
in prior microfluidic tubes, the flow resistance caused by the
friction between the interior wall and the liquid leads to a
reduced velocity of the liquid close to the interior wall as it
travels through the channel. The velocity of the liquid at
different distances from the interior wall of the channel is
represented by the velocity vector profile 1801. This vector
profile 1801 shows that liquid in the center of the channel travels
fastest (longer velocity vector), and liquid immediately adjacent
to the wall travels slowest (shorter velocity vector). As a result
of this flow resistance at the inner wall larger pumps that consume
relatively high amounts of power are required to pump the liquid
through the microfluidic channel 1802. FIG. 18B shows how, by
disposing a nanostructure feature pattern 1803 on the internal wall
of the microfluidic channel 1802, the flow resistance exerted on
the liquid is advantageously reduced. This is represented by the
velocity vector profile 1804 of the liquid, which shows how the
velocity of the liquid adjacent to the walls of the channel 1802 is
approximately equal to the velocity of the liquid in the center of
the channel. Due to the lowering of the flow resistance resulting
from the use of nanostructure feature pattern 1803, the pumps
required to pump the liquid through the channel are smaller and
advantageously require relatively low power to operate.
[0065] In microfluidic applications it is often desirable to mix
different liquids traveling through two or more channels. For
example, it is useful to mix DNA and a reagent traveling in
separate channels for use in integrated microfluidic biochemical
analysis systems. In such systems, this mixing precedes the
electrophoresis process through which genetic information is
derived from the DNA sample. Prior efforts to mix multiple channels
have, disadvantageously, taken a relatively long distance to fully
accomplish this mixing. FIG. 18C shows a mixer in accordance with
the principles of the present invention. This mixer is useful if,
for example, it is desired to combine the flows of two different
liquids flowing, illustratively, in directions 1809 and 1810. As
discussed above, by disposing a nanostructure feature pattern 1803
on the internal walls of the channel, a relatively low flow
resistance is achieved through the channel. As discussed above, the
flow resistance at certain areas, such as areas 1806, along the
wall can be increased by selectively causing the liquid to
penetrate the nanostructure feature pattern by, for example,
increasing the temperature of discrete areas 1806 of the
nanostructured feature pattern or, alternatively, creating a
voltage differential between the liquid and areas 1806 of the
feature pattern. Thus, the flow 1805 through the mixer of FIG. 18C
is characterized by areas of low flow resistance, with areas of
high flow resistance to the liquid adjacent to areas 1806 of the
microfluidic channel 1802. Therefore, strongly disturbed flow, such
as that represented by flows 1808, are created. This disturbed flow
greatly enhances the mixing process and accomplishes mixing of two
or more liquids over a relatively short distance. Additionally,
since the mixer of FIG. 18 enables dynamic control of the interface
properties between the liquid and the surface, such as flow
resistance, the mixer of FIG. 18 can be actively tuned.
[0066] FIG. 19 shows another embodiment of the present invention
whereby a nanostructured feature pattern is disposed on the
internal walls of a channel, such as that advantageously used in a
heat dissipation device. Heat dissipation is a primary concern in
many applications, such as dissipating the heat generated by
electronic devices. The performance and overall life of an
electronic device is often adversely affected by excess heat.
Device 1901 is an exemplary heat-generating device, such as a
central processing unit or other processor in a computer. Channel
1902 is placed adjacent to or, preferably, in contact with device
1901. When no heat is generated by device 1901, the liquid in
channel 1902 experiences low flow resistance, as shown in FIG. 18B.
Illustratively, the liquid is chosen such that, when a
predetermined amount of heat is experienced, the surface tension of
the liquid drops and the liquid wets the surface of wall 1904. One
skilled in the art will recognize that other methods of wetting the
nanostructured surface may also be utilized, such as the
aforementioned electrowetting method illustrated in FIGS. 9A and
9B.
[0067] When device 1901 generates a sufficient amount of heat, the
heat is transferred through the channel wall 1904. The surface
tension of the liquid traveling within the channel in direction
1903 drops and, as a result, the liquid penetrates the
nanostructure feature pattern on the interior walls of the channel
in area 1905 of the nanostructure feature pattern. As such, the
liquid in area 1905 comes in direct contact with wall 1904 and more
efficiently transfers heat from the wall to the liquid flowing in
direction 1903. As will be recognized by one skilled in the art,
the disturbed flow 1906 that results from the penetration of the
liquid in area 1905 is more conducive to heat dissipation than is
an undisturbed laminar flow. While prior liquid-based heat
dissipation attempts (not relying on a nanostructured feature
pattern) were adequate for dissipating heat to a certain extent,
the embodiment of FIG. 19 is advantageously capable of dissipating
significantly more heat.
[0068] The foregoing merely illustrates the principles of the
invention. It will thus be appreciated that those skilled in the
art will be able to devise various arrangements which, although not
explicitly described or shown herein, embody the principles of the
invention and are within its spirit and scope. For example, one
skilled in the art, in light of the descriptions of the various
embodiments herein, will recognize that the principles of the
present invention may be utilized in widely disparate fields and
applications. For example, moving a droplet of liquid and causing
it to remain in a desired stationary location is useful in the
self-assembly of various devices, such as microlenses. By using the
principles disclosed herein, a microlens can be placed on a surface
and will autonomously move to a desired location at which point it
remains stationary. In another potential embodiment of the present
invention, the nanostructured or microstructured surfaces of the
present invention are used in a display device. By controlling the
movement of one or more liquids inside the display through the use
of the principles disclosed herein, different images can be
displayed.
[0069] All examples and conditional language recited herein are
intended expressly to be only for pedagogical purposes to aid the
reader in understanding the principles of the invention and are to
be construed as being without limitation to such specifically
recited examples and conditions. Moreover, all statements herein
reciting aspects and embodiments of the invention, as well as
specific examples thereof, are intended to encompass functional
equivalents thereof.
* * * * *