U.S. patent application number 11/168779 was filed with the patent office on 2007-01-11 for controlling electrolytically generated gas bubbles in in-plane electroosmotic pumps.
Invention is credited to Alan M. Myers, Jonathan D. Posner, Juan Santiago, Shuhuai Yao.
Application Number | 20070009366 11/168779 |
Document ID | / |
Family ID | 37618464 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070009366 |
Kind Code |
A1 |
Myers; Alan M. ; et
al. |
January 11, 2007 |
Controlling electrolytically generated gas bubbles in in-plane
electroosmotic pumps
Abstract
An "in-plane" electroosmotic pump may reduce deterioration of
performance due to electrolytic gas generation. By controlling the
flow of gas generated at the electrodes, while allowing ionic
current, the gas may be prevented from fouling the narrow slots
which act as pumping channels.
Inventors: |
Myers; Alan M.; (Menlo Park,
CA) ; Santiago; Juan; (Fremont, CA) ; Yao;
Shuhuai; (Stanford, CA) ; Posner; Jonathan D.;
(Menlo Park, CA) |
Correspondence
Address: |
TROP PRUNER & HU, PC
1616 S. VOSS ROAD, SUITE 750
HOUSTON
TX
77057-2631
US
|
Family ID: |
37618464 |
Appl. No.: |
11/168779 |
Filed: |
June 28, 2005 |
Current U.S.
Class: |
417/48 |
Current CPC
Class: |
F04B 37/02 20130101 |
Class at
Publication: |
417/048 |
International
Class: |
F04B 37/02 20060101
F04B037/02 |
Claims
1. A method comprising: controlling gases formed by an electrode of
an electroosmotic pump while allowing the passage of liquid.
2. The method of claim 1 including surrounding the electrode with a
membrane which passes ions but controls gases.
3. The method of claim 2 including surrounding said electrode with
a proton exchange membrane.
4. The method of claim 3 including providing spacers between a
tubular membrane and said electrode.
5. The method of claim 2 including providing a port to eject gases
from within said membrane.
6. The method of claim 5 including providing a membrane over said
port, which membrane passes gas but prevents liquid from
passing.
7. The method of claim 6 including providing a membrane including
Gortex.RTM. fabric.
8. The method of claim 1 including passing a liquid through a
series of slots in said pump.
9. The method of claim 8 including forming a pair of reservoirs on
either side of the series of slots in a semiconductor die.
10. The method of claim 9 including forming a sidewall around said
reservoirs and forming a slot at the top of said sidewall to
receive an electrode.
11. An electroosmotic pump comprising: a pair of electrodes; a
plurality of liquid passages between said electrodes; and a sheath
around at least one of said electrodes which sheath passes ions and
reduces the passage of gas.
12. The pump of claim 11 wherein said sheath is a proton exchange
membrane.
13. The pump of claim 11 including a semiconductor die with
reservoirs formed therein, and passages formed between said
reservoirs, said electrode positioned in one of said reservoir.
14. The pump of claim 13 including slots in said die to receive
said electrode.
15. The pump of claim 12, said sheath including a doubled tube of
proton exchange membrane material surrounding said electrode.
16. The pump of claim 11 including spacers to space said sheath
from said electrode.
17. The pump of claim 11 wherein said sheath is tubular and has a
material which allows gas from within the sheath to pass outwardly
of said pump while preventing liquid from within said sheath from
passing through said material.
18. The pump of claim 17 wherein said material includes Gortex.RTM.
brand fabric.
19. The pump of claim 17 wherein said material is positioned over
said sheath on the outside of said pump.
20. The pump of claim 19 wherein said electrode passes through said
material.
21. The pump of claim 19 wherein the passage of said electrode
through said material is sealed with adhesive.
22. A system comprising: an integrated circuit; a cooler associated
with said circuit; and an electroosmotic pump coupled to said
cooler, said pump including an electrode covered by a sheath to
pass ions and to control gas.
23. The system of claim 22 including a heat exchanger coupled to
said pump.
24. The system of claim 22 wherein said cooler, said integrated
circuit, and said pump are formed in one integrated circuit
package.
25. The system of claim 24 wherein said sheath includes a proton
exchange membrane.
26. The system of claim 25 wherein said sheath is a Nafion.RTM.
tube.
27. The system of claim 22 including a semiconductor die with
reservoirs formed therein, and slots formed between said
reservoirs, and electrodes in said reservoirs.
28. The system of claim 22 including spacers to space said sheath
from said electrode.
29. The system of claim 22 wherein said sheath is tubular and has a
material which allows gas from within the sheath to pass outwardly
of said pump while preventing liquid from within said sheath from
passing through said material.
30. The system of claim 29 wherein said material includes
Gortex.RTM. fabric.
Description
BACKGROUND
[0001] This invention relates generally to electroosmotic pumps and
in particular to "in-plane" electroosmotic pumps. These are pumps
where fluid flow is induced in multiple slots formed in a planar
structure.
[0002] Existing in-plane electroosmotic pumps that produce
relatively high flow rates are prone to formation of gas bubbles.
These bubbles result from electrolytic decomposition of the pumping
fluid at the pump electrodes. As an example, if the pumping liquid
is water, hydrogen gas is produced at the cathode and oxygen gas is
produced at the anode. These bubbles displace the fluid in the
pumping channels of in-plane electroosmotic pumps, reducing pumping
performance after a short period of time. Bubbles can also lead to
poor electrochemical coupling.
[0003] Ultimately, the effectiveness of high flow rate in-plane
electroosmotic pumps is severely limited by the presence of the
bubbles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is an enlarged, horizontal, cross-sectional view of
one embodiment of the present invention;
[0005] FIG. 2 is an enlarged, partial cross-sectional view taken
generally along the line 2-2 in accordance with one embodiment of
the present invention;
[0006] FIG. 3 is a cross-sectional view taken generally along the
line 3-3 in FIG. 1 in accordance with one embodiment of the present
invention;
[0007] FIG. 4 is a schematic depiction of one of the electrodes
shown in FIG. 1 in accordance with one embodiment of the present
invention;
[0008] FIG. 5 is a schematic depiction of the other electrodes in
accordance with one embodiment of the present invention; and
[0009] FIG. 6 is a schematic depiction of a system in accordance
with one embodiment of the present invention.
DETAILED DESCRIPTION
[0010] Referring to FIG. 1, an electroosmotic pump 10 may be
fabricated in silicon or other semiconductor material, in one
embodiment, using semiconductor fabrication techniques. The pump 10
is capable of pumping a fluid, such as a cooling liquid, through a
row of slots 20 formed in a semiconductor. In one embodiment, the
row of slots 20 may be formed by either wet or dry etching
techniques. A pair of opposed electrodes 32 may generate an
electrical field that results in the transport of a liquid through
the row of slots 20. In one embodiment, the electrodes are
fabricated from platinum. All areas of the pumping surface may be
coated in an insulating material to prevent current leakage through
the conducting semiconductor substrate. In one embodiment, the
insulating material may be silicon dioxide, silicon nitride or
multiple layers of these materials.
[0011] This process by which fluid pumping occurs is known as the
electroosmotic effect. In such a case, hydrogen from the hydroxyl
groups on the walls of the slots 20 deprotonate, resulting in an
excess of protons near the wall surface. The excess hydrogen ions
move in response to the electric field applied between the
electrodes 32 in the direction of the arrows A (from anode to
cathode). The non-charged water atoms also move in response to the
applied electric field because of the drag forces that exist
between the ions and the water atoms.
[0012] As a result, a pumping effect may be achieved without any
moving parts in some embodiments. In addition, the structure may be
fabricated in silicon at extremely small sizes, making such devices
applicable as pumps for cooling integrated circuits and many other
applications.
[0013] Referring to FIG. 2, which shows a cross-section of the row
of slots 20 in FIG. 1, the row of slots 20 may be composed of a
series of vertical walls 54 separated by trenches 56 which define a
plurality of parallel channels for fluid and charge flow between
the electrodes 32. The electrode 32a may act as the anode 28a and
the electrode 32b may act as the cathode 28b.
[0014] Also provided in the liquid W may be a buffer which adjusts
the pH of the liquid. In one embodiment, sodium borate may be used
as a buffer to improve the zeta potential which is a measure of the
excess ion charge near a solid surface in the fluid. For example,
0.5 mM of sodium borate buffer may be utilized in water.
[0015] Relatively high flow rates may be achieved in some
embodiments. However, eventually, the flow rates diminish in
conventional embodiments because of the displacement of the fluid
by gas in the narrow channels by bubbles produced at each of the
electrodes 32.
[0016] Thus, referring to FIG. 5, at the anode 28b, oxygen gas is
generated by the electrode 32b. At the cathode 28a, shown in FIG.
4, hydrogen gas is generated. These gases could eventually fill the
surrounding area and, ultimately, displace the fluid in the
trenches 56 in the row of slots 20.
[0017] Referring to FIG. 1, in order to contain the bubbles and to
prevent them from being entrained within the row of slots 20, a
closed, tubular sheath 30 may be provided around each electrode 32
to form the anode 28b and cathode 28a. The sheath 30 may be made of
a material that passes liquid and ions or charge, but blocks
bubbles and gas. Instead, the collected gas inside the sheath 30
passes outwardly of the pump 10 through an appropriate material 34.
That is, when the gas pressure builds up inside the sheath 30, the
gas passes outwardly through the material 34. Thus, not only is the
gas prevented from fouling the row of slots 20, but excess gas is
discarded from the system.
[0018] While many proton exchange membranes may be used for the
sheath 30, in some embodiments, the sheath 30 may be a Nafion brand
material made by E.I. DuPont de Nemours & Co. of Wilmington,
Del. The specific form of Nafion.RTM. material used in some
embodiments is a tube which may be obtained from Perma Pure LLC of
Toms River, N.J. 08754.
[0019] Nafion.RTM. material is a copolymer of
perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and
tetrafluoro-ethylene. Thus, Nafion.RTM. material has a Teflon.RTM.
backbone with side chains of another fluorocarbon. Those side
chains may terminate in a sulfonic acid. The Nafion.RTM. material
may function as an ion exchange resin. Each sulfonic acid group may
absorb up to thirteen molecules of water. The sulfonic acid groups
create, effectively, ionic channels through the polymer so that
water is very readily transported through the channels, while gas
is not.
[0020] In some embodiments, a doubled tube of Nafion.RTM. material
may be utilized as the sheath 30 to better contain the gas. In
addition, spacers 40 may be provided between the electrodes 32 and
the sheaths 30 to prevent gas outflow. It has been found by the
present inventors that if the electrodes 32 contact the sheaths 30,
gas may escape. Thus, spacers 40 may be provided along the length
of each electrode 32 to space the sheath 30 away from the electrode
32. In one embodiment, the spacers 40 may be formed of globules of
epoxy adhesive attached to the electrode surface.
[0021] The material 34, which allows the gas to flow outwardly of
the pump 10, may be Gortex.RTM. brand fabric. The material 34
prevents loss of pumping liquid while allowing gas to escape
outwardly from the electroosmotic pump 10. In one embodiment, the
electrodes 32 may simply pass through the material 34. In another
embodiment, a Nafion.RTM. tube may be connected to a manifold block
that contains the material 34.
[0022] In some embodiments, relatively high flow rates (such as
high as 10 milliliters per minute per square centimeter of planar
pumping structure) with high pressures (such as 0.5 pounds per
square inch) may be obtained. These flow rates may be continuous in
that they are not prone to substantial bubble fouling in some
embodiments.
[0023] The row of slots 20 may be patterned and etched to form an
individual pump semiconductor die 12. The wafer 12 may consist of
walls 59 (FIG. 1), liquid filled reservoirs 62 (FIG. 1), and a row
of trenches 56 (FIG. 2) which act as the pumping medium. The pump
wall 59 may be between 0.1 and 1 millimeter thick in one
embodiment. The trenches 56 may have a depth between 10 and 300
microns, in some embodiments, with a width between 1 and 10 microns
and a length between 5 and 100 microns.
[0024] The pump walls 54 (FIG. 2) between trenches 56 may have a
width between 5 and 100 microns in one embodiment. A slot 60 (FIG.
1) may also be etched in the wall 12 of each liquid reservoir 62 at
the same time the trenches 56 are formed. A sheathed electrode 32
is then inserted into each slot 60 in a post-silicon processing
step. The slot 60 width may be on the order of 0.1 to 1 millimeter
in one embodiment.
[0025] The pump die 12 may be coated with an insulating liner
material 58 (FIG. 2) prior to electrode 32 insertion. The liner
material 58 insulates the walls 54 to prevent current leakage into
the conductive silicon and achieves an appropriate final slot 56
width (by reducing the slot width). Appropriate materials for
covering silicon walls 54 include, without limitation, oxide
produced by thermal oxidation, low pressure chemical vapor
deposition silicon nitride, low pressure chemical vapor deposition
polysilicon silicon that is then oxidized.
[0026] Referring to FIG. 3, the die 12 may be covered by a cover
plate 26. The cover plate 26 may have holes 36 formed in it to
communicate with tubing connectors 22. Liquid may be drawn into the
pump 10 and expelled from the pump 10 through the connectors 22.
The cover plate 26 may, for example, be formed of glass or silicon,
to mention a few examples. For example, and without limitation,
bonding techniques, such as anodic bonding, may be used for glass,
metallic bonding for silicon, or direct bonding for silicon may be
used to bond the cover plate 26 to the die 12. Typically, the
tubing connectors 22 may be secured by adhesive 24, such as
epoxy.
[0027] The electrodes 32 may be formed of platinum and are inserted
within the sheaths 30. The sheathed electrodes 32 are then inserted
into the electrode slots 60 (FIG. 1) that span the length of a
trench row and run parallel to the length of the slots 20. Close
proximity of the electrodes 32 to the row of slots 20 improves pump
performance, as this reduces resistive losses. That is, the
electrode-to-trench row potential drop is reduced. However, if the
sheath 30 is positioned too close to the trench row of slots 20 or
if the sheath 30 tube diameter is almost the same dimension as the
reservoir 62 height, fluid flow may be restricted.
[0028] The material 34 may be a polytetrafluoroethylene or
Gortex.RTM. brand membrane that allows the gas from inside the tube
to escape while trapping water inside the sheath 30. The sheath 30
traps electrolytic gases inside the sheath. Electrolytic gas
generated within the sheaths 30 may not enter the pump reservoirs
62, thereby interfering with the electroosmotic pumping action.
Outside the pump 10, the electrode 32 passes through either the
membrane 34 at the end of the sheath 30 or through the sheath 30
tube itself or into a manifold. An electrode via may be sealed in
place with an adhesive, such as epoxy, in some embodiments.
[0029] Thus, referring to FIG. 6, the electroosmotic pump 10 may be
supplied with a potential as indicated. In some embodiments, the
liquid may be pumped to a mechanical cooler 48 that cools a
semiconductor integrated circuit 50. The flow then proceeds through
channels 44 to a radiator 46 which removes excess heat in some
embodiments.
[0030] In some cases, a semiconductor package 52 may be formed with
the pump 10, cooler 48, integrated circuit 50 to be cooled. The
integrated circuit may be a microprocessor, for example. Then, the
radiator 46 may be secured by conventional techniques to the
package 52. However, the present invention need not be limited to
semiconductor cooling embodiments.
[0031] References throughout this specification to "one embodiment"
or "an embodiment" mean that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one implementation encompassed within the
present invention. Thus, appearances of the phrase "one embodiment"
or "in an embodiment" are not necessarily referring to the same
embodiment. Furthermore, the particular features, structures, or
characteristics may be instituted in other suitable forms other
than the particular embodiment illustrated and all such forms may
be encompassed within the claims of the present application.
[0032] While the present invention has been described with respect
to a limited number of embodiments, those skilled in the art will
appreciate numerous modifications and variations therefrom. It is
intended that the appended claims cover all such modifications and
variations as fall within the true spirit and scope of this present
invention.
* * * * *