| Controlling electrolytically generated gas bubbles in in-plane electroosmotic pumps -> Monitor Keywords |
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Controlling electrolytically generated gas bubbles in in-plane electroosmotic pumpsRelated Patent Categories: Pumps, Electrical Or Getter TypeControlling electrolytically generated gas bubbles in in-plane electroosmotic pumps description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070009366, Controlling electrolytically generated gas bubbles in in-plane electroosmotic pumps. Brief Patent Description - Full Patent Description - Patent Application Claims
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. 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