Layered electroosmotic structure

Abstract

A layered structure for pumping fluid by electroosmotic transport includes a first layer, wherein the first layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough, and wherein the openings in the first layer that permit the fluid to flow therethrough create a porosity of less than 10%; and a second layer, wherein the second layer is an electroosmotic layer. The layered structure has a net fluid flow direction that extends through the first layer and the second layer, wherein the layered structure has a region that permits fluid to flow in a direction that is non-parallel to a net fluid flow direction, and wherein the region is located between the first layer and the surface of the second layer that is furthest from the first layer. An electroosmodialysis apparatus may also be provided that includes two layered structures.

Claims

1. A layered structure for pumping fluid by electroosmotic transport, the layered structure comprising: a first layer, wherein the first layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough, and wherein the openings in the first layer that permit the fluid to flow therethrough create a porosity of less than 10%; and a second layer, wherein the second layer is an electroosmotic layer, wherein the layered structure has a net fluid flow direction that extends through the first layer and the second layer, wherein the layered structure has a region that permits fluid to flow in a direction that is non-parallel to a net fluid flow direction, and wherein the region is located between the first layer and the surface of the second layer that is furthest from the first layer.

2. A layered structure according to claim 1, wherein the second layer is on a downstream side of the first layer.

3. A layered structure as claimed in claim 1, wherein the openings in the first layer that permit the fluid to flow therethrough have a width dimension of 0.1 to 10 m.

4. A layered structure as claimed in claim 3, wherein the openings in the first layer that permit the fluid to flow therethrough have a width dimension of 0.5 to 2 m.

5. A layered structure as claimed in claim 1, wherein the second layer comprises the region that permits fluid to flow in a direction that is non-parallel to the net fluid flow direction.

6. A layered structure as claimed in claim 1, wherein the region that permits fluid to flow in a direction that is non-parallel to the net fluid flow direction is provided by a third layer located between the first layer and the second layer.

7. A layered structure as claimed in claim 1, wherein the region that permits fluid to flow in a direction that is non-parallel to the net fluid flow direction has a thickness that is at least 2 times the average distance between the openings of the first layer.

8. A layered structure as claimed in claim 1, wherein the electroosmotic layer has a fixed surface charge that is the same sign as the sign of the ions to which the material of the first layer is perm selective.

9. A layered structure as claimed in claim 1, wherein the first layer is provided on only one side of the second layer.

10. A layered structure as claimed in claim 1, wherein the layered structure is arranged so that, when electric current is passed through the layered structure in a first direction, the electrolyte concentration in the second layer will be decreased and, when electric current is passed through the layered structure in a second direction opposite to the first direction, the electrolyte concentration in the second layer will be increased.

11. A layered structure as claimed in claim 10, wherein the first electric current direction corresponds to the movement of ions, for which the material of the first layer is perm selective away from the second layer and towards the first layer.

12. A layered structure as claimed in claim 10, wherein when a given amount of charge flows in the first direction more electroosmotic transport of the fluid will occur through the layered structure than when the same amount of charge flows in the second direction.

13. A method of pumping a fluid through a layered structure by electroosmotic transport, the method comprising: providing a layered structure comprising: a first layer, wherein the first layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough, and wherein the openings in the first layer that permit the fluid to flow therethrough create a porosity of less than 10%; and a second layer, wherein the second layer is an electroosmotic layer; wherein the layered structure has a net fluid flow direction that extends through the first layer and the second layer, wherein the layered structure has a region that permits fluid to flow in a direction that is non-parallel to a net fluid flow direction, and wherein the region is located between the first layer and the surface of the second layer that is furthest from the first layer; and applying an AC signal across the layered structure, wherein when a given amount of charge flows through the layered structure in a first direction more electroosmotic transport of the fluid occurs than when the same amount of charge flows through the layered structure in a second, opposite direction.

14. An electroosmodialysis apparatus, the apparatus comprising: a first layered structure comprising a first layer, wherein the first layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough; and a second layer, wherein the second layer is an electroosmotic layer; and a second layered structure comprising a third layer, wherein the third layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough; and a fourth layer, wherein the fourth layer is an electroosmotic layer; and a flow path between the first and second layered structures into which fluid is transported when a current is passed through the first and second layered structures.

15. An electroosmodialysis apparatus as claimed in claim 14, wherein the apparatus is arranged so that when a current is passed through the first and second layered structure fluid transported into the flow path is of a lower electrolyte concentration than fluid that has not been transported into the flow path.

16. An electroosmodialysis apparatus as claimed in claim 14, wherein the ion perm selective material of the first layer of the first layered structure and the ion perm selective material of the third layer of the second layered structure are of opposite polarity.

17. An electroosmodialysis apparatus as claimed in claim 14, wherein the second layer has a surface charge of the same sign as the charge to which the first layer is perm selective and the fourth layer has a surface charge of the same sign as the charge to which the third layer is perm selective.

18. A method of performing electroosmodialysis, the method comprising: providing an electroosmodialysis apparatus according to claim 14; and passing a current through the first and second layered structures to cause electroosmodialysis.

19. A layered structure for pumping fluid by electroosmotic transport, the layered structure comprising: a first layer, wherein the first layer is made from an ion perm selective material having openings therein that permit the fluid to flow therethrough; and a second layer, wherein the second layer is an electroosmotic layer, wherein the layered structure has a net fluid flow direction that extends through the first layer and the second layer, wherein the layered structure has a region that permits fluid to flow in a direction that is non-parallel to a net fluid flow direction, and wherein the region is located between the first layer and the surface of the second layer that is furthest from the first layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic of an exemplary layered structure;

(3) FIG. 2 is a schematic of another exemplary layered structure;

(4) FIG. 3 is a schematic to help illustrate how the layered structure works; and

(5) FIG. 4 is a schematic of an electroosmodialysis device.

DETAILED DESCRIPTION

(6) FIG. 1 shows an electroosmotic layered structure 1. The structure 1 comprises a first layer that is an ion perm selective material layer 2 and a second layer that is an electroosmotic layer 4.

(7) The ion perm selective material layer 2 is a layer made from an ion perm selective material. The perm selective material is a material that has significantly higher conductivity and diffusivity of ions of one sign. For example, the conductivity and diffusivity of ions of one sign is at least two times, or at least 10 times the conductivity and diffusivity of ions of the opposite sign.

(8) The ion perm selective material has microscopic pores 6 therethrough that permit fluid to flow through the layered structure. The fact that the pores 6 are microscopic pores may mean that they have a width (dimension in a direction parallel to the surface of the structure 1 and perpendicular to the direction in which fluid flows through the structure 1) between 0.1 and 10 microns.

(9) The electroosmotic layer 4 is a layer that has a fixed surface charge and will cause transport of liquid by electroosmotic flow when a voltage is applied across the layer 4.

(10) When a voltage is applied across the layered structure 1 fluid will flow in a net direction from the ion perm selective material layer 2 to the electroosmotic layer 4 as illustrated by arrow 8.

(11) The electroosmotic layer 2 may have a structure that permits fluid to flow in a direction that is parallel to the net fluid flow direction 8 and to flow in a direction (i.e. a lateral direction) that is non-parallel to the net fluid flow direction 8.

(12) As shown in FIG. 2, the layered structure 1 may comprise a third layer, a mixing layer 10 between the ion perm selective material layer 2 and an electroosmotic layer 4. The mixing layer 10 may be a porous structure that permits fluid to flow in a lateral direction (i.e. a direction that is non-parallel to the net fluid flow direction 8). This may be particularly beneficial when the electroosmotic layer 4 only has pores that permit fluid flow in a direction substantially parallel to the net fluid flow direction.

(13) The ion perm selective material layer 2 and the electroosmotic layer 4 may have opposite fixed charges. The fixed surface charge of the electroosmotic membrane 4 may be the same sign as the sign of the ions for which the ion perm selective material is perm selective. For example, the electroosmotic membrane 4 may have a negative fixed surface charge and the material of the ion perm selective material layer 2 may have higher conductivity and diffusivity of anions or the electroosmotic membrane 4 may have a positive fixed surface charge and the material of the ion perm selective material layer 2 may have higher conductivity and diffusivity of cations.

(14) The structure 1, 1 may be provided on either side with an electrode that permits a voltage to be applied across the structure 1, 1, i.e. structure 1, 1 may be located between two electrodes.

(15) When a voltage of a first polarity is applied across the layered structure 1, 1, a current flows through the structure 1 in a first direction. A given current flow direction (the direction being dependent on the sign of the ion to which the ion perm selective material layer 2 is permselective) induces a concentration polarisation at the surface of the ion permselective material layer 2 that extends into the electroosmotic layer 4.

(16) This concentration polarisation reduces the electrolyte concentration in the electroosmotic layer 4 and that increases the rate of electroosmotic transport.

(17) When a voltage of a second opposite polarity is applied across the layered structure 1, 1, a current flows through the structure 1 in a second opposite direction. This increases the electrolyte concentration in the electroosmotic layer 4 and that decreases the rate of electroosmotic transport. This allows the structure 1, 1 to have a net electroosmotic transport through the structure 1, 1 even when there is no net charge flow (i.e. over a period of time the same amount of charge will flow in each opposite direction) such as when an AC signal is used.

(18) FIG. 3 illustrates schematically the effect of the two structures together.

(19) This shows the ion perm selective material layer 2 with openings 6 therein that permit bulk fluid flow through. The electroosmotic layer 4 has openings in the structure that allow fluid to flow in a direction that is non-parallel to the net fluid flow direction 8. As illustrated by multi headed arrow 12, fluid can flow through the ion perm selective material layer 4 and into the electroosmotic layer 4 where it can flow in directions non-parallel to the net fluid flow direction so as to permit mixing of the fluid flowing through the openings 6 with the fluid at or near the surface of the ion perm selective material layer 2 where there are no openings.

(20) As shown in the upper most pore of the electroosmotic layer 4, there are negative surface charges of the pores of the electroosmotic layer 4. This results in movement of positive movable ions (illustrated schematically at 14) near the surface of the electroosmotic layer pores to compensate the negative charge (the fixed surface charge and the movable ions together form an electric double layer). The positive ions move in response to the electric field imposed by electrodes (not shown) and results in electroosmosis.

(21) As shown schematically at arrow 16, the positive ions are blocked by the perm selective material whilst, as shown at arrow 18, the negative ions can pass through the perm selective material. As shown at 20, ions of both sign can pass through the openings in the perm selective material.

(22) The perm selectivity of layer 2 and the restrictions to mixing with bulk liquid (as the lateral pores do not allow complete mixing) caused by the electroosmotic membrane 4 results in the formation of a concentration polarization zone 19. This zone 19 extends into the thickness of the electroosmotic layer 4. The triangle shape of this zone 19 is purely schematic and just indicates the direction in which the zone will grow (and not the real shape of the zone). Depending on the electroosmotic layer 4, the depleted zone 19 might reach part of or the whole of its thickness at steady conditions.

(23) The concentration polarisation 19 reduces the electrolyte concentration in the electroosmotic layer 4. This results in an increased rate of electroosmosis.

(24) When a voltage of opposite polarity is applied across the structure 1, 1, a charge will flow in an opposite direction. This will increase the electrolyte concentration in the electroosmotic layer 4. This results in a decreased rate of electroosmosis. Thus, the structure 1, 1 may have an asymmetric (and therefore net) electroosmotic flow.

(25) The layered structure may also be used to provide an electroosmodialysis device 100 as shown in FIG. 4.

(26) The electoosmodialysis device 100 may comprise a first layered structure 102 and a second layered structure 104. The first and second layered structures 102 and 104 may be layered structures as described above. The first and second layered structures 102, 104 are separated by a volume 106.

(27) The first layered structure 102 comprises a first ion perm selective material layer 108 with openings 110 therein and a first electroosmotic layer 112.

(28) The second layered structure 104 comprises a second ion perm selective material layer 114 with openings 116 therein and a second electroosmotic layer 118.

(29) The sign of the ions to which the first ion perm selective material layer 108 is perm selective is opposite to the sign of the ions to which the second ion perm selective material layer 114 is perm selective. Accordingly (given that the fixed surface charges of the electroosmotic layer is the same sign as the sign of the ions to which its respective ion perm selective material layer is perm selective) the fixed surface charges of the electroosmotic layers 112, 118 are also opposite to each other.

(30) The fixed surface charge of the electroosmotic layer is the opposite sign as the fixed charge of the respective ion perm selective material.

(31) The two layered structures 102, 104 may therefore be regarded as a mirror image of each other.

(32) When a voltage of a certain polarity is applied across the device 100 such that a current flows across the device, fluid will be transported by means of electroosmosis from the liquid surrounding the device 100 through one of the layered structures 102, 104, into the volume 106 between the layered structures 102, 104. A solution of reduced salinity will be pumped electroosmotically into the volume 106 thereby performing electrodialysis.