ELECTROWETTING DISPLAY

Abstract

An electrowetting optical element for enabling powering of a first and a second electrode layer for rearranging a polar liquid relative to a non-polar liquid on a hydrophobic surface of an insulating layer. The electrowetting optical element includes a first electrode layer stack, a second electrode layer stack, one or more cell walls extending between the first and second electrode stacks, and a formed containment space containing a polar liquid and a non-polar liquid wherein the liquids are immiscible with each other. Each of the one or more cell walls are mounted on a second interface surface of the second electrode layer stack and extend towards the first electrode layer.

Claims

1-17. (canceled)

18. An electrowetting optical element configured for enabling powering of a first electrode layer and a second electrode layer for rearranging a polar liquid relative to a non-polar liquid on a hydrophobic first interface surface of an insulating layer, the electrowetting optical element comprising: a first electrode layer stack comprising a substrate, the first electrode layer, and the insulating layer having the hydrophobic first interface surface; a second electrode layer stack comprising a superstrate, and the second electrode layer having a hydrophobic second interface surface having a lower hydrophobicity than the hydrophobic first interface surface; one or more cell walls extending between the first electrode stack and the second electrode stack; and a containment space formed between the hydrophobic first interface surface of the first electrode layer stack and the hydrophobic second interface surface of the second electrode layer stack, and the one or more cell walls defining sides of the containment space, and the containment space containing at least a polar liquid and a non-polar liquid, wherein the polar liquid and the non-polar liquid are immiscible with each other; wherein each of the one or more cell walls are fixedly mounted on the hydrophobic second interface surface of the second electrode layer stack and extend towards the first electrode layer; and wherein an end face of each of the one or more cell walls, opposite and facing the first electrode layer stack, comprises a hydrophilic surface.

19. The electrowetting optical element according to claim 18, wherein the one or more cell walls are fixedly mounted on the hydrophobic second interface surface and extend with a free end thereof towards the first electrode layer, and wherein the one or more cell walls have a height that at least extends beyond a maximum distance between the hydrophobic second interface surface and the interface between the polar and the non-polar liquid in a disabled powering modus of the first electrode layer and the second electrode layer.

20. The electrowetting optical element according to claim 18, wherein a height of the one or more cell walls extending from the hydrophobic second interface surface corresponds with a distance between the hydrophobic first and second interface surfaces.

21. The electrowetting optical element according to claim 20, wherein the end face of each of the one or more cell walls forming a top surface of a cell wall loosely contacts the hydrophobic first interface surface.

22. The electrowetting optical element according to claim 18, wherein the end face of each of the one or more cell walls forming a top surface of a cell wall contacts the hydrophobic first interface surface thereby forming a sealing contact between the top surface of the cell wall and the hydrophobic first interface surface for sealing the polar and the non-polar liquids within the containment space.

23. The electrowetting optical element according to claim 18, wherein the one or more cell walls comprise a compressible compound or an expandable compound in a top part of the one or more cell walls.

24. The electrowetting optical element according to claim 23, wherein the compressible compound or the expandable compound comprises a porous structure.

25. The electrowetting optical element according to claim 24, wherein the porous structure is configured for imbibition with the polar liquid.

26. The electrowetting optical element according to claim 24, wherein the porous structure comprises pores configured to absorb liquid during an immersing process.

27. The electrowetting optical element according to claim 26, wherein the liquid is the polar liquid.

28. The electrowetting optical element according to claim 26, wherein the immersing process is performed at a predetermined elevated temperature for a predetermined time period.

29. The electrowetting optical element according to claim 18, wherein side walls of each of the one or more cell walls comprise a hydrophilic surface.

30. The electrowetting optical element according to claim 18, wherein each of side walls and an end face of each of the one or more cell walls comprise a hydrophilic surface.

31. The electrowetting optical element according to claim 30, wherein the hydrophilic surface of the side walls and the end face are formed from a continuous layer.

32. A method of manufacturing an electrowetting optical element configured for enabling powering of a first electrode layer and a second electrode layer for rearranging a polar liquid relative to a non-polar liquid, the method comprising the steps of: providing a first electrode layer stack comprising a substrate, the first electrode layer, and an insulating layer having a hydrophobic first interface surface; providing a second electrode layer stack comprising a superstrate and the said second electrode layer having a hydrophobic second interface surface having a lesser hydrophobicity than the hydrophobic first interface surface; fixedly mounting cell walls on the hydrophobic second interface surface of the second electrode layer stack thereby forming a containment space defined by the hydrophobic second interface surface and the cell walls, wherein an end face of each of the cell walls facing the first electrode layer stack comprises a hydrophilic surface; filling the containment space with a polar liquid and a non-polar liquid, wherein the polar liquid and the non-polar liquid are immiscible with each other; and covering the containment space with the first electrode layer stack such that the hydrophobic first interface surface faces the hydrophobic second interface surface.

33. The method according to claim 32, wherein: the step of mounting the cell walls comprises fixedly mounting the cell walls with a height of the cell walls extending with a free end towards the first electrode layer, wherein the cell walls have a height that at least extends beyond a maximum distance between the hydrophobic second interface surface and an interface between the polar liquid and the non-polar liquid in a disabled powering modus of the first electrode layer and the second electrode layer; and the step of covering the containment space comprises covering the containment space with the first electrode layer stack such that the end face of each of the cell walls contacts the hydrophobic first interface surface thereby forming a sealing contact between the top surface of the cell walls and the hydrophobic first interface surface for sealing the polar liquid and the non-polar liquid within the containment space.

Description

[0056] The invention will further be described with reference to the enclosed drawings wherein embodiments of the invention are illustrated, and wherein:

[0057] FIG. 1 shows in an illustrative manner, an electrowetting element according to the prior-art;

[0058] FIG. 2 shows in an illustrative manner, an unreliable effect of an electrowetting element according to the prior-art;

[0059] FIG. 3 shows in an illustrative manner, another unreliable effect of an electrowetting element according to the prior-art;

[0060] FIG. 4 shows in an illustrative manner, an electrowetting element according to a first aspect of the invention.

[0061] FIG. 1 shows an electrowetting optical element or further referred to as electrowetting element, according to the state of the art. The element consists of two electrode layer stacks, one at the bottom and one at the top. The stack at the bottom, as seen from a viewing path, being the first electrode layer stack, and consists of respectively a substrate 12, a first electrode layer 13, an electrically insulating hydrophobic layer 14 or an insulating layer 14 having a hydrophobic surface 15. The hydrophobic surface 15 interfaces with at least a non-polar liquid 20 capable of displacing a polar liquid 21 from the hydrophobic surface 15 through its preferential wetting of the hydrophobic surface 15. These liquids are immiscible with each other and contained in a containment space. The containment space defines one cell and is formed by the first stack at the bottom, the second stack at the top, and a set of cell walls 16 which are disposed in parallel at a certain distance from each other.

[0062] The second electrode layer 11 of the superstrate 10 and the first electrode layer 13 of the substrate 12 could be formed to only apply a voltage across one single cell, but are preferably, as also shown in FIG. 1, continued for multiple cells. In this way, multiple cells together may form one single pixel element which is independently addressable.

[0063] The element in FIG. 1 further comprises a second electrode layer stack which consists of a second electrode layer 11 and a superstrate 10. The second electrode layer 11 interfaces with at least a polar liquid 21 in the containment space. As indicated, seen from a viewing path, the first electrode stack, at the bottom of the element can be denoted as the first stack comprising the first electrode layer. The second electrode stack at the top of the element can be denoted as the second stack and comprising the second electrode layer. The containment space comprising a polar liquid and a non-polar liquid separates the first stack from the second stack.

[0064] In order to define the cells and to keep the liquids inside their cells in the containment space, the element contains cell walls 16. As known, these cell walls are fixedly mounted at one end 17 to the second interface surface 11 of the second electrode layer stack. The opposite end 19 of the cell walls is not fixedly mounted at that end to the first interface surface 15 of the first electrode layer stack, but rather has a free and thus non-fixed end 19. This free end comprises a hydrophobic top 19. The non-polar liquid 20 is attracted to both the hydrophobic end face surface 19 of the cell wall 16 and to the hydrophobic first interface surface 15 in both the non-powered and the powered-up state of the electrowetting cell. This enables the non-polar liquid 20 to be entrained more easily into the slit between the end face 19 and the first interface surface 15. As such, the polar liquid 21 is trapped inside the containment space and is prevented from spreading from one cell to another. In order to obtain such a hydrophobic surface on the top or free end 19 of the cell walls 16, both a coating step with a hydrophobic material and a subsequent anneal step is required.

[0065] With this anneal step, the hydrophilic properties of the sides 18 of the cell walls 16 are reduced. As a result, the non-polar liquid 20 may stick to the sides 18 of the cell walls 16 which prevents backflow or reduces the speed of backflow of the non-polar liquid 20 from the sides 18 of the cell walls 16 to and across the first interface surface 15 when the electrowetting element is switched from a powered-up modus back to a disabled powering modus of the first and second electrode layer, thereby inducing an unreliable electrowetting effect. This effect is clearly shown in FIG. 2 in which the element is in a disabled powering modus and wherein thus no voltage is applied between the first and second electrodes 11, 13. The non-polar liquid 20 should preferably be distributed evenly as a lens-shaped liquid film across the first interface surface 15 in the cell as illustrated in the non-powered modus of the electrowetting element shown in FIG. 1 in order to be most effective as an optically absorbing liquid film when the non-polar liquid 20 is optically absorbing across at least part of the visible wavelength region. In FIG. 2 however, it can be seen, that the non-polar liquid 20 of the prior-art element also shown in FIG. 1 climbs up a side of the cell wall 16 because of the cell wall's annealing-induced increased hydrophobicity which promotes its wetting with the non-polar liquid 20. The resulting uneven distribution of the non-polar liquid 20 across the cell in FIG. 2 induces an unreliable and non-reproducible electrowetting effect. Because part of the non-polar liquid 20 in the cell has climbed the side 18 of the cell wall 16, the resulting non-uniform coverage of the hydrophobic interface surface 15 with optically-absorbing non-polar liquid 20 leads to an undesired increased optical transmission of incident light through the cell.

[0066] In FIG. 3, another more extreme example is shown of an unreliable electrowetting effect in the disabling power modus. Here, most of the non-polar liquid 20 adheres to the side of the cell wall 16, thereby effectively depleting the hydrophobic interface surface 15 from most of the non-polar liquid 20. This leads to an undesirable almost unhindered transmission of incident light through the cell when the non-polar liquid 20 is optically absorbing across at least part of the visible wavelength region.

[0067] In FIG. 4 an electrowetting optical element according to an aspect of the disclosure is shown. It is arranged for enabling powering of a first 13 and a second electrode layer 11 for rearranging a polar liquid 21 relative to a non-polar liquid 20 on a hydrophobic surface 15 of an insulating layer 14. The electrowetting optical element comprises a first electrode layer stack with a substrate 12, the first electrode layer 13, and the insulating layer 14 having said hydrophobic first interface surface 15. The substrate 12 may be made from glass or another substrate material which is transparent across the visible light spectrum. The interface surface or interface layer thus covers the electrode layer 13, which is transparent. The interface layer 15 or at least its surface is preferably a made from a fluoropolymer. To improve the adhesion of the fluoropolymer 15 to the insulator layer 14, the substrate 12 may preferably also be provided with an adhesion promoting layer which is disposed between the fluoropolymer 15 and the insulator layer 14.

[0068] The electrowetting optical element further comprises a second electrode layer stack comprising a superstrate 10, and the second electrode layer 11 which has a second interface surface having a lower hydrophobicity then the first interface surface. Similar to the first substrate 12, the second substrate 10 or also known as the superstrate 10 may also be constructed from glass. On top of the glass superstrate, from a viewing path, the second electrode layer 11 is formed as a transparent electrode, formed from electrically conductive material such as indium tin oxide. However, also conductive organic materials known in the art may be used which possess a suitable lower hydrophobicity than the hydrophobic interface surface 15 of the first stack.

[0069] The element also contains one or more cell walls, in which a set or multiple cell walls define a cell, depending on the form of the cells. These could be cubic, but also hexagonal shaped or any other suitable shape. The cell walls 16 extend between the first and second electrode stack. Between the cell walls and the first and second stack a containment space is formed. The containment space at least contains polar liquid 21 and non-polar liquid 20 which are immiscible with each other.

[0070] The containment space is thus provided with a polar liquid 21. Every suitable polar liquid may be applied, however, being readily available and at low-cost, water, glycols, glycerine or mixtures thereof may be used as the polar liquid of the present embodiment of the invention. In addition to the polar liquid, the cell or containment space further comprises a non-polar liquid 20. The polar liquid and non-polar liquid are immiscible forming a polar-non-polar liquid interface. The oil used as non-polar liquid 20 may be decane or another suitable liquid, such as selected from a group comprising mineral oils, animal and vegetable oils, high-boiling hydrocarbons, higher fatty acids, silicone liquids, in particular alkanes such as octane, decane, dodecane, vaseline, spindle oil, castor oil, olive oil and liquid paraffin.

[0071] As shown in FIG. 4, each of the cell walls is fixedly mounted on the second interface surface 11 of said second electrode layer stack and is directed with its other end 19 towards the first electrode layer. The end face 19 of each of the one or more cell walls facing the first electrode layer stack comprises a hydrophilic surface. This surface may be formed as a skin on the cell walls 16, and may or may not be continuous with the sides 18 of the cell walls 16. The surface may however also be continuous over the whole volume of the cell wall 16, which defines a monolithic cell wall with hydrophilic properties and thus a hydrophilic surface on the top 19 and sides 18 of the cell walls 16.

[0072] The top 19 or free end of the cell walls 16, which is at least free and non-fixed before the final assembly of the element, is hydrophilic. Due to these hydrophilic properties, the polar liquid 21 may have a tendency to diffuse from one cell to another and, as a result, the non-polar liquid 21 will also have a tendency to migrate to other cells. Consequently, the volume ratio between the polar and non-polar liquid in individual cells becomes affected and a non-uniform distribution of non-polar liquid will result in an unreliable electrowetting effect. Therefore, to avoid an unreliable electrowetting effect, the free end 19 of the cell wall should preferably form a sealing contact with the interface surface 15 of the first stack.

[0073] In order to achieve a reliable seal between the top 19 of the cell wall 16 and the interface surface 15 of the first stack, a part of the cell wall 16, preferably a top section 19 (but this may also be a thin layer formed as a skin on the top and/or on the side walls 18 or part of the side wall 18 near the top 19 thereof) is made from a material which can expand under particular circumstances. Preferably, the expandable material is a material which is arranged for imbibition. Imbibition is the ability of the material to absorb a fluid through solvation which results in a swelling of the material. The swelling will assure an effective seal between the top 19 of the cell wall 16 and the interface surface 15. The degree of swelling and thus expanding can be selected or defined by one or a combination of the amount of material with imbibition ability in the cell wall, or by the location thereof. For example, the cell wall may be formed from a material with imbibition ability combined with another material lacking the ability to imbibe, wherein the ratio between both materials defines the net degree of imbibition. More preferably however, the material with imbibition ability may be located solely at the top part 19 of the cell wall 16, e.g. at the top 1, 2, 5, 10, 15, 20, 25 or 30% of the height of the cell wall 16. The degree of swelling and thus expanding may however also be defined by the duration of soaking of the cell walls 16 or the tops 19 thereof in a liquid. The imbibed liquid is preferably the polar liquid 21 which is also present in the containment space. The soaking is preferably done at an elevated temperature of the liquid, which elevation not only improves the imbibition effect on the material, but also decreases the time needed to achieve a pre-defined swelling volume. As such, the swelling volume may be predefined, and for example set to achieve a swelling of approximately 1, 2, 3, 4, 5, 7 or 10% of the height of the cell wall 16.

[0074] As will be appreciated by the person skilled in the art, the present invention may be practised otherwise than as specifically described herein. Obvious modifications to the embodiments disclosed, and specific design choices, will be apparent to the skilled reader. The scope of the invention is only defined by the appended claims.