WO2009072866A1

WO2009072866A1 - Electrowetting element

Info

Publication number: WO2009072866A1
Application number: PCT/NL2007/000303
Authority: WO
Inventors: Hermanus Feil; Bente Adriaan Bordes; Eric Paul Van Der Pols
Original assignee: Miortech Holding B.V.
Priority date: 2007-12-06
Filing date: 2007-12-06
Publication date: 2009-06-11
Also published as: WO2009072871A1

Abstract

The present invention is directed to an electrowetting element comprising a first electrode layer having a hydrophobic surface and a second electrode layer opposite said hydrophobic surface, a containment space formed between said first and said second electrode layer, and pixel walls mounted on said hydrophobic surface for forming at least one pixel element in said containment space between said pixel walls. The containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity. Said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non-transmissive state. In accordance with the invention, said hydrophobic surface is comprised of a substance having chemical properties that enable said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

Description

Title: Electrowetting element

DESCRIPTION

Field of the Invention

The present invention relates generally to electrowetting elements, and in particular to an electrowetting element comprising a first electrode layer having a contact surface and a second electrode layer opposite said contact surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said contact surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non-transmissive state.

The present invention further relates to a method of manufacturing an electrowetting element comprising a first electrode layer having a contact surface and a second electrode layer opposite said contact surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said contact surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non- transmissive state.

Background of the Invention Electrowetting technology is based on modification of an energy balance between on one hand surface tension forces of liquids and wetting properties of a solid surface, and on the other hand electrostatic forces induced by an applied voltage over a capacitor arrangement comprising said boundary layer.

An electrowetting element may subsequently from bottom to top be comprised of respectively a first electrode layer, an electrically insulating hydrophobic layer (i.e. having a hydrophobic surface on a side opposite the side adjacent or nearest to the first electrode layer), a mixture of two liquids comprising at least a polar liquid and a non-polar liquid, and a second electrode in contact with at least the polar liquid. In practice, the liquids are contained in between for example pixel walls forming a containment tray and a top glass plate.

Various materials can be used- for the electrically insulating hydrophobic layer, e.g. teflon"¹¹ (Polytetrafluoroethyline (PTFE)) is a suitable material having suitable optical and electrical properties. Materials which are to a large extend hydrophobic are considered preferrable in the field of technology. As a non-polar liquid, one may use an oil such as decane. The selection criteria for selecting a suitable non-polar liquid include (apart from the liquid being non-polar), dielectric constant sufficiently large (the liquid is preferably a good isolator, or at least a poor conductor) and having an optical transmission coefficient that is suitable for the application wherein it is used (in practice the liquid will have low transmissibility, but in the present invention, a certain (small) degree of transmissibility may be advantageous, though not essential). Optical properties may be modified or adapted by introducing a small percentage of a dye in the non-polar liquid as an additive. The polar liquid preferably has good conductive properties, and should additionally be selected with respect to its optical properties. Preferably, the polar liquid is optically transmissive.

The principles of operation of an electrowetting element are as follows. In an unpowered state, i.e. when no voltage is applied over the first and second electrode, the lowest energetic state of the system is where the non-polar liquid forms a boundary layer between the polar liquid and the hydrophobic surface of the insulating layer. This is because the polar liquid is repelled by the hydrophobic layer. The poor transmissibility of the non-polar liquid then forms an obstruction to light that penetrates the system. When a voltage is applied over the electrodes, the lowest energetic state of the system becomes the situation wherein the (poorly conductive or insulating) non-polar liquid is pushed aside by the (conductive) polar liquid, and the polar liquid thereby being in direct contact with the insulating hydrophobic layer. Note that the voltage must be large enough for the electrostatic forces to overcome the repellent and surface tension forces that separate the polar liquid from the hydrophobic surface. In this situation, light that penetrates the system has rather unobstructed access to the insulating hydrophobic layer because of the well transmissibility of the polar liquid and the non-polar liquid being pushed aside.

Upon switching the electrodes from the powered state back to the unpowered state, by taking away the voltage that is applied to the electrodes, the equilibrium of forces present in the powered state is destroyed and the system will commence to turn back to the lowest energetic state of the system that was present before the electrodes were powered. A problem experienced with prior art electrowetting elements is that pixels of the system may not close after being opened. This problem is in the prior art resolved by increasing the porosity of the hydrophobic contact surface of the element. By increasing the porosity, the non- polar and often oily liquid is able to entrain the pores or microvoids in the hydrophobic contact layer, enabling the oil in each pixel to open and close more easily such as to prevent the above problem to occur.

A disadvantage of this method is that the porosity of the surface cannot be controlled efficiently in the manufacturing process. Therefore, the quality of electrowetting elements produced based on porose hydrophobic surface layers is variable. In case the porosity of the surface layer is too low, e.g. in some regions of the surface of the electrowetting elements, the probability of non-closing pixels of the electrowetting element produced, is larger in such an area and the produced electrowetting element may in that case be defective.

Another disadvantage is that by increasing the porosity of the hydrophobic contact surface, during use the polar liquid (e.g. water) may quite easily fill the pores or microvoids of the surface once a voltage is applied to the electrodes of the electrowetting elements and the polar liquid is in contact with the contact surface. The polar liquid may remain in the pores once the voltage is again turned off. This will deteriorate the electrowetting element, because the contact surface becomes hydrophilic in those areas of the surface wherein the polar liquid has entrained the microvoids. Therefore, over time it may occur that an electrowetting element that used to function perfectly directly after it was manufactured, deteriorates over time, leading to more and more non-closing pixels over time. This deteriorates the display quality of the electrowetting element and shortens the lifetime thereof.

Summary of the Invention

The present invention has for its objects to resolve the above- mentioned problems of the prior art, and provide an improved electrowetting element.

This and other objects is achieved in that there is provided an Electrowetting element comprising a first electrode layer having a hydrophobic surface and a second electrode layer opposite said hydrophobic surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said hydrophobic surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non- transmissive state, characterized in that, said hydrophobic surface is comprised of a substance having chemical properties that enable said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

The present invention is based on the insight that the contact angle between the non-polar liquid and the hydrophobic contact surface can be influenced by proper selection of the chemical properties of the surface, viz. by proper selection of the substance comprised by the hydrophobic contact surface.

According to a preferred embodiment of the present invention, the hydrophobic contact surface is comprised of a substance having chemical properties such that an interfacial tension between the non-polar liquid and the hydrophobic surface is approximately equal to the difference between an interfacial tension between the polar liquid and the hydrophobic surface and interfacial tension between the polar liquid and the non-polar liquid for minimizing a contact angle of an interface between the non-polar liquid and the polar liquid relative to the hydrophobic surface.

The invention is based on the insight that the problem of non-closing pixels is created by a local energetic minimum caused by an equilibrium of surface tension forces between the interfaces surface-polar liquid, surface non-polar liquid and polar liquid-non-polar liquid. By properly selecting the chemical composition of the surface such that the interfacial tension between the non-polar liquid and the surface substantially equals the difference between the interfacial tensions between the polar liquid and the hydrophobic surface on one hand and the interfacial tension between the polar liquid and the non-polar liquid on the other hand, the establishing of a local force equilibrium between the surface tension forces will not occur until the non-polar liquid covers the complete hydrophobic surface of the pixel element of the electrowetting element. This is regardless of whether or not the pores or microvoids are present in het hydrophobic surface.

According to another preferred embodiment of the invention, the hydrophobic surface is comprised of a substance having chemical properties such that an interfacial tension between said non-polar liquid and said hydrophobic surface is at least one order of a magnitude smaller than at least one of an interfacial tension between said non-polar liquid and said polar liquid and an interfacial tension between said polar liquid and said hydrophobic surface.

It has been observed that proper selection of the substance to be comprised in the hydrophobic surface which matches this criterium, i.e. it matches with the chemical properties of the non-polar liquid to such an extend that the interfacial tension is relatively small (at least one order of a magnitude smaller) compared to the other interfacial tensions involved, the effects of the present invention can be achieved easily. According to another embodiment of the present invention, the chemical properties of the hydrophobic surface match with chemical properties of said non-polar liquid for enabling said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state. In particular, where the non-polar liquid is an oily substance, the hydrophobic surface may be particularly oilphilic.

Surprisingly good results have been achieved by a hydrophobic contact surface comprising a organosilane layer. Organosilane molecules are comprised of a silane group which readily adheres or bonds to e.g. a silica insulating layer, and at the same time the rest of the molecule may be matched to the molecular structure of the non-polar liquid used. If for example the non-polar liquid comprises a hydrocarbon compound, such as decane, the organosilane layer may comprise an organosilane molecular structure which comprises said hydrocarbon compound, such as decylsilane. More generally, for alkane-based oils, the use of an alkylsilane coating or surface layer is preferred in accordance with the invention.

The non-polar liquid used in the electrowetting element according to the present invention, may be selected from a group comprising mineral oils, animal and vegetable oils, esters, high-boiling hydrocarbons, higher fatty acids, higher alcohols and polyolefins, in particular alkanes such as decane, vaseline, spindle oil, castor oil, olive oil, liquid paraffin and polybutene. Oilphilic coatings, or surface layers, such as an alkylsilane surface layer or another silane layer matching with the molecular structure of the non-polar liquid use, can be applied in order to achieve the effects of the present invention.

According to a second aspect of the present invention there is provided a Method of manufacturing an electrowetting element comprising a first electrode layer having a hydrophobic surface and a second electrode layer opposite said hydrophobic surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said hydrophobic surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non-transmissive state, characterized in that said method comprises a step of selecting a substance for forming said hydrophobic surface such that the chemical properties of said substance enable said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state. Brief Description of the Drawings

The invention is further explained by means of specific embodiments and examples thereof, with reference to the enclosed drawings, wherein: figure 1 illustrates a pixelelement in an electrowetting element according to the present invention; and figure 2 Illustrates a molecule of an alkylsilane monolayer coating present on a silica substrate surface.

Detailed description of Embodiments of the Invention

The invention is directed to an electrowetting element and a method for manufacturing thereof. The enclosed figure 1 discloses a pixel element (generally indicated by 1 ), comprising a containment space 4 between a first substrate 2 and a second substrate 3. Substrate 3 may for example be comprised of a glass layer, while substrate 2 may be a reflective substrate or another glass layer, which may be contiguous to backlighting means of a display. Substrate 2 may also comprise a specular reflective surface such that the electrowetting element may be applied to a mirror for dimming the reflectivity thereof. This may for example be advantageous for use in self-dimming car mirrors. The invention is however not limited to this application, and the teachings may be applied to electrowetting elements in general, regardless of their specific application and use.

Contiguous to substrate 2 and 3 electrodes 5 and 6 may be present at either side of containment space 4. Between electrodes 5 and 6 a voltage may be applied such as to switch the pixel element 1 on and off, as will be explained below.

On top of electrode 5 insulating layer 9 provides electric insulation between the containment space and said electrode. On top of insulating layer 9 a hydrophobic contact layer 10 is provided which is selected such that in an unpowered state the polar liquid is repelled from the hydrophobic surface, enabling the non-polar liquid to cover the complete surface of the pixel element 1.

In containment space 4, on top of insulating silica layer 9, pixel walls 13 separates pixel element 1 from contiguous pixel elements (not shown) of the electrowetting element. The pixel walls are preferrably of hydrophilic nature. The containment space 4 is filled with polar liquid 15. Every suitable polar liquid may be applied, however being a popular and readily available low-cost polar liquid, water is used as the polar liquid of the present embodiment of the invention. In addition to the polar liquid, the pixel element further comprises a non-polar liquid 16. The polar liquid and non-polar liquid are immissible forming a polar - non-polar liquid interface 17. The interface between the non-polar liquid and the hydrophobic contact surface is indicated with reference numeral 18. The interface between the polar liquid and the hydrophobic contact layer is generally indicated by reference numeral 19.

The pixel element 1 illustrated in figure 1 is in a transitional mode wherein, after being switched on, it has just been switched off (by releasing the voltage applied to electrodes 5 and 6), and the pixel element is closing, transiting to the unpowered equilibrium state wherein the pixel element is optically not transmissive. The non-polar liquid 16 is turning back to the 'off state wherein the non-polar liquid fully covers the hydrophobic contact surface 10, separating surface 10 from polar liquid 15.

In the embodiment disclosed in figure 1 , insulating layer 9 is comprised of a silica material layer, which is covered with a monolayer coating of an alkylsilane, for example decylsilane. The oil used as non-polar liquid 16 may be decane, while, as mentioned above, the polar liquid is water.

An schematic enlargement of a silica-based hydrophobic surface layer 10 of a pixel element 1 , comparable to the embodiment illustrated in figure 1 , is illustrated in figure 2. The enlargement is at microscopic level, disclosing two individual decylsilane molecules 25 which are attached to the silica substrate layer 9 of the pixel element. As can be seen, the a decylsilane molecule comprises a silane group 27 which easily adheres to the silica layer 9. A decyl-group 28 of the molecule forms the interface between the non-polar liquid and the surface 10. The hydrophobic (oilphilic) surface thus comprises a molecular structure which is similar to the molecular structure of decane. Because of this property, the wetting properties of hydrophobic contact layer 10 are dominated by the wetting properties of decane because of the similarity of the molecular structure at the surface. Bonding forces between decane molecules are thus comparable to the bonding forces between the decane molecules and the decylsilane surface molecules. Therefore, if decane is used as non-polar liquid 16, the interfacial tension between the decane and the alkylsilane mono layer of hydrophobic surface layer 10 will be negligible such that the non-polar liquid easily spreads across the surface of the pixel element 1 , after the pixel element is switched off to the unpowered state. This enables the pixel element to easily turn back into its unpowered equilibrium state wherein the non-polar liquid fully covers the hydro-phobic surface layer en separates this layer from the polar liquid 15.

In the embodiment described, since the wetting properties of layer 10 are determined by the decyl-group in the decylsilane film, the interfacial tension between the polar liquid and the surface 10 (interface 19) are comparable to the interfacial tension at interface 17, between the non-polar liquid and the polar liquid. Therefore, the interfacial tension between the polar liquid and the non-polar liquid is substantially equal to the interfacial tension of between the polar liquid and the hydrophobic surface. It has been discovered that such an interfacial tension renders the contact surface 10 sufficiently oilphilic for the electrowetting element to be operable. In addition, due to these comparable surface tension forces and the negligible surface tension between the non-polar liquid and the hydrophobic surface, the problem of non-closing pixel elements is effectively resolved since an equilibrium of surface tension forces will not occur until the non-polar liquid fully covers the hydrophobic surface. This is even the case when the hydrophobic surface is smooth, without microvoids or pores.

If θ is the contact angle of the non-polar liquid on the hydrophobic surface, and the roughness parameter R provides an indication of the roughness (such as porosity) of the surface (wherein R= 1 for a smooth surface), and the interfacial tensions between the non-polar liquid (O), the surface (S), and the polar liquid (W) are respectively given by γ_os. Ysw . and γ_ow, the contact angle at which surface tension equilibrium may occur is provided by:

_{R QOsθ =} ∑swZloi_

Yow

For the above described case, the contact angle will thus be close to 0°, i.e. the non- polar liquid will fully cover the hydrophobic surface, thereby achieving the effects of the invention.

Although the invention has been described with reference to the embodiments disclosed in figures 1 and 2, the invention may be practised differently, as will be apparent to the skilled reader. The scope of the invention is not limited by the embodiments disclosed, but is determined by the scope of the appended claims.

Claims

1. Electrowetting element comprising a first electrode layer having a hydrophobic surface and a second electrode layer opposite said hydrophobic surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said hydrophobic surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non-transmissive state, characterized in that, said hydrophobic surface is comprised of a substance having chemical properties that enable said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

2. Electrowetting element according to claim 1 , wherein said hydrophobic surface is comprised of a substance having chemical properties such that an interfacial tension between said non-polar liquid and hydrophobic surface is approximately equal to the difference between an interfacial tension between said polar liquid and hydrophobic surface and an interfacial tension between said polar liquid and said non-polar liquid for minimizing a contact angle of an interface between said non-polar liquid and said polar liquid relative to said hydrophobic surface.

3. Electrowetting element according to any of the previous claims, wherein said hydrophobic surface is comprised of a substance having chemical properties such that an interfacial tension between said non-polar liquid and said hydrophobic surface is at least one order of a magnitude smaller than at least one of an interfacial tension between said non-polar liquid and said polar liquid and an interfacial tension between said polar liquid and said hydrophobic surface.

4. Electrowetting element according to any of the previous claims, wherein said chemical properties of said hydrophobic surface match with chemical properties of said non-polar liquid for enabling said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

5. Electrowetting element according to claim 4, wherein said hydrophobic surface is oilphilic, and said non-polar liquid is an oily substance.

6. Electrowetting element according to claim 4 or 5, wherein said hydrophobic surface comprises a organosilane layer for matching said chemical properties of said non-polar liquid with said chemical properties of said hydrophobic surface.

7. Electrowetting element according to claim 6, wherein said non-polar liquid comprises a hydrocarbon compound, and said organosilane layer comprises an organosilane based on said hydrocarbon compound.

8. Electrowetting element according to claim 6 or 7, wherein said hydrophobic surface comprises an alkylsilane.

9. Electrowetting element according to any of the previous claims, wherein said non-polar liquid is selected from a group comprising mineral oils, animal and vegetable oils, esters, high-boiling hydrocarbons, higher fatty acids, higher alcohols and polyolefins, in particular alkanes such as decane, vaseline, spindle oil, castor oil, olive oil, liquid paraffin and polybutene.

10. Method of manufacturing an electrowetting element comprising a first electrode layer having a hydrophobic surface and a second electrode layer opposite said hydrophobic surface, a containment space formed between said first and said second electrode layer, pixel walls mounted on said hydrophobic surface for forming at least one pixel element in said containment space between said pixel walls, wherein said containment space and said at least one pixel element formed therein comprise a mixture of at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, and wherein said electrowetting element further comprises powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said at least one pixel element between an optically transmissive and optically non-transmissive state, characterized in that said method comprises a step of selecting a substance for forming said hydrophobic surface such that the chemical properties of said substance enable said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

11. Electrowetting element according to claim 10, wherein said substance is selected having chemical properties such that an interfacial tension between said non-polar liquid and hydrophobic surface is approximately equal to the difference between an interfacial tension between said polar liquid and hydrophobic surface and an interfacial tension between said polar liquid and said non-polar liquid for minimizing a contact angle of an interface between said non-polar liquid and said polar liquid relative to said hydrophobic surface.

12. Method according to claim 10 or 11 , wherein said substance is selected having chemical properties such that an interfacial tension between said non-polar liquid and said hydrophobic surface is at least one order of a magnitude smaller than at least one of an interfacial tension between said non-polar liquid and said polar liquid and an interfacial tension between said polar liquid and said hydrophobic surface.

13. Method according to any of the claims 10-12, wherein said substance is selected such that said chemical properties of said hydrophobic surface match with chemical properties of said non-polar liquid for enabling said non-polar liquid to spread across said hydrophobic surface upon switching said electrode layers from a powered to an unpowered state.

14. Method according to any of the claims 10-13, wherein said hydrophobic surface is an oilphilic surface.

15. Method according to any of the claims 10-14, wherein said substance comprises an organosilane compound.

16. Method according to claim 15, wherein said organosilane compound comprises an alkylsilane.

17. Method according to any of the claims 14 or 16, wherein said non- polar liquid comprises a hydrocarbon compound, and said organosilane compound comprises an organosilane based on said hydrocarbon compound.