WO2010062163A1

WO2010062163A1 - Electrowetting optical element and mirror system comprising said element

Info

Publication number: WO2010062163A1
Application number: PCT/NL2008/000257
Authority: WO
Inventors: Hermanus Feil
Original assignee: Miortech Holding B.V.
Priority date: 2008-11-25
Filing date: 2008-11-25
Publication date: 2010-06-03

Abstract

The present invention provides an electrowetting optical element comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, and pixel walls mounted on said first electrode layer for forming at least one pixel element in said containment space between said pixel walls. The first electrode layer comprises a hydrophobic interface with said containment space, and said first electrode layer comprises a conductive material layer for powering said first electrode layer. The containment space and the at least one pixel element formed therein comprise at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity. The polar liquid and the non-polar liquid are immiscible with each other. The electrowetting optical 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 different states of optical transmittance. The conductive material layer in said pixel element comprises at least one electrically non-conductive pixel area portion for leaving at least a part of said first electrode layer unpowered during said powering. The invention further relates to a mirror system comprising such an electrowetting element.

Description

Title: Electrowetting optical element and mirror system comprising said element.

Technical field

The present invention relates to an electrowetting optical element comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first electrode layer for forming at least one pixel element in said containment space between said pixel walls, said first electrode layer comprising a hydrophobic interface with said containment space, and said first electrode layer comprising a conductive material layer for powering said first electrode layer, wherein said containment space and said at least one pixel element formed therein comprise at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, said at least one polar liquid and said at least one non- polar liquid being immiscible with each other, and wherein said electrowetting optical 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 different states of optical transmittance.

The present invention further relates to a mirror system comprising an electrowetting optical element as descibed above.

Background

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), at least a polar liquid and a non-polar liquid immiscible with each other, 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.

The electrowetting element is mainly transparent, except for the non-polar liquid in each of the pixels formed by the pixel walls. The non-polar liquid is often non-transparent or has a low optical transmission coefficient. The transmission coefficient of the non-polar liquid typically depends on the application of the electrowetting element. In a colour display, an electrowetting element comprising coloured non-polar liquids may be used.

It is preferred to use dark or non-transparent pixel walls in order to prevent the appearance of artefacts in the image reflected by the mirror, or to prevent blurring of the image provided by the specular reflecting surface.

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.

In the powered up state, when voltage is applied over the electrodes, the electrowetting element is transmissive, and a reflected image is transmitted by the mirror system for presentation thereof to a viewer of the mirror system. A disadvantage is that because of symmetries of the optical system, interference will occur between the light rays transmitted by the mirror system. A viewer which is watching the mirror system presenting a reflection of a light source, will see a strong diffraction pattern around the light source which is destructive to the reflective image. As will be understood, such a diffraction pattern is an undesired artefact of a mirror, especially for a mirror that is used in a vehicle, e.g. for use as rear view mirror.

Summary of the invention

It is an object of the present invention to provide an electrowetting optical element wherein problems with diffraction are resolved.

This and other objects of the present invention are achieved in that there is provided an electrowetting optical element comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first electrode layer for forming a plurality of pixel elements in said containment space between said pixel walls, said first electrode layer comprising a hydrophobic interface with said containment space, and said first electrode layer comprising a conductive material layer for powering said first electrode layer, wherein said containment space and said plurality of pixel elements formed therein comprise at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, said at least one polar liquid and said at least one non-polar liquid being immiscible with each other, and wherein said electrowetting optical 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 different states of optical transmittance, wherein said conductive material layer in at least part of said plurality of pixel elements comprises at least one electrically non-conductive pixel area portion for leaving at least a part of said first electrode layer unpowered during said powering, and wherein said non- conductive pixel area portions of said at least part of said plurality of pixel elements comprise a mutually randomized geometry for reducing symmetry of said pixel arrangement during said powering.

By providing the conductive material layer of the first electrode layer with an electrically non-conductive pixel area portion, the interface locally at the non- conductive portion of the pixel will be left uncharged when the first electrode layer is powered up. The electric force that drives the polar liquid to the interface is not present locally in the electrically non-conductive pixel area portion. Therefore, locally in the non-conductive pixel area portion the repellent forces of the hydrophobic interface on the polar liquid cause the local force equilibrium to repel the polar liquid, and to concentrate the non-polar liquid on the non-conductive pixel area portion of the hydrophobic interface. Often, a suitable oil is used as non-polar liquid, which in the present invention will form an oil pocket on the location of the electrically non- conductive pixel area portion. In other words, the preferred location where the non- polar liquid will concentrate in the powered up state can be controlled by providing the conductive material layer with electrically non-conductive pixel area portions such as proposed by the present invention. This provides direct control over the periodic nature of the boundary between the polar and non-polar liquid in the powered up state, and enables the electrowetting optical elements to be designed such as to remove the periodic nature, and prevent the occurrence of diffraction. The invention is based on the insight that the issues with diffraction may be resolved by gaining control over the behaviour of the liquids during use. In particular the invention recognises the regularity of the geometric forms of non-polar liquid pockets in the powered-up state of the element, and the issues this may cause in terms of optical interference. By providing a preferred location within each cell or pixel within an electrowetting element for the non-polar liquid to retreat during the powered-up state, regularity may be controlled. In the invention, the regularity is eliminated by mutually randomizing various geometric properties of said non- conductive portions.

The non-conductive pixel area portion in the conductive material layer may simply be formed by a gap in the conductive material layer at the location of the non-conductive pixel area portion. During manufacturing the first electrode of the electrowetting optical element, such a gap may be created by means of etching, cutting, or by an other suitable method.

In an other embodiment of the present invention, the gap comprises at least one of a group comprising a vacuum, and a non-conductive material, such as a non-conductive gas, a non-conductive liquid or a non-conductive solid. In case the conductive material layer is coated with an insulating layer, the material of the insulating layer may also fill the internal space of the gap in the conductive material layer. According to a preferred embodiment of the present invention, the electrowetting optical element comprises a plurality of pixel elements, and the conductive material layer in at least part of said plurality of pixel elements comprises at least one electrically non-conductive pixel area portion.

Because the geometry of the non-conductive pixel area portions is mutually randomised between the pixels, the symmetry of the pixel arrangement during the powered up state can be effectively reduced or eliminated. The present invention is based on the insight that the occurrence of diffraction patterns is caused by the periodic nature of structures within the pixels that form a pixel arrangement. These structures include, in case of electrowetting optical elements, the regular structures of the boundary between the polar and non-polar liquid, e.g the oil-water interface, in the powered-up state. The amount of periodicity in the pixel arrangement determines the amount of diffraction. Strong diffraction patterns are achieved when pixel arrangements are used having a very regular periodic nature. When the amount of symmetry in the pixel arrangement is reduced gradually, the diffraction pattern will gradually disappear likewise.

In a conventional electrowetting display, in the powered-state, the non-polar liquid has a preference of concentrating near the pixel wall and providing free access for the polar liquid to contact the first electrode layer. This is a result of the energy balance in the powered up state, wherein electric forces on the dipolar molecules of the polar liquid exceed the repellent forces between the hydrophobic interface and the polar liquid. In each pixel cell, the non-polar liquid of contrasting colour, will have a regular shaped boundary with the polar liquid due to this energy balance. This regularity gives rise to the occurrence of diffraction. The occurrence of diffraction patterns may effectively be reduced by reducing the symmetry of the pixel arrangement as much as possible. This is achieved in the present invention by mutually randomising the geometry of the non- conductive pixel area portions in each pixel (or at least in a substantial number of the available pixels). This randomising can be done in various ways, as will be described further below.

In respect of the above it is noted that the term "geometry" used in the present description refers to the mathematical study of the properties of, and relations between points, lines, angles, surfaces and solids in space, including the dimensional relations within or between objects. The mutual geometry of non- conductive pixel area portions thus includes orientation of these pixel area portions amongst each other, the size, the shape, and the dimensions of non-conductive pixel area portions, the relative location of non-conductive pixel area portions in each pixel, and the number of non-conductive pixel area portions per pixel, as will be understood by the skilled person. In addition it is noted that the term "symmetry" includes specific forms of symmetry such as translation symmetry, point symmetry, line symmetry, periodicity, etc.

According to an embodiment of the invention, the mutually randomised geometry of the non-conductive pixel area portions comprises non- conductive pixel area portions having a randomised shape. This shape may include any random, unstructured shape, but may also include various known shapes such as squares, rectangles, crosses, circles, ovals, triangles, poly-angles and so forth. The orientation of the shapes may be randomised as well.

According to another embodiment of the invention, the mutually randomised geometry of the non-conductive pixel area portions comprises the non- conductive pixel area portions having a randomised size. By randomising the size of the non-conductive pixel area portions, the periodicity of the structure may also be decreased to some extent.

According to another embodiment of the invention, the relative location of the non-conductive pixel area portions within the pixels is randomised. Yet in an other embodiment of the invention the non-conductive pixel area portions include a number of pixel area portions having a substantially elongated shape, wherein the orientation of the elongated shape is randomised.

In a second aspect of the invention, there is provided a mirror system comprising an electrowetting optical element in accordance with the first aspect described above, wherein the mirror system comprises a viewing surface for being operable as a mirror, further comprising a specular reflective surface and means for controlling an amount of light transmitted between said specular reflective surface and said viewing surface, said means for controlling said amount of light comprising said electrowetting optical element. According to an embodiment of the mirror system of the second aspect, it further comprises backlighting means arranged for providing light to said specular reflective surface for transmission through said mirror system to said viewing surface, wherein said specular reflective surface comprises a transmissive area portion, wherein said transmissive area portion within a pixel of said electrowetting optical element at least partially coincides with said non- conductive pixel area portion.

Brief description of the drawings The invention will be explained and illustrated further by means of some specific examples thereof, with reference to the enclosed drawings, wherein: figure 1 illustrates a schematic cross section of an electrowetting optical element according to the present invention; figure 2 schematically illustrates a pixel arrangement of an electrowetting optical element according to the present invention.

Detailed description

Figure 1 illustrates a cross section of an electrowetting optical element 1 according to the present invention. In figure 1 the electrowetting optical element comprises a first electrode layer 3 and a second electrode layer 5 which are connected to control means 28 for controlling powering up and powering down of the electrowetting optical element. By controlling the powering up and down of the electrowetting optical element, the element can be switched between different states of optical transmittance, e.g. an optical non-transmissive state wherein the element has a very low optical transmittance, and an optically transmissive state, wherein the optical transmittance of the electrowetting element is relatively large. The first electrode layer 3 and the second electrode layer 5 may be of an optically transmissive material, such as indium tin oxide (ITO) which is often used for this purpose. The skilled person will appreciate that the first and second electrode layers may also be fabricated from different materials that are either optically transmissive or, for example, sufficiently thin, in order to have transmissive properties.

First electrode layer 3 comprises a conductive material layer 4 and an insulating layer 8. Insulating layer 8 comprises a hydrophobic interface 9 with containment space 15. The containment space 15 comprises a plurality of pixel walls 22, 23 that divide the full area of the electrowetting element in pixels. Second electrode layer 5 is disposed underneath a glass layer 25, which provides a viewing surface 26 to be observed by a user, such that electrode layer 5 is in contact or comprises a conductive interface with the said containment space 15.

Underneath the first electrode layer 3 an insulating layer 10 separates the first electrode layer from a reflective surface 12. Reflective surface 12 acts as a mirror component for using said electrowetting optical element 1 in an automatically dimmable mirror system, such as an automatically dimmable car mirror. Each pixel of the electrowetting element comprises an amount of a non-polar liquid, such as oil pockets 18, 19 and 20. The interior of the containment space 15 is, in addition to the non-polar liquid, filled with a polar liquid such as a salt water solution 16.

In the conductive material layer 4 of the first electrode layer 3, a plurality of gaps 30, 31 and 32 are present underneath each of the pixels in the electrowetting element. The gaps are schematically illustrated in cross section, but have a shape in the 2-dimensional plane of the conductive material layer which is randomised from pixel to pixel. If the first electrode layer is charged during powering up of the electrowetting element, the gap portions 30, 31 and 32 in the conductive material layer will remain uncharged. Therefore, the non-polar liquid, i.e. the oil, in each pixel will concentrate on the surface 9 at the location of the gaps 30, 31 and 32. This is because the electric forces acting on the polar liquid will pull the polar liquid to the "charged" parts of the surface 9, and the local energy balance of the surface 9 at the location each of the gaps 30, 31 and 32 causes the polar liquid to be repelled from the hydrophobic surface enabling the non-polar liquid to concentrate there. This enables control on where and how the non-polar liquid concentrates in each pixel in the powered up state of the electrowetting element. This feature can therefore be used to control the shape of the boundary between the polar and non-polar liquid, and to reduce the overall periodicity of the electrowetting optical element in the powered up state. The reflectivity of the specular reflective surface 12, in the electrowetting element 1 of figure 1 , has been removed (or at least reduced) in spots 35, 36 and 37. Therefore, surface 12 is transmissive in spots 35, 36 and 37. Backlighting means 40 (schematically illustrated) have been disposed on the backside of electrowetting element 1 (the viewing surface of the electrowetting element 1 is located near the second electrode layer 5). In the powered up state (relatively thick) oil pockets 18, 19 and 20 prevent light from the backlighting means 40 to transmit through the electrowetting element 1 via for example spot 36 and gap 31. When the electrowetting element is powered down, the non-polar liquid, e.g. oil pocket 18, spreads covering the full surface of the pixel. In the powered down state, the thickness of the non-polar liquid layer covering the hydrophobic surface 9 is sufficiently thin for enabling the light from the backlighting means to penetrate through openings 35, 36 and 37, and gaps 30, 31 and 32 respectively, such as to penetrate through the oil layer covering each pixel, to the viewing surface of the electrowetting element. In the powered down state, element 1 of figure 1 will not reflect light incident on the viewing surface, since this will have to travel through the non-polar liquid layer twice, and will be attenuated too much to be visible at a mirror reflection. In other words, in the unpowered state only the backlight will be visible through the electrowetting element. This feature may be used for providing information through the mirror system, for example for displaying information icons through the display.

Figure 2 schematically illustrates a pixel arrangement of an electrowetting element according to the present invention. The pixel arrangement comprises a plurality of pixels such as pixels 43, 44 and 45, separated by means of pixel walls. As indicated by reference numerals 48-80, underneath the pixels in the conductive material layer of the first electrode layer, the pixels comprise a non- conductive pixel area portion for concentrating the non-polar liquid in the unpowered state of the electrowetting element. The shape of the non-conductive pixel area portions is randomised from pixel to pixel. The shapes of these pixel area portions may be completely random such as non-conductive area portions 48, 49, 50, 52, 55, 56, 63, 64, 65, 67, 68, 69, 70, 71 and 80. The shape of the non-conductive pixel area portions may also comprise an arbitrary chosen regular shape, such as circle 51 , any of the ovals 60, 62, 74 or 75, square 76, triangle 77, or cross 78. Also the orientation of the non-conductive pixel area portions may be mutually randomised, such as is done for ovals 60, 62 and 74. An other option for mutually randomising the non- conductive pixel area portions is varying the size of these portions, for example compare the size of pixel area portion 75 to the size of pixel area portion 64. Yet a further option for randomising the geometry is to randomise the relative location of the non-conductive pixel area portion in the pixel, for example compare pixel area portion 49 located in the upper right corner of the pixel with pixel area portion 63 located in the lower left portion of the pixel. Non-conductive pixel area portions may also be formed near the walls of a pixel, such as is done for pixel area portion 55. Also the number of non-conductive pixel area portions in each pixel may be different, for example compare pixels 44 and 45, comprising no non-conductive pixel area portions at all, to pixel area portions 64 and 65 in a single pixel and to for example pixel 43 having a single non-conductive pixel area portion 55.

The amount of geometrical irregularity that can still by followed by the boundary between the polar and non-polar liquid is determined by the properties of the liquids used, such as the viscosity of the non-polar and polar liquid and the surface tension. Good results have been achieved with a water-salt solution as polar liquid, and a non-polar liquid such as, but not limited to, decane.

The invention may be practised otherwise than as specifically described herein, and is only limited by the appended claims.

Claims

1. Electrowetting optical element comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first electrode layer for forming a plurality of pixel elements in said containment space between said pixel walls, said first electrode layer comprising a hydrophobic interface with said containment space, and said first electrode layer comprising a conductive material layer for powering said first electrode layer, wherein said containment space and said plurality of pixel elements formed therein comprise at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, said at least one polar liquid and said at least one non-polar liquid being immiscible with each other, and wherein said electrowetting optical 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 different states of optical transmittance, wherein said conductive material layer in at least part of said plurality of pixel elements comprises at least one electrically non-conductive pixel area portion for leaving at least a part of said first electrode layer unpowered during said powering, and wherein said non-conductive pixel area portions of said at least part of said plurality of pixel elements comprise a mutually randomized geometry for reducing symmetry of said pixel arrangement during said powering.

2. Electrowetting optical element according to claim 1 , wherein said non-conductive pixel area portion is formed by a gap in said conductive material layer.

3. Electrowetting optical element according to claim 2, wherein said gap comprises at least one of a group comprising a vacuum, and a non-conductive material, such as a non-conductive gas, a non-conductive liquid or a non-conductive solid.

4. Electrowetting optical element according to any of the previous claims, wherein said mutually randomized geometry of said non-conductive pixel area portions comprises said non-conductive pixel area portions having a randomized shape.

5. Electrowetting optical element according to claim 4, wherein said mutually randomized geometry of said non-conductive pixel area portions comprises said non-conductive pixel area portions having a randomized size.

6. Electrowetting optical element according to any of the previous claims, wherein a relative location of said non-conductive pixel area portions within said pixels is randomized.

7. Electrowetting optical element according to any of the previous claims, wherein at least a plurality of said non-conductive pixel area portions comprise a substantially elongated shape, wherein orientation of said elongated shapes of said non-conductive pixel area portions is randomized.

8. Mirror system comprising an electrowetting optical element according to any of the claims 1-7, said mirror system comprising a viewing surface for being operable as a mirror, further comprising a specular reflective surface and means for controlling an amount of light transmitted between said specular reflective surface and said viewing surface, said means for controlling said amount of light comprising said electrowetting optical element.

9. Mirror system according to claim 8, further comprising backlighting means arranged for providing light to said specular reflective surface for transmission through said mirror system to said viewing surface, wherein said specular reflective surface comprises a transmissive area portion, wherein said transmissive area portion within a pixel of said electrowetting optical element at least partially coincides with said non-conductive pixel area portion.