HK1097941B - Electro- wetting displays - Google Patents

Description

Electrowetting display

The present invention relates to electrowetting displays.

It has been known a century ago that the interfacial tension between two immiscible dielectrics can be controlled by applying an electrical potential between the dielectrics; see, for example, Lippmann, m.g., ann.chim.phys., 5, 494 (1875). It has long been known that the mathematical relationship between the applied potential (V) and the resulting surface tension (γ) can be expressed by the Lippmann formula:

γ＝γ⁰-0.5cV²

wherein, γ⁰Is the surface tension of the solid-liquid interface at substantially zero charge (i.e., when there is no charge on the surface of the solid), c is the capacitance per unit area, assuming that the charge layer can be modeled as a symmetric helmholtz capacitor. Electroosmotic and electrocapillary displays have also been developed; all of these types of displays rely on a change in the wetting properties of the liquid in the presence of an electric field. See, for example, Sheridon, N.K., "Electrical Imaging devices for Display and Data Storage", Xerox Display Journal1979, 4, 385 386; and U.S. patent nos. 5,956,005; 5,808,593, respectively; 5,757,345, respectively; 5,731,792, respectively; 5,659,330, respectively; 4,569,575, respectively; 6,603,444, respectively; and 6,449,081. Richard b.fair and his colleagues developed various displays using this principle at Duke university; see, for example, http:// www.ee.duke.edu/Research/microfluidics.

Recently, it has been found that a thin dielectric layer between the electrodes and the liquid in an electrowetting device (thus forming an "electrowetting on dielectric" device) can emulate an electrical double layer in a conventional electrowetting device. The dielectric layer can block electron transport while maintaining a high electric field at the interface that results in charge redistribution upon application of a voltage. The use of hydrophobic dielectrics and aqueous liquids provides a large initial contact angle and a large room for change in contact angle upon electrowetting. Moreover, by employing a dielectric layer between the liquid and the electrode, virtually any kind of liquid can be used, regardless of the polarity of the interface; see "Low voltage electric-on-dielectric" published by Moon, H.et al, J.Appl. Phys, 2002, 92, 4080.

Researchers at Philips research laboratory, Eindhoven, the netherlands, describe an electrowetting display that states that it can be used for video rate applications; see Nature, 425, 383(2003) and international patent application WO 2004/068208; WO 2004/027489; and WO 03/071346. The display is of the dielectric type electrowetting, using at its bottom a cell with transparent electrodes arranged on a white substrate. The electrodes are covered by a hydrophobic dielectric layer. The unit also contains a pigmented (colored) oil and water. When no voltage is applied, the colored oil wets the hydrophobic dielectric, so the color appears the color of the oil. However, when a voltage is applied between the transparent electrode and the second electrode in contact with water, the oil moves to a small portion of the pixel, so the main portion of the pixel assumes the white color of the substrate. A CMYK colour scheme can be achieved by dividing the pixel into three sub-pixels, each having a white substrate but each having two oil layers of different colours, for example cyan and magenta.

There are a number of problems with this type of display. The display is not bi-stable because the confinement of the oil to a small portion of the pixel is maintained only when a field is applied. This is not a serious problem when the display is used to continuously display video, but there are application situations where, particularly in portable devices, the user wishes to pause the video and check the individual frames, which is advantageous if the display is bi-stable so that the check of each individual frame can be done without continuously consuming power drain in the battery. The visibility of the oil in a small fraction of the pixels reduces the contrast of the display. The use of dyes dissolved in oil can lead to long-term problems, since most dyes in solution will be adversely affected by long-term exposure to radiation, which typically leads to discoloration of the dye. This can be a particular problem in displays that rely on the use of different coloured oils, which fade at different rates, so that over time the colour displayed can shift.

The present invention is directed to various improvements in electrowetting displays that reduce or eliminate the above-mentioned problems.

In one aspect, the present invention provides a display comprising:

a substrate;

a first fluid adjacent to the substrate, the first fluid absorbing at least one optical wavelength;

a light-transmissive second fluid immiscible with the first fluid;

at least one electrode for applying an electric field to the first fluid; and

a concealing member spaced from the substrate and made of a substantially opaque material,

in the absence of an electric field, the first fluid is caused to cover a first area of the substrate, but upon application of an electric field to the first fluid via the at least one electrode, the first fluid moves to a second area smaller than the first area and is substantially confined between the concealing member and the substrate, so that the concealing member substantially conceals the first fluid from view by an observer viewing the display from the opposite side of the concealing member from the substrate.

The term "light-transmissive" as used herein means that the second fluid must transmit sufficient light to be visible to an observer observing the movement of the first fluid through the second fluid (in the case of a display being a machine reading non-optical wavelengths, the term "light-transmissive" must of course be understood to mean that the wavelength of the electromagnetic radiation reading the display is transmissive, and other terms used below in relation to light should be understood accordingly). Typically, the light transmissive second fluid is transparent, but we do not exclude the possibility that some color may be present in the second fluid to adjust the color displayed. For example, many people prefer "white" with a somewhat bluish hue rather than pure white, so, for example, in displays of the type described below with reference to fig. 1 and 2 in which the color changes from white to black, it may be advantageous to inject some slight blue so that the white state appears slightly bluish-white.

For convenience, this display may be referred to hereinafter as the "hidden member display" of the present invention. In such a display, the substrate may comprise a dielectric surface adjacent to the first fluid and/or may comprise a coloured or reflective layer. In a preferred form of such a display, the substrate has a substantially planar surface and the concealing member comprises a substantially planar portion extending substantially parallel to but spaced from the substantially planar surface of the substrate.

In another aspect, the present invention provides a display comprising:

a substrate having at least first and second portions having first and second optical characteristics that are different from each other;

a first fluid that absorbs at least one wavelength of light and has a third optical characteristic that is different from at least one of the first and second optical characteristics;

a light-transmissive second fluid immiscible with the first fluid; and

a first electrode adjacent to a first portion of the substrate and a second electrode adjacent to a second portion of the substrate,

such that by controlling the potentials applied to the first and second electrodes the first fluid may occupy a first position in which the first fluid substantially covers the second portion of the substrate leaving the first portion uncovered, and a second position in which the first fluid substantially covers the first portion of the substrate leaving the second portion uncovered.

For convenience, this display will be referred to hereinafter as the "color shifting display" of the invention. In such a display the first fluid can occupy a third position, wherein the first fluid covers the first and second portions of the display.

In a color shifting display, the substrate may have more than two portions of different colors. For example, the substrate may have a third portion having an optical property different from the first, second and third optical properties, the display may further comprise a third electrode adjacent the third portion of the substrate, such that by controlling the electrical potential applied to the first, second and third electrodes, the first fluid may occupy a third position in which the first fluid substantially covers one of the at least first and second portions of the substrate leaving the third portion uncovered. For example, the first, second and third portions of the substrate may be randomly arranged red, green and blue, or yellow, cyan and magenta. Furthermore, the substrate may have a fourth portion having optical properties different from the first, second and third optical properties and different from the optical properties of the third portion of the substrate, the display further comprising a fourth electrode adjacent to the fourth portion of the substrate such that the first fluid may be caused to occupy a fourth position by controlling the electrical potential applied to the first, second, third and fourth electrodes, wherein the first fluid substantially covers at least one of the first, second and third portions of the substrate leaving the fourth portion uncovered. For example, the first, second, third and fourth portions of the substrate may be any arrangement of red, green, blue and black, or yellow, cyan, magenta and black.

In the color shifting display of the present invention, generally, the first and second portions (and the third and fourth portions, if present) of the substrate will be coplanar. These portions may exhibit different geometries. For example, the portions may have the form of substantially equilateral triangles. Alternatively, the first and second portions may have a substantially circular form, the substantially circular first and second portions being connected by a neck portion having a width less than the diameter of each substantially circular portion. Electrodes may be disposed on or adjacent to the neck portion.

In another aspect, the present invention provides a display comprising:

a first substrate through which an observer can view the display; a second substrate spaced apart from the first substrate; and at least one sidewall extending between the first and second substrates, the first and second substrates and the sidewall together defining a cavity having a first substrate surface, a second substrate surface, and at least one sidewall surface;

a first fluid disposed in the cavity, the first fluid absorbing at least one wavelength of light;

a light transmissive second fluid immiscible with the first fluid and disposed in the cavity;

a first electrode adjacent to a second substrate surface of the cavity;

a second electrode disposed adjacent to a sidewall surface of the cavity; and

a third electrode extending into the cavity and in electrical contact with the second fluid,

by controlling the potentials applied to the first, second and third electrodes, the first fluid may occupy a first position in which the first fluid is adjacent to the second substrate surface of the cavity and a second position in which the first fluid is adjacent to the sidewall surface of the cavity.

For convenience, such displays will be referred to hereinafter as "microcell displays" of the present invention. In such a display, the substrate may comprise a dielectric surface adjacent to the first fluid and/or may comprise a coloured or reflective layer. The display also includes an insulating block adjacent to a junction between the second substrate surface and the sidewall surface of the cavity, the third electrode passing through the insulating block.

In another aspect, the present invention provides a display comprising:

a fluid;

a substrate having an exposed surface that is resistant to wetting by the fluid;

at least three electrically conductive vias extending through the substrate and terminating near the exposed surface; and

a cap member covering an end of the conductive via adjacent the exposed surface, the cap member being formed of a material wetted by the fluid.

For convenience, such displays will be referred to hereinafter as "conductive via displays" of the present invention. In such a display, the conductive vias may be arranged in a two-dimensional array. Furthermore, the fluid may be aqueous, the exposed surface hydrophobic, and the cap member formed of a hydrophilic material.

In another aspect, the present invention provides a display comprising:

a substrate;

a first fluid disposed adjacent to the substrate, the first fluid absorbing at least one light wavelength;

a light-transmissive second fluid immiscible with the first fluid; and

at least one electrode for applying an electric field to the first fluid,

in the absence of an electric field, causing said first fluid to cover a first area of the substrate, but upon application of an electric field to said first fluid via the at least one electrode, said first fluid moves to a second area smaller than said first area,

wherein the first fluid is coloured by pigment particles or nanoparticles.

For convenience, this display will be referred to hereinafter as the "pigment/nanoparticle display" of the invention.

Finally, the present invention provides a display comprising:

first and second spaced apart electrodes, the second electrode being light transmissive; and

first and second fluids confined between the first and second electrodes, the first and second fluids being immiscible with each other, the first and second fluids being opaque and having different colors,

the display has a first stable state in which the first fluid is adjacent the first electrode such that the colour of the second fluid is visible to an observer viewing the display through the second electrode, and a second stable state in which the first fluid is adjacent the second electrode such that the colour of the first fluid is visible to an observer.

For convenience, the display will be referred to hereinafter as the "bi-color fluid display" of the invention. In such a display, the first fluid may comprise oil and the second fluid may comprise water. The display further comprises first and second dielectric layers arranged between the first electrode and the fluid and the second electrode and the fluid, respectively.

FIG. 1 of the drawings is a schematic side view of a hidden-member display of the present invention with a second fluid covering a large first area of the substrate;

similar to fig. 1, fig. 2 shows a schematic side view of a second fluid being confined to a smaller second region of the substrate.

Fig. 3 is a top view of a substrate for a four color-shifting display of the present invention.

Fig. 4 is a schematic side view of a bistable two-color fluid electrowetting display of the invention.

Fig. 5 is a schematic side view of a microcell display of the present invention.

FIG. 6 is a top view of a second color-shifting display of the present invention.

Fig. 7 is a schematic side view of a conductive via display of the present invention.

As previously mentioned, the present invention has many different aspects. Various aspects will be described separately below, but it should be understood that a single display may utilize multiple aspects of the invention. For example, the microcell display of the present invention the pigment/nanoparticle according to the present invention may use a first fluid in which the pigment particle or the nanoparticle is colored.

In the present display, the first (moving) fluid is typically oil, while the second fluid is typically aqueous. For ease of understanding, the following description uses the terms "oil" and "water" in place of the first and second fluids, but these terms should not be construed in a limiting sense.

First, as described above, the present invention provides a concealing member display having a concealing member for concealing oil when an electric field is applied; the invention also provides a method of operating such a display. A particular masking member display of the present invention is shown in fig. 1 and 2 of the drawings. As shown in fig. 1, an electrowetting display (only a single pixel is shown) comprises a substrate 102 (typically coloured white), a transparent first electrode 104 and a dielectric layer 106. A layer of colored first fluid (oil) is spread over a large first area of the dielectric layer 106 (across the dielectric 106 as shown), and a transparent second fluid (water) 110 is located over the oil layer 108. The electrowetting display further comprises a concealing member 112 having a first portion extending away from the dielectric layer 106 and a second portion extending parallel to the planar dielectric layer 106; the surface of the second portion of the concealing member 112 supports a second electrode (not shown).

Fig. 1 shows an electrowetting display in which no electric field is applied between the two electrodes, so that the coloured oil layer 108 spreads evenly over the surface of the dielectric layer 106. Thus, a single pixel is shown displaying the color of the oil. Figure 2 shows the display when an electric field is applied between two electrodes. The oil layer no longer spreads out evenly over the surface of the dielectric layer 106 but collects in compact droplets 108 'covering the second area under the masking member 112 so that the white substrate 102 is mainly visible to a viewer viewing the display in a given direction (from the opposite side of the masking member 112 from the substrate 102, i.e. from above in fig. 1 and 2), the droplets 108' are hidden by the masking member 112 and are not visible to the viewer.

It will be apparent that the contrast of the pixel can be varied by varying the color of the visible surface of the concealing member 112 (i.e., the surface away from the dielectric layer 106). For example, making the visible surface white will increase the brightness (brightness) of the white state of the pixel (as shown in FIG. 2), although the blackness of the black state (as shown in FIG. 1) will be lost. Alternatively, maximum contrast can be obtained by making the visible surface an intermediate gray shade (gray shade).

Various variations of the display shown in fig. 1 and 2 are possible. For example, the second electrode need not be located on the concealing member, so long as the second electrode is in electrical contact with the water 110. In practice, each pixel of the display need not have a separate second electrode; instead, the display may use an electrode structure similar to a conventional active matrix display, each pixel having a common front electrode (shaped to guide movement of the first fluid, as described below) extending across a large number of pixels (typically across the entire display), and a separate first electrode 104. Furthermore, one optical state of the display need not show the color of the substrate. For example, the substrate may be transparent so the display functions as a light modulator. Alternatively, the colored filters or reflectors may be arranged in any suitable location.

Fig. 3 is a highly schematic top view of one pixel of the substrate of the four-color-shifting display of the present invention. As can be seen from the figure, the pixel is an equilateral triangle, consisting of four equilateral triangle sub-pixels, the central sub-pixel being black (K), the others being red (R), green (G) and blue (B); it will be apparent that cyan, yellow and magenta sub-pixels may be used in place of the red, green and blue sub-pixels if desired. The pixel is provided with four electrodes (not shown), one at each vertex of the triangular pixel, the fourth electrode being in its centre. (if desired, electrodes may be provided on a concealing member similar to that of figures 1 and 2, the exposed surface of the concealing member being coloured to match the portion of the underlying pixel.) the pixels are used with a black oil to operate in a similar manner to the display shown in figures 1 and 2. When an electric field is not applied through any of the electrodes, the oil spreads uniformly over the entire pixel, so that black is displayed all over. If a voltage is applied only to the central electrode, oil collects in the central black sub-pixel electrode, exposing the red, green and blue pixels, so that the full appearance of the pixel appears as "white pigment" (effectively grey). For example, if a voltage is applied using the center electrode and an electrode adjacent to the red subpixel, the ink will cover the black and red subpixels and will display cyan. It is clear that by applying voltages to one, two or three electrodes, a plurality of colors can be displayed on the pixel.

It is also possible to make bistable electrowetting displays. Recent prior art electrowetting displays are only monostable, as they are only stable in the absence of an applied electric field; the other state (similar to that shown in fig. 2) is only maintained during the application of the electric field. However, a two-color fluid electrowetting display of the present invention can be prepared having two states, each state being similar to that shown in fig. 1. Figure 4 shows a pixel in the form of such a bi-stable display of two-colour fluid. The display includes a substrate 102 (which is not necessarily colored for reasons explained below), an electrode 104 (which is not necessarily transparent), and a dielectric layer 106, all of which are similar to corresponding numbers in fig. 1, as shown. The display further comprises a coloured oil layer 108 and a coloured aqueous layer 110 ', which layer 110' has a colour different from that of the oil layer 108. There are no masking members, but the display of FIG. 4 includes a front transparent dielectric layer 114 and a transparent front electrode 116; indeed, it may be desirable to provide a front substrate (not shown) to provide mechanical support and protection for the display.

As in the condition shown in fig. 1, in the condition shown in fig. 4, the oil layer 108 spreads evenly over the dielectric layer 106, and the pixel displays (for an observer who observes the display through the electrode 116 and the dielectric layer 114, i.e., an observer who observes the display from above in fig. 4) the color of the aqueous layer 110 ', which aqueous layer 110' makes the color of the oil layer 108 obscure. However, by suddenly applying a voltage between the electrode 104 and an electrode (not shown) in contact with the aqueous layer 110 ', the oil layer 108 can be de-wetted from the dielectric layer 106, forming a partially elliptical droplet similar to the droplet 108' in fig. 2, and from there (when the drive voltage is removed) wetting the front transparent dielectric layer 114, thereby entering a second stable state generally similar to that shown in fig. 4 except that the oil layer is adjacent to the front transparent dielectric layer 114. In this second stable state, the pixel displays the color of the oil layer, which obscures the color of the aqueous layer 110'. It will be apparent that the pixel can be switched between two stable states whenever required. Moreover, the display has a threshold for switching, because a considerable voltage has to be applied to the display to switch it between its two stable states; such thresholds enable such displays to be driven using passive matrix methods rather than the more complex active matrix methods.

If dyes are used to color the oil layer 108 and the aqueous layer 110' in the display of fig. 4, it is important for the long-term stability of the display that these dyes do not migrate between the two layers. In practice, this does not pose great difficulties, since a wide variety of dyes are available which are soluble in water but insoluble in oil, or which are soluble in oil but insoluble in water. However, it is advantageous to use pigment particles or nanoparticles instead of dyes as colorants. Such pigment particles or nanoparticles may be provided with a coating (see, e.g., U.S. published patent application No.2002/1085378) to make them strongly aqueous or lipophilic so that they will not migrate between the oil and water layers.

Figure 5 of the accompanying drawings is a schematic side view, generally similar to figure 4, showing a microcell electrowetting display that operates in a manner similar to figure 4. Fig. 5 shows a single microcell of a display having a back wall (second substrate) 502, sidewalls 504 and 506, and a front wall (first substrate) 508 through which the display is viewed. The microcell further comprises a back transparent electrode 510, a back dielectric layer 512, similar to the corresponding numerals in fig. 1 and 4, adjacent to the second substrate surface of the microcell. However, the microcell also includes a side (second) electrode 514 (which need not be transparent) adjacent the sidewall surface and an associated side dielectric layer 516. The electrodes 510 and 514 are insulated from each other by an insulating block 518 from which a third electrode 520 is immersed in the colorless aqueous medium 110, the colorless aqueous medium 110 substantially filling the microcells. The microcells also comprise a colored oil phase 522.

The first stable state of the microcell shown in fig. 5 is similar to the first stable state in fig. 1 and 4; when no voltage is applied between the two electrodes, the colored oil phase 522 wets the back dielectric layer 512 so that an observer viewing the microcell through the front wall 508 sees the color of the oil phase 522 through the colorless aqueous medium 110. However, upon sudden application of a voltage between the back electrode 510 and the third electrode 520, the oil phase 522 will stop wetting the back dielectric layer 512 and will form a droplet that will pass through the electrode 520 and end up in the second stable state where it wets the side dielectric layer 516. In this second stable state, the color of the rear electrode 510 or rear dielectric layer 512 (either) is visible to an observer viewing the microcell through the front wall 508, either of which is colored (the oil phase 522 adjacent the side dielectric layer 516 occupies only a small portion of the cross-section of the microcell, substantially invisible to the observer). Alternatively, both the back electrode 510 and the back dielectric layer 512 may be colorless, and a colored backing or reflector may be provided behind the microcell, so that the microcell may operate in a "shutter mode," see U.S. Pat. Nos. 6,130,774 and 6,172,798.

It will be appreciated that in addition to the embodiments described above with reference to figures 1 to 5, a number of other types of colour electrowetting displays can be produced using a coloured oil phase which is moved to cover electrodes of other colours or possibly of different areas. It is known to apply an electric field between adjacent electrodes in the same plane to move a droplet by applying a voltage to the electrode adjacent to the electrode on which the oil phase resides, thereby causing the oil phase to move to the electrode to which the voltage is applied. This movement can of course be reversed. By using electrodes with different colors, alternative sizes, and colored oil phases, various effects can be produced. A simple monochrome display can be provided by moving the black oil phase between a large white electrode and a small black electrode; it is clear that when the black oil phase covers the white electrode, both the black and white electrodes appear black, whereas when the black oil phase is confined to a small black electrode, the overall appearance of the pixel is substantially white. More complex effects, including chromatic colors, can be created by using, for example, an oil phase having substantially the same color as the small electrode, while the adjacent large electrode is of a complementary color. In this way, a color display may utilize individual pixels with the following oil/electrode combinations:

red oil/small red electrode/large cyan electrode;

green oil/small green electrode/large magenta electrode; and

blue oil/small blue electrode/large yellow electrode.

Of such displays, the second color-shifting display of the present invention shown in FIG. 6 is useful. The display is substantially dumbbell-shaped with the dielectric surface having two circular portions 602 and 604 connected by a central substantially rectangular "neck" portion 606. There are three independently controllable electrodes, two circular electrodes 608 and 610 centered on the circular portions 602 and 604, respectively, and one rectangular electrode 612 centered on the neck portion 606.

The display shown in fig. 6 operates in the following manner. Assuming that the colored oil phase is disposed on circular portion 602, the aqueous phase is located above the colored oil phase and extends into contact with rectangular electrode 612 and circular portion 604. If it is desired to move the oil phase to occupy the circular portion 604, a voltage is applied to the electrode 608, thereby rendering the electrode hydrophilic, and no voltage is applied to the electrode 612, so the electrode becomes hydrophobic. Thus, the oil phase moves from portion 602 to neck portion 606. Next, a voltage is applied to the electrode 612 while maintaining the voltage applied to the electrode 608, but no voltage is applied to the electrode 610. Thus, the oil phase moves from the neck portion 606 to the circular portion 604. The formulation of the oil phase in the circular portion 604 is generally stable so once the oil phase is on the portion 604, no voltage is required to be applied to any of the electrodes.

Although circular portions 602 and 604 are shown in fig. 6 at the same size, these portions may of course vary in shape (e.g., a portion may be elliptical rather than circular), size, and/or color. Furthermore, one circular portion may be equipped with a masking member, similar to masking member 112 in FIG. 1, to obscure the presence of oil phase on the circular portion.

Fig. 7 is a schematic side view of a conductive via electrowetting display of the present invention. The display uses an aqueous (typically coloured) medium as its working fluid. The display of fig. 7 includes a substrate 702 formed of a hydrophobic dielectric material. Very high K dielectrics are preferably used for this purpose, for example ceramic high K dielectric suspensions, for example barium titanate in polar polymers, for example poly (vinylidene fluoride). Assuming that the entire substrate 702 is insulating, only the properties of the exposed upper surface of the substrate 702 (as shown in fig. 7) affect the operation of the display for the following reasons. As such, the substrate 702 may include a high-K hydrophobic dielectric exposed surface layer on the bottom of a low-K material, such as a polymer such as polyethylene or poly (ethylene terephthalate).

A plurality of spaced apart conductive vias 704 extend through the substrate 702 and terminate near its exposed upper surface. Each via 704 is covered by a thin cap member 706 in the form of a hydrophilic coating, which cap member 706 covers the end of the conductive via 704 adjacent the exposed upper surface of the substrate 702. Although only three vias 704 are shown arranged in a row in fig. 7, in practice a larger number may be used and the vias may be arranged in a two-dimensional array.

The aqueous working fluid, shown as droplet 708, rests on the exposed surface of substrate 702. In the absence of a voltage across any of the vias 704, the droplet 708 will not wet the hydrophobic surface of the substrate 702, but rather "dome" the cap member 706 surrounding the via 704 (which is not the state shown in fig. 7). However, when a voltage is applied across two adjacent vias 704 (say the middle and right vias in fig. 7), the surface portion of substrate 702 between these vias becomes less hydrophobic, so that droplet 708 spreads across the cap portion of the two vias to which the voltage is applied and the intervening portion of substrate 702, as shown in fig. 7. Given the appropriate choice of the characteristics of the exposed surfaces of the cap member 706 and the substrate 702, the droplet 708 will remain stable in the position shown in fig. 7, i.e., the droplet will remain in the same position even when the voltage is removed from between the two electrodes, since the droplet is "pinned" at the ends of the two vias by the cap member 706.

To move the droplet 708 to a different position, a voltage can be applied to, say, the middle and left vias 704. This causes the exposed surface portion of the substrate 702 between the through holes to be less hydrophobic, so the fluid will flow to the less hydrophobic portion of the surface, which assumes the form 708' shown in fig. 7. It is clear that more delicate manipulation of the aqueous fluid is possible, in particular using a two-dimensional array of through holes.

As previously mentioned, the invention also extends to the use of pigments and nanoparticles as colorants in electrowetting displays. Although electrowetting displays have hitherto used dyes dissolved in an oil and/or aqueous phase, the dyes in solution are susceptible to long-term effects of electromagnetic radiation, in particular ultraviolet radiation, which tend to lead to discoloration and/or discolouration of the dyes, which effects may limit the working life of the electrowetting display. The use of pigments or nanoparticles instead of dissolved dyes increases the working life. The use of pigments or nanoparticles also allows the surface properties of the pigments or nanoparticles to be controlled, for example, by forming charged or chargeable groups or polymers thereon. (see, for example, U.S. published patent application No. 2002/0185378).

Claims (4)

1. A display, comprising:

a substrate (102);

a first fluid (108) adjacent to the substrate (102), the first fluid (108) absorbing at least one light wavelength;

a light-transmissive second fluid (110) immiscible with the first fluid (108); and

at least one electrode (104) for applying an electric field to the first fluid (108);

in the absence of an electric field, such that the first fluid (108) covers a first area of the substrate (102), but upon application of an electric field to the first fluid (108) via the at least one electrode (104), the first fluid (108) moves to a second area smaller than the first area,

the display is characterized in that:

a concealing member (112) spaced from the substrate (102) and made of a substantially opaque material,

such that upon application of an electric field to the first fluid (108), the first fluid (108') becomes substantially confined between the concealing means (112) and the substrate (102), so that the concealing means (112) substantially conceals the first fluid (108) from view to an observer viewing the display from the opposite side of the concealing means (112) from the substrate (102).

2. A display according to claim 1, wherein the substrate (102) comprises a dielectric surface layer (106) adjacent to the first fluid (108).

3. A display according to claim 1, wherein the substrate (102) comprises a coloured or reflective layer.

4. The display of claim 1, wherein the substrate (102) has a substantially planar surface, the concealing member (112) comprising a substantially planar portion that extends along a substantially planar surface that is substantially parallel to the substrate (102) but spaced from the substantially planar surface of the substrate (102).