WO2006021912A1

WO2006021912A1 - Display device based on immiscible fluids

Info

Publication number: WO2006021912A1
Application number: PCT/IB2005/052695
Authority: WO
Inventors: Robert A. Hayes; Ivo G. J. Camps; Bokke J. Feenstra; Ronald M. Wolf; Anthony R. Franklin
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-08-26
Filing date: 2005-08-16
Publication date: 2006-03-02
Also published as: TW200619685A

Abstract

Display device based on immiscible fluids located between a transparent substrate and a second substrate. One of the fluids is electroconductive or polar. The transmission of the display elements is determined by voltages applied between the electroconductive or polar fluid and electrodes provided on the transparent substrate. In a reflective display device based on layer break-up or layer displacement having at least two different states. One of the fluids (e.g. oil) in a first state adjoins at least a first support plate and in the second state the other fluid at least partly adjoins the first support plate. The picture elements have a modified shape to facilitate control of the first fluid in one of the extreme transmission states.

Description

DISPLAY DEVICE BASED ON IMMISCIBLE FLUIDS

The invention relates to a display device comprising in a display element at least one first fluid and a second fluid immiscible with each other within a space between a first transparent support plate provided with electrodes and a second support plate, the second fluid being electroconductive or polar, the transmission of the display clement being controlled by a voltage between the second fluid and the electrodes on the first transparent support plate.

Such a (color) display device is used especially, but not exclusively in paper- like applications, but the invention is also applicable to faster (video) screens.

If the first fluid is a (colored) oil and the second fluid is water a two-layer system is provided which (due to interfacial tensions) comprises a water layer and an oil layer. However, if a voltage is applied between the water and an electrode on the first support plate the oil layer moves aside or breaks up due to electrostatic forces. Since parts of the water now penetrate the oil layer the picture element becomes partly transparent.

Display devices based on this principle (electro-wetting) have been described in PCT- Application WO 03/00196

When used in a reflective display the optical performance of a picture element depends a. o. on the optical intensity of the colored film in the non-reflective state (off state) Dyes (or sometimes pigments) are added to the oil (usually hydrocarbon but also possibly silicone or fluorocarbon), to provide sufficient optical intensity. Using the electro-wetting display principle the brightness in the reflective state (on state) is dependent on the scattering properties of the (underlying) substrate and the area fraction occupied by residual oil.

In a display device picture elements typically have a square shape. In this case, in a display device based on the electro-wetting principle it has been observed that, when a voltage is applied the oil generally moves to one of five positions, viz. the four picture element corners or to the center of the picture element.

It is one of the objects of the present invention to overcome at least partly the above-mentioned requirement.

The invention is based on the understanding that the geometry of the picture elements may be chosen in such a way that, viewed perpendicular to the first transparent support plate, an area of the first fluid in one of the extreme transmission states is defined unambiguously.

Now, when a voltage is applied the oil splits in each picture element is the residue reproducibly throughout an array of picture elements. For all picture elements a region is unambiguously created now, where the first fluid contracts itself. This region can be used, if the first fluid is non-transparent to hide a switching element. Moreover when using, in a color display, a stack of three monochrome displays (e.g. yellow, cyan, magenta) these regions are placed above each other to enhance aperture.

A single oil residue per reservoir is however preferred. The inventors surprisingly have observed that this can indeed be achieved by in fact reducing the symmetry of the picture elements, rendering a defined location of the oil reservoirs. One example of this is to use "delta" (trapezoid) shaped picture elements. In this case the oil was found to exhibit the tendency to migrate preferentially to the shortest side or "point" of the picture element. Increasing the ratio of the lengths of the parallel sides enhanced this effect. For example a ratio 3:1 gives better control of the fluidic motion than 2:1. Arrays of isosceles triangles also offer more advantageous fluidic control. Changes to the shape of the picture element, in particular the picture element corners, also offer a manner of controlling the fluidic behavior. The shape of the picture element corners is modified to facilitate the fluidic control. Especially rounded corners facilitate movement, while corners defined by lines result in pinning of the oil film. It is noted here that such corners in fact are a mathematical idealization, because in reality the forms of such corners are defined by finite elements, not by ideal lines. In this respect it is also noted that the above-mentioned ratio for isosceles triangles theoretically have an infinite value. In reality, due to processing and other effects the ratio for isosceles triangles has a finite value. In a preferred embodiment at least a part of the edge of a picture element viewed perpendicular to the first transparent support plate has a curvature. Very good results are obtained with picture element being substantially hexagonal layouts.

On the other hand also picture element having a substantially circular form (if necessary being cut off along by a substantially straight line) showed good results. In a further embodiment the picture elements viewed perpendicular to the first transparent support plate have an extension near the area of the first fluid in said extreme transmission state. Now a reservoir is provided near the "sharp corner" of the picture element. In this way the display aperture, and thus optical performance of the display can be maximized Again a region is created in which for all picture elements a region, where the first fluid contracts itself, is unambiguously defined. This region can be used favorably again to hide a switching element or in a stack of three monochrome displays.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:

Figure 1 is a diagrammatic cross-section of a part of a display device, to show the principle on which a display device according to the invention is based, Figures 2 shows a further diagrammatic cross-section of a part of such a display device,

Figure 3 is a diagrammatic plan view of a part of a display device according to the invention, while

Figure 4 are diagrammatic plan views of parts of further display devices according to the invention and

Figures 5, 6 and 7 are diagrammatic plan views of parts of further display devices according to the invention and

The Figures are diagrammatic and not drawn to scale. Corresponding elements are generally denoted by the same reference numerals.

Fig. 1 shows a diagrammatic cross-section of a part of a display device 1 which shows the principle on which a display device according to the invention is based. Between two transparent substrates or support plates 3, 4 a first fluid 5 and a second fluid 6 are provided, which are immiscible with each other. The first fluid 5 is for instance an alkane like hexadecane or as in this example colored oil. The second fluid 6 is electroconductive or polar, for instance water or a salt solution (e.g. a solution of KCl in a mixture of water and ethyl alcohol).

In a first state, when no external voltage is applied (Fig. Ia, off state) the fluids 5, 6 adjoin the first and second transparent support plates 3, 4 of e.g. glass or plastic. On the first support plate 3 a transparent electrode 7, for example indium (tin) oxide is provided and an intermediate less wettable (hydrophobic) layer 8, in this example an amorphous fluoropolymer (AF 1600). When a voltage is applied (voltage source 9) via interconnections 20, 21 the layer 5 moves aside or breaks up into small droplets (Fig. Ib, on state). This occurs when the electrostatic energy gain is larger than the surface energy loss due to the creation of curved surfaces. As a very important aspect it was found that reversible switching between a continuous film 5 covering the support plate 3 and a film adjoining the wall 2 is achieved by means of the electrical switching means (voltage source 9).

Figure 2 shows an embodiment of a display device according to the invention, in which walls between separate picture elements have been omitted for the greater part for the sake of clarity. In this embodiment the picture element walls 13 do not extend across the whole picture element thickness. Such walls may be obtained by offset printing or other printing techniques known in the art. It appears that the oil film 5 is very stable, which is enhanced even further as the picture element size decreases. So during switching the oil remains confined in each area. The other reference numerals have the same meaning as those in Figure 1. The display has been made reflective by adding a reflector 14 as shown in

Figure 2. The optical performance of a display picture element depends essentially on the optical intensity of the colored film in the off state as well as the reflectivity and exposed part of the reflector 14 upon switching. Dyes (or sometimes pigments) are added to the oil (usually hydrocarbon but also possibly silicone or fluorocarbon), to provide sufficient optical intensity.

Figure 3, in a plan view, shows picture elements 15 of a part of a display, having a "delta" (trapezoid) shaped layout. The layout may be obtained by either the process (e.g. a lithographic process) defining the picture element walls 13 or by the lay-out (defining of the indium (tin) oxide) of the transparent electrodes 7. As mentioned in the introduction, using this layout offers less freedom of oil movement, than a rectangular picture element which itself again offers less freedom of oil movement than a square picture element. The oil in a rectangular picture element was found to exhibit the tendency to migrate preferentially to the shortest side of the picture element, when a voltage was applied. In fact oil motion was found to be parallel to the longer picture element side and because of the tendency for the oil to become pinned at one, or both, picture element end(s) the oil residue is now restricted to one or two positions in the picture element.

By reducing the symmetry of the oil reservoirs further for example by using the "delta" (trapezoid) shaped picture elements of Figure 3 a single oil residue per picture element is realized. Now the oil migrates preferentially to the shortest side or "point" of the picture element (Fig. 3, arrows 16). This effect can be enhanced as the ratio of the length h of the picture elements with respect to their width w is increased. For example a ratio 3:1 gives better control of the fluidic motion than 2:1.

In the embodiment of Figure 3 the aspect ratio may affect both the maximum white area attainable (and thus display brightness and contrast) and also gray level control. Figure 4 shows some picture element patterns based on essentially square picture elements 15. Essentially the shape of the picture element corners is modified to facilitate the fluidic control. Rounded corners facilitate movement, while sharp corners result in pinning of the oil film. At the area where the oil is pinned a switching element, such as a TFT -transistor may be placed below the electrode. The switching element normally is defined next to the electrode. The dimensions generally are such that the oil covers the region of the switching element in both states, since this switching element is covered by the intermediate less wettable (hydrophobic) layer 8.

The separation between picture elements (typically 5 m) is exaggerated relative to the picture element size (typically 250 or 160 m) in the figure to facilitate its explanation. The arrows 16 in separately shown picture elements 15 ' designate the direction of oil movement. Figure 4A shows a single curved picture element corner, whereas Figure 4B shows a decreasing curvature gradient between initiation corner 17, intermediate corners 18 and residue corner 19. In the example of Figure 4C a reservoir 22 is provided at the "sharp corner" 19 of the picture element, which can, if necessary, be accommodated by the curvature in the corner of the adjacent picture element. In this way the display aperture, and thus optical performance of the display can be maximized.

Figure 5, in a plan view, shows picture elements 15 of a part of a display, having a hexagonal layout. The hexagon may have only one rounded corner, as shown by broken liners 17 and the rounded line 19, or more rounded corners as shown by line 18 Using this lay-out offers greater aperture. The layout of the picture elements 15 of Figure 6 has substantially circular form being cut off along by a substantially straight line 23. Figure 7 shows a further possible layout. The reference numerals in Figures 5, 6, 7 have the same meaning as those in Figures 3,4.

Several variations of the principle are possible. Although a reflective device has been described the invention also applies to transmissive display devices.

The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A display device comprising in a display element at least one first fluid (5) and a second fluid (6) immiscible with each other within a space between a first transparent support plate (3) provided with electrodes and a second support plate (4), the second fluid being electroconductive or polar, the transmission of the display element being controlled by a voltage between the second fluid and the electrodes on the first transparent support plate the geometry of the picture elements viewed perpendicular to the first transparent support plate defining the area of the first fluid in one of the extreme transmission states.

2. A display device according to Claim 1 the picture elements viewed perpendicular to the first transparent support plate having a trapezoid or triangular shape.

3. A display device according to Claim 3 the ratio of the length h of the picture elements with respect to their width w being at least 2: 1.

4. A display device according to Claim 1 the picture elements viewed perpendicular to the first transparent support plate having a substantially rectangular corner opposite to a rounded corner.

5. A display device according to Claim 1, 2 or 4 the picture elements viewed perpendicular to the first transparent support plate having an extension near the area of the first fluid in said extreme transmission state.

6. A display device according to Claim 1 at least a part of the edge of a picture element viewed perpendicular to the first transparent support plate having a curvature.

7. A display device according to Claim 6 the picture element being substantially hexagonal.

8. A display device according to Claim 7 the picture element having at least one rounded corner.

9. A display device according to Claim 6 the picture element having a substantially circular form.

10. A display device according to Claim 6 the substantially circular form being cut off along by a substantially straight line.

11. A display device according to Claim 6 the picture elements viewed perpendicular to the first transparent support plate having an extension near the area of the first fluid in said extreme transmission state.

12. A display device according to Claim 1 the picture elements the area of the first fluid in the one of the extreme transmission states overlying a switching element.