Two groups of researchers have advanced the use of electrowetting droplets for displays, with one demonstrating a new way of exploiting the optical geometry of the liquid used, and another developing video-speed performance and a new strategy for incorporating color. The first team, from the University of British Columbia (UBC; Vancouver, B.C., Canada) has developed a novel approach that is based on the fact that the hemispherical droplets used in electrowetting systems can be used as reflectors. The second approach, demonstrated at Philips Research Laboratories (PRL; Eindhoven, The Netherlands), exploits the fact that the technique can be used to maximize and minimize the coverage of the display with a colored "ink." Both approaches have the advantage of high brightness and contrast, which may eventually make the technology suitable for implementing electronic paper.
In electrowetting, a voltage is placed across a liquid droplet on some surface, causing the latter's wettability to change. As a result, the contact area of the droplet is altered, changing the geometry of the droplet (for another use of this effect, see Laser Focus World, November 2003, p. 17). If the conditions are right, the repeatability of this geometrical change is very high, thus allowing the droplet to be exploited as a kind of switch.
Droplet as partial retroreflector
In the UBC work, the switching takes place by using the droplet as a partial retroreflector.1 When there is no voltage across the device, the transparent surface is quite hydrophobic. As a result, the droplet minimizes its contact and sits higher above it, forming a shape that is nearly spherical. When the voltage is applied, however, the surface area increases, turning the droplet into a hemisphere. In this case, light comes in through the surface and droplet, reaches the liquid-air interface, and is totally internally reflected (as long as it enters at less than the critical angle). It then continues to be reflected each time it hits the interface until it leaves the device and is returned to the eye of the viewer. Because total internal reflection is 100% efficient, the reflectivity of such devices is very high; however, the angular dependence does mean the pixel shape is annular.
A different approach
The Philips approach is very different.2 When their system is off, a layer of colored oil prevents water from having any contact whatsoever with a white substrate; thus, the color of the pixel is the color of the oil. When voltage is used to change the wettability, however, the water does make contact and the oil droplet is forced to minimize the area it takes up. The result is that the color is essentially shoved into a corner, and the white below is allowed to show through. Because there is no filter between the viewer and the substrate, the reflectance is more than 40%; the reflectance of paper is 60%. Likewise, with a contrast ratio of 11, the electrowetting technology is not far behind paper at 15. Further, with a response time of just 10 ms, the technique is fast enough to show video content.
To make a multicolor display, the team has found a way of including cyan, magenta, and yellow (the subtractive colors) in a single filter. Each pixel consists of a passive colored filter (C, M, or Y), with the other colors as oil droplets at the top and bottom of the pixel. These two active filters can be switched independently, allowing the three colors to be mixed as required. The different types of pixel, with the different passive filters, would be distributed across the pixel array to provide the full color display.
REFERENCES
- V. H. Kwong et al., Applied Optics 43(4) (Feb. 1, 2004).
- R. A. Hayes and B. J. Feenstra, Nature 425, (Sept. 25, 2003).
