LIQUID crystal flakes are the latest contender in the race to produce full-colour electronic paper.
The aim is to come up with a roll-up display that combines the versatility of a computer screen with the readability of paper. Now Kenneth Marshall at the University of Rochester in New York says he has found a way of making flakes of a liquid crystal polymer that can reflect either red, green or blue light when switched on. This gives them one important advantage over competing technologies, says Marshall: they naturally create high-resolution colour without the need for complex or clumsy colour filters.
Each flake is made up of many layers of rod-like molecules. In each layer, the molecules lie parallel to each other, but crucially the direction of the molecules shifts slightly from one layer to the next, forming a helix as you progress through the molecule (see Graphic). The colour that a particular flake reflects depends on the pitch of the helix-the length of a full 360掳 rotation. The pitch is determined in manufacturing through the choice of polymer.
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Like all liquid crystal displays, Marshall鈥檚 flakes work by interacting with polarised light. Each helix interacts with light in a narrow range of frequencies. First the light is circularly polarised. 鈥淚t is broken down into two components because it interacts with the macroscopic helical structure,鈥� explains Marshall. Half of the circularly polarised light is reflected back while the rest passes through the flake.
So far Marshall鈥檚 team has made simple devices with flakes suspended in an oily fluid sandwiched between two glass plates. The flakes are switched on or off by applying a voltage across the fluid which causes the flakes to change their orientation.
鈥淭he only downside is that you are only going to use 50 per cent of the light,鈥� says Marshall. But this could be easily improved, he believes, by having polarising reflectors behind the flakes reversing the polarity of light and allowing some of the remaining light to pass through the flakes again.
Marshall鈥檚 technique will have to compete with e-paper from companies such as Massachusetts-based E Ink and Gyricon, a spin-off from Xerox. 鈥淭he other folks have been at it for five or six years now,鈥� he says. 鈥淲e are just beginning.鈥� The materials have not yet been optimised, he says, leaving plenty of room for improvement.
The technology could have a variety of other uses, too. Marshall envisages applications such as adaptive camouflage that changes the colour of a vehicle to suit its surroundings, or paint that will change the colour of a room at the flick of a switch. But, despite a lot of hype, no one has yet produced coloured e-paper that is anywhere close to being foldable. All the attempts so far, including Rochester鈥檚, are complex multilayer constructions involving glass substrates.