A multilayered solution

E-READERS, such as Amazon’s Kindle, have been a commercial success. They have not, however, revolutionised the publishing industry in quite the way that many predicted they would. In part, that is because their displays are black and white, and they seem to many readers to be slow, grainy and—if truth be told—a little archaic. Better screens might make the difference between e-readers being intriguing gadgets and killer apps, and Shin-Hyun Kim and David Weitz, who work at the Experimental Soft Condensed Matter group at Harvard University, think they may have found a way to build those better screens.

Unlike conventional display screens, which are lit from behind, e-readers use reflected light in a way similar to paper. Letters and other characters on the screen are formed out of ink that has a high optical contrast with the background, making them easy to read. The difference is that, rather than being printed into permanent shapes like normal ink, electronic ink is held in small capsules that can reveal it or hide it as required. 

The result is legible even in bright sunlight. But it often takes more than half a second to “turn” the page of an e-book (so displaying the 25 images a second needed for video is out of the question). And, although the size (roughly 100 microns across) of the elements, known as pixels, that make up the display is fine for monochrome reading, they would need to be a third of that or less to create sub-pixels of the three primaries (red, green and blue) that colour displays require. The answer proposed by Dr Kim and Dr Weitz, in a paper in Angewandte Chemie, is to change the way e-ink is manufactured.

At the moment, such ink is composed of small, transparent spheres containing black and white particles suspended in a clear fluid. Half the particles are white and positively charged. The other half are black and negatively charged. When an electric field is applied, one lot is drawn towards it while the other is repelled. A negative charge attracts the positive particles, making the pixel appear black. A positive charge does the reverse.

The problem, according to Dr Kim and Dr Weitz, is that the densities of the black and white particles are different, and therefore cannot both be made to match that of the fluid in which they are immersed. This slows down their movement, and thus the speed at which a screen can refresh its image. A better solution would be to immerse the black particles in one fluid and the white particles in another (so that in both cases their densities match that of the suspending liquid)—yet, at the same time, to continue to package both types of particle in a single sphere. 

To do so, the pair turned to a technology called microfluidics, which borrows from the techniques used to make computer chips to produce devices that mix small amounts of liquid in precise ways. Their own, particular device uses tiny channels to force two different liquids (one of which contains ink particles) in one direction down a channel, through a nozzle, thus bringing them into contact with two other streams travelling in the opposite direction. As the four streams collide they are forced into a third channel, forming layered droplets as they go. 

Normally, this sort of single-step mixing would not work, because of the difficulty of getting two liquids to flow stably through one channel. However, by using an oily liquid and an aqueous one, and by covering one side of the channel with a substance that repels water and the other side with one that attracts water, this can be avoided. The result is a “Russian-doll” droplet that, if the correct oily and aqueous liquids are chosen, can be made permanent by curing some of its layers into transparent polymers using ultraviolet light.

To demonstrate, Dr Kim and Dr Weitz created what they call magnetic ink. This consists of an oily core containing magnetic particles mixed with carbon black, which is suspended within a watery layer that contains white polystyrene particles. That, in turn, is suspended in a transparent oily fluid. 

Like those in e-reader displays, the black and white particles can be drawn towards or away from the viewing surface (in this case using a magnetic field, rather than an electric one). They move much faster than those in traditional displays, though, because their densities are closer to those of the suspending media. If the new droplets can be incorporated into real screens, that will deal with the slow refresh rate.

The next stage is to include all three primary colours in a single droplet. That is some way off. But if it proves possible, it will deal with the black-and-whiteness problem, too, by providing full-colour pixels that have the same number of droplets as monochrome ones.

Turning this invention into a screen will take time. Indeed, it may never come to pass, for many other groups are approaching the e-reader problem from different directions. But whatever happens to this specific idea, Dr Kim’s and Dr Weitz’s invention is likely to have larger ramifications. It might, for example, be used to package together drugs in slow-release capsules of greater sophistication than is now possible.