E-readers, such as the Kindle and iPad, are quickly becoming an everyday part of our lives. However their two familiar screens operate quite differently. While the iPad uses a conventional LCD display, the Kindle’s screen is based on the phenomenon of electrowetting. Electrowetting (EW) describes how liquids spread out over a hydrophobic surface when an electric field is applied; by manipulating colored liquids suspended in oils, images can be produced. While backlit LCD displays sap power, EW e-readers can hold an image without expending energy, and come close to replicating the experience of reading from paper, as they do not need to be lit from behind.
Unfortunately the technology is not particularly cheap compared to traditional print, and EW e-readers have some way to go before the experience is truly comparable to reading from a page. However, it has now been demonstrated that the electrowetting properties of paper can be as good to those of glass, meaning that the next generation of electronic paper devices, could be constructed from paper [Kim and Steckl, ACS Appl Mater Interfaces, (2010) 2, 3318].
Kim and Steckl investigated three different types of commercially available paper, by depositing thin layers of copper, a dielectric (parylene) and a hydrophobic fluroploymer (FluoroPel) onto each. Each system was then immersed in oil, while a deionized water droplet was added for measurement. The key parameter for electrowetting devices is the angle between the sides of the droplet and the contact surface. They found that the roughest paper exhibited the largest change in contact angle with applied voltage: a change that was even larger than that observed for glass. However, the smoothest paper was found to provide the best overall surface for a paper based device, thanks to the high saturation voltage and low hysteresis. It is critical that and hysteresis effects are small if the device is to be used to display video. The switching time for the smooth paper was also incredibly good, and only slightly larger than the value for glass.
As well as reducing cost and helping replicate a more traditional reading experience, the disposal of paper based devices should also be much easier and environmentally friendly; not that you should be throwing away your old e-reader just yet. It’s unlikely that we’ll see these devices in time for next Christmas, but perhaps within the next decade we’ll be able to watch color video on bright, cheap, disposable screens.

Stewart Bland