Water, electricity, and between … On electrowetting and its applications

Imagine a drop of water lying on a surface, pulled into a ball by surface tension. With electricity it is possible to change the shape of the drop and cause it to flatten out. This is electrowetting, a physical phenomenon which has aroused great interest in recent years as it has found new applications. Here we will describe the phenomenon and two of its applications: variable-focus liquid lenses and paper-thin, video-rate, reflective color displays. The physics of wetting Go into the kitchen and do a small experiment: place a drop of water on a smooth, clean glass surface (a plate, for example), and another drop on a Teflon frying pan or on greased baking paper. You will be able to see the difference in the behavior of the drops: on the glass the drop flattens out whereas on the Teflon or the greased paper it turns into a ball. We say the drop wets the glass, whereas on a hydrophobic (‘‘water hating’’) surface such as Teflon the wetting is only partial (Fig. 1). For a general review see Ref. 1