Electric fields move water droplets

It is usually a good idea to keep water away from electrical equipment but researchers in Japan have discovered a new effect by breaking this rule. Masahide Gunji and Masao Washizu of the University of Tokyo have shown that electric fields can be used to move water droplets around a solid surface.

Their work could lead to new ways to perform chemistry experiments much faster than is possible at present.

The Japanese team began by making a pair of parallel-strip electrodes on a glass substrate that had been covered by a thin electrically insulating hydrophobic layer and then placing a water droplet on the surface. The droplet, which had a volume of between about 1 nanolitre and 1 microlitre, adopted an almost spherical shape due to a combination of surface tension and wetting effects.

To their surprise, the team found that the droplet began moving at speeds of up to 10 centimetres per second when a voltage was applied across the electrodes. The initial direction of motion was unpredictable and the droplet continued to move until it reached the edge of the device or the voltage was switched off. Moreover, when the electrodes were made into the shape of a racetrack, the droplet kept travelling around the track.