A microfluidics path to harvesting mechanical energy

Imagine a drop of water on a solid, pulled up into a ball by surface tension. The simple application of voltage between liquid and solid alters the interfacial energy and prompts the droplet to spread out, an effect known as electrowetting; the greater the voltage, the greater the spread. In the past decade, researchers exploiting the effect have developed, among other applications, liquid lenses with voltage-tunable focal lengths and microfluidic circuits that store and steer droplets without the need for pumps or mixers. Tom Krupenkin and Ashley Taylor at the University of Wisconsin–Madison have now developed an approach that runs the process in reverse—converting the mechanical energy of liquid motion into electrical current. In one implementation, they pressurized a fluidic channel to force a train of mercury droplets past dielectric-coated electrodes connected to a bias voltage on the order of tens of volts. As the overlap area between the droplets and electrodes changed, so did the charge stored at their interface, giving rise to an alternating current that can drive a load. The researchers measured a few milliwatts from a channel containing 22 droplets. But from their model of the process they calculate that average powers of 1 W or more could easily be generated in a fluidic device with 1000 flowing droplets. The devices are small enough to fit into a pair of shoes; with each step, fluid is squirted back and forth between the heel and toe. (T. Krupenkin, J. A. Taylor, Nat. Commun. 2, 448, 2011 .)—R. Mark Wilson