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Self-propulsion of a water droplet in an electric field

In this paper, we describe electric field driven self-propulsion of a water droplet on a solid surface using co-planer parallel-strip electrodes fabricated on a substrate and covered by a hydrophobic coating. A water droplet dispensed on the electrodes, ranging from about 1 nl to 1 µl in volume, takes a nearly spherical shape owing to surface tension and wetting conditions. When the electrodes are energized by either dc or low-frequency single-phase ac voltage, elongation due to the electrostatic force occurs, but translational motion is not expected because the electrostatic potential energy is constant regardless of the droplet's position. However, under certain conditions we observe that the droplet starts moving along the electrodes at speeds of as high as 10 cm s−1. The initial direction of the motion is unpredictable, but once started, it persists until the droplet reaches the end of the electrodes or the voltage is turned off. The motion is of a self-propelling nature; the water droplet leaves behind a moisture layer on the substrate surface, which shields the electric field on the droplet's trailing edge, thus producing an imbalance in the Maxwell stress, which leads to the driving force. The motion seems to be most controllable and regular when the droplet is driven at its hydrodynamic resonance. The mechanism is experimentally verified by real-time measurements of the conductance of the moisture layer.