We report the rotary motion of an aqueous microdroplet in an oil phase under a stationary direct current electric field. A droplet exhibits rotary motion under a suitable geometrical arrangement of positive and negative electrodes. Rotary motion appears above a certain critical electric potential and its frequency increases with an increase in the potential. A simple theoretical model is proposed to describe the occurrence of this rotary motion, together with an argument for the future expansion of this micro rotary motor system.

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See supplementary material at http://dx.doi.org/10.1063/1.3358385 for a movie of the rotary motion of two droplets, and detailed description for the numerical simulation.

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