Energy consumption and conversion efficiency for a micromotor under DC voltage

A water droplet works as a novel kind of micromotor exhibiting back-and-forth motion between two needle shaped electrodes in an oil phase under a stationary direct-current voltage. In the present study, a quantification method was adapted to estimate the kinetic energy output of back-and-forth motion of a water droplet, together with the electric energy input to this system. It was observed that the volume of the droplet gradually decreased accompanied by the periodic motion, which is attributed to electrolysis. Energy consumption owe to the electrolysis vs. the generation of kinetic energy of the moving droplet experimentally evaluated. The kinetic energy was deduced based on Stokes' drag equation. Electric current was measured at the pA level. Then, the energy conversion efficiency of this micromotor system was able to be calculated. As far as we aware, this is the first report on the conversion efficiency for the autonomous motion under stationary DC voltage. Until now, various kinds of micromotor systems have been individually developed by different study group, and the comparison was difficult due to the lack of universal reliable index of the efficiency. The quantification method reported here evaluates micromotor system on its energy conversion efficiency, irrespective of experiment conditions. This quantification method can be a universal tool for micromotor system comparison, and is expected to play the role as the guidelines for future developments.