Nonconductive droplet oscillations drive flows through alternating current electrowetting

A micro-underwater robot driven by an alternating current (AC) electrowetting mechanism has gained significant attention in recent years due to its strong maneuverability, simple structure, and low energy consumption. Currently, several micro-actuators based on the AC electrowetting principle have been developed globally. However, the propulsion processes of these micro-actuators generally occur at the gas–water interface, which differs from the ideal working environment of underwater robots. To further facilitate the advancement of micro-underwater robots, this work proposes a potential underwater propulsion method. The method utilizes AC electrowetting to periodically drive oil droplets attached to solid surfaces underwater, causing oscillation of the oil–water interface and generating capillary waves, which in turn produce thrust. In this work, we investigate the oscillation behavior of oil droplets driven by electrowetting in a water environment, analyze the effect of surface charge trapping on the oscillation process, and confirm the oscillation model of the oil droplets as well as the secondary flow propagation model generated by the oscillation. Furthermore, a theoretical method is proposed for calculating the momentum transfer capacity of oscillatory flow under low-frequency oil droplet oscillation. Calculations show that the maximum thrust generated by the oscillation of a 5 μl oil droplet can reach 4.2 μN. Based on the proposed propulsion method, a cylindrical prototype with a total mass of approximately 0.3 g was designed, which achieved a maximum speed of 1.25 mm/s underwater.