Capillary-driven water-walking actuators*

We introduce simple water-walking actuators that operate in both active and passive modes. The active water-walking actuator is inspired by the water-skiing motion of Microvelia, which moves on water surfaces without mechanical motion by secreting body fluid from its rear to generate Marangoni thrust. This active actuator is equipped with a reservoir containing a volatile liquid with low surface tension at the rear. The vapor emanating from the liquid lowers the surface tension of the adjacent water surface, enabling the actuator to move forward. The passive water-walking actuator is inspired by the movements of beetle larvae, which move across water surfaces due to the Cheerios effect. By placing solid objects or immiscible liquid droplets onto water surfaces channeled through a narrow conduit, we found that these objects or droplets could spontaneously advance on the water surface due to the Cheerios effect. We use a high-speed camera to visualize the motions of both active and passive water-walking actuators and analyze their distinct features by combining experimental and theoretical approaches.