Microscale engine swimming underwater powered by marangoni convection

This study proposes a prototype of a microscale engine swimming underwater powered by Marangoni convection. The engine is a layered disk with a hole and holds a bubble ring in a gap between the layers. When the liquid-gas interface at the hole edge has the surface tension gradient, the engine gains the thrust force as the reaction of Marangoni convection. In a temperature Marangoni convection experiment, one side of the engine was heated by a laser, to make temperature gradient on the liquid-gas interface. It was confirmed that Marangoni convection was generated while the expanding bubble plugged the hole, to prevent the flow penetrating through the hole. In a concentration Marangoni convection experiment, pure water and acetic acid were injected toward the hole at each side, to make concentration gradient. The engine successfully generated jet-like flow through the hole to drive itself by 1.2 mm. PIV analysis visualized the flow field around the engine and the velocity profile of the jet. The jet direction was not stable because of the non-uniformity of the concentration on the gas-liquid interface and the magnitude of the jet velocity gradually diminished with the diffusion of acetic acid. The thrust force of the engine was estimated as 270 nN by calculating the momentum conservation equation of the flow around the engine.