Conversion Mechanism of Front-Side Marangoni Convection to Droplet Motion in Continuously Moving Self-Propelled...

When a 1-hexanol droplet is laid down on a hexanol aqueous solution surface, it exhibits a self-propelled motion. The Marangoni flow in front of the droplet plays an important role in the self-propulsion, but it was so far unclear how the flow is converted into the driving force. In this study, the time-resolved surface tension, surface flow speed, water surface height, and droplet position of a unidirectionally moving hexanol droplet were simultaneously measured and converted to position distributions. The flow in the aqueous solution was also visualized by using particles. The results of the surface tension difference between the front and the rear of the droplet were not dependent on the droplet speed, whereas the surface flow speed and the water level difference between the front and the rear of the droplet were clearly dependent on the droplet speed. Because 90% of the relative flow below the droplet was directed backward, the viscous force at the bottom of the droplet mainly decelerated the droplet. These results suggest that the self-propelled motion originated by the Marangoni convection at the front of the droplet was converted into the pressure difference caused by the change in water level between the front and the rear of the droplet.