Deformation dynamics of an oil droplet into a crescent shape during the intermittent motion

A paraffin droplet containing camphor and oil red O dye floating on the water surface exhibits spontaneous motion with deformation by generating a surface tension gradient around the droplet. We focused on the intermittent motion with a pronounced deformation into a crescent shape observed at specific concentrations of camphor and oil red O. We quantitatively analyzed the time changes in the droplet deformation and investigated the role of the oil red O by measuring the time-dependent oil/water interfacial tension with the pendant drop method. The observed effect can be explained by adsorption of the oil red O molecules at the oil/water interface: the interfacial tension decreases gradually after the interface formation, resulting in the dynamic deformation of the droplet. The combination of the decrease in interfacial tension and the decrease in driving force related to camphor outflow generates intermittent motion with dynamic deformation into a crescent shape.