Spontaneous oscillations of a sessile lens

Abstract

When an oil drop is placed on a water surface, it assumes the form of a sessile lens. We consider the curious behaviour that may arise when the oil contains a water-insoluble surfactant: the lens radius oscillates in a quasi-periodic fashion. While this oscillatory behaviour has been reported elsewhere, a consistent physical explanation has yet to be given. We present the results of an experimental investigation that enable us to elucidate the subtle mechanism responsible. Videomicroscopy reveals that the beating behaviour is generated by a subtle process of partial emulsification at the lens edge and sustained by evaporation of surfactant from the water surface.