Abstract

Interfaces control many aspects of the thermodynamics and dynamics of multiphase materials. Not surprisingly, questions about the dynamics of interfaces, e.g. their flow and response to forces, occur in colloid science, fluid mechanics and other areas of science and engineering. In this talk we describe two distinct problems that we have studied where interfaces control the mechanical response of the system. First, we describe an experiment where the degree of miscibility of an amphiphile controls the area over which a surface-tension-driven flow is observed at the interface of water and air. We report universal features of the surface velocity distribution and identify a scaling law for the area over which the flow is observed. An analytical description is offered for some of these observations. Second, we study bubble bursting at a compound air/oil/water-with-surfactant interface and document the hitherto unreported formation and dispersal into the water column of submicrometre oil droplets. We show that dispersal results from the detachment of an oil spray from the bottom of the bubble towards water during bubble collapse. Perhaps surprisingly, we provide evidence that the droplet size is selected by physicochemical interactions between oil molecules and the surfactants rather than by hydrodynamic effects.