Surfactant-assisted spreading of an emulsion on a liquid bath

We studied experimentally the spreading dynamics of a drop of a surfactant-stabilized oil-in-water emulsion over the free surface of a layer of pure water. The dynamics display three regimes. After the deposition of the emulsion drop, oil droplets are advected by a Marangoni flow, due to the difference in surfactant concentration between the emulsion and the liquid surface, and spread at the air/liquid interface. The oil droplets eventually stop, forming a dense ring, whose diameter is constant as long as oil droplets are transported by Marangoni flow. During this stage, oil droplets are moving rapidly on a surface with a low droplet concentration. Once the initial drop is empty, the ring collapses on itself, a phenomenon not yet reported experimentally to our knowledge. Spreading and retraction occur on a few hundred milliseconds while the ring stage lasts a few seconds. We show that these dynamics result of the out-of-equilibrium desorption of the surfactant from the surface of the layer which depends on the affinity of the surfactant with water. We also bring direct evidence that the surface of the layer is deformed during spreading with a jump of a few hundred microns in the layer thickness at the location of the ring. We discuss the analogy between our system and a hydraulic jump