Spontaneous Cyclic Dimpling in Emulsion Films Due to Surfactant Mass Transfer between the Phases

A fascinating spontaneous cyclic phenomenon was encountered during observations of aqueous emulsion films between oil phases, Nonionic surfactants (Tweens) initially dissolved in the water phase were used as stabilizers. The films (with typical diameter of 300 μm) were formed in a capillary and were illuminated in reflected monochromatic light. After the films thin down to their equilibrium thickness, a dimple (lens-shaped thicker formation) spontaneously forms and starts to grow around their center. Upon reaching a certain size the dimple forms a channel to the periphery and flows out, leaving a plane-parallel film behind. Immediately afterward a new dimple starts to grow, and so on. This cyclic process could go on for hours. We have proved that the driving force of the process is the surfactant redistribution between the phases. We presume that similar phenomena, resulting from the coupling of the processes of surfactant redistribution and interfacial and hydrodynamic motion inside the films, could be important for the stability of newly formed emulsions.