Spontaneous non-linear oscillations of interfacial tension at oil/water interface

Three particular systems are considered where transfer of a surfactant across the interface between two immiscible liquids, water and oil, is accompanied by spontaneous oscillations of relaxation type with an abrupt decrease of interfacial tension followed by its gradual increase. These oscillations cannot be explained in the frameworks of linear stability analysis, because they are related to essentially non-linear effects. The oscillations characteristics depend on the properties of a surfactant (interfacial activity, solubility, partition coefficient, density difference between the surfactant solution and pure solvent), other solutes present in one or both liquid phases, and, usually, also on the system geometry. If the transferred surfactant is an ionic one, then, the oscillations of interfacial tension are synchronised with the oscillations of electric potential across the interface. The available hypothesis about oscillations mechanism are discussed, in particular, the model proposed recently for oscillations due to Marangoni instability by surfactant transfer from a point source located in one of the liquid bulk phases.

Spontaneous oscillations due to solutal Marangoni instability: air/water interface

Systems far from equilibrium are able to self-organize and often demonstrate the formation of a large variety of dissipative structures. In systems with free liquid interfaces, self-organization is frequently associated with Marangoni instability. The development of solutal Marangoni instability can have specific features depending on the properties of adsorbed surfactant monolayer. Here we discuss a general approach to describe solutal Marangoni instability and review in details the recent experimental and theoretical results for a system where the specific properties of adsorbed layers are crucial for the observed dynamic regimes. In this system, Marangoni instability is a result of surfactant transfer from a small droplet located in the bulk of water to air/water interface. Various dynamic regimes, such as quasi-steady convection with a monotonous decrease of surface tension, spontaneous oscillations of surface tension, or their combination, are predicted by numerical simulations and observed experimentally. The particular dynamic regime and oscillation characteristics depend on the surfactant properties and the system aspect ratio.