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.
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