Retarded transfers of an emulsified two-component oil phase

We study oil evaporation from oil in water emulsions, whose oil phases are mixtures of two alkanes of different solubilities in water. The aqueous phase is a colloidal clay suspension that exhibits a yield stress, in which the oil droplets are immobilized. A surfactant-free emulsion is thus formed, providing a convenient model system in which the oil transfer solely occurs through the aqueous phase. When the emulsion is left in contact with the atmosphere, oil evaporation results in the formation of a droplet-free region in the emulsion, and that region grows diffusively with time. We show that the transfer of the more soluble alkane is dramatically retarded when the mixture ratio of the less soluble alkane is increased, as a consequence of entropy loss associated with oil demixing. Using a simple transfer model, which captures well the experimental trends, we study the alkane concentrations both within the oil droplets and within the aqueous phase of the emulsion. We report overshoots in the concentration profiles of the less soluble alkane both in the emulsion and in the droplets, and explain their formation using a qualitative argument.