Hindered coalescence in the presence of insoluble surfactants*

For moderate Capillary numbers, Ca=(0.05-0.3), simulations and scaling of two viscous drops undergoing a flow-induced head-on collision in a viscous matrix (Stokes flow limit) showed that coalescence is hindered, i.e. the film between the drops reached a steady state thickness hss. Here,  we analyze this phenomenon in the presence of insoluble surfactants both for high and low surfactant interfacial  concentrations (C), via the boundary integral method [1].

We find that hss in the presence of surfactant is smaller than hss for the clean interface case and it decreases as the surfactant interfacial diffusivity Ds decreases. Contrary to intuition, hss decreases with increasing C, i.e. high C systems are less stable to coalescence than low C ones, for the same Ds. Moreover, for high C , as Ds decreases, the film drains continuosly with time, without an oscillatory behavior. A comparison between surfactant cover drops and clean drops, characterized by different viscosity ratios with respect to the matrix, shows that drops with surfactants, even in trace amounts, behave like highly viscous drops, with viscosity ratios O(100).

[1]Vannozzi, C. Physics of Fluids 2012, 24.