Hindered Coalescence in the Presence of Insoluble Surfactants*

For moderate capillary numbers, i.e. Ca=O(0.05÷0.3), simulations and theory for two viscous drops undergoing a flow induced head-on collision in a viscous matrix in the Stoke flow limit showed that coalescence is hindered, i.e. the film reached a steady state thickness hss. We analyze this phenomenon in the presence of insoluble surfactants both for high (C=60% Cmax, where Cmax is the maximum surfactant concentration) and low (C=1.6% Cmax) surfactant concentrations through the use of boundary integral calculations of the full drop problem, [1] .
We find that hss in the presence of surfactant is smaller than hss of a clean interface system and it decreases as the Ds decreases. Contrary to intuition, hss decreases with increasing C, i.e. high surfactant concentration systems are “less” stable against coalescence than low surfactant concentration systems, for the same surfactant interfacial diffusivity Ds. Moreover, for high C, as Ds decreases, the film drains continuously with time. A comparison between surfactant covered drops and clean drops with different viscosity ratio show that drops with surfactants, even if present in traces amounts, behave like very high viscosity ratio dropsO(100).
[1] Vannozzi, C., Phys. Fluids.2012, 24.