Flow dynamics of surfactant-laden droplets with complex interface rheology

The dynamics of individual droplets in viscous flows has received considerable attention owing to its significance in understanding the rheology of emulsions. Notably, the use of surfactants is often involved in real applications, but the influence of surfactants on drop dynamics and the underlying mechanisms are not fully understood, especially considering that many complex interface rheology have been revealed. We have developed a numerical methodology for the simulation of an insoluble surfactant-laden droplet in flows, in which the rheology of the drop surface is modeled via a Boussinesq–Scriven constitutive law with both the surface tension and surface viscosity depending strongly on local surfactant concentration. Our results show that the surface viscosity exhibits non-trivial effects on the surfactant transport, i.e., both dilatational and shear surface viscosity tend to eliminate the non-uniformity of surfactant concentration over the drop surface. As a consequent, the coupling between the surface viscosity and surfactant transport induces non-negligible effects in the dynamics of the whole droplet.