Interfacial Flows and Surfactants

Surfactant adsorb at interfaces, reducing the interfacial tension. They can be swept toward converging stagnation points or sites of surface contraction, and accumulate there if the rates of mass transfer which restore equilibrium are slower than these surface convective fluxes. Thus, the kinetics of surfactants at fluid interfaces plays a central role in determining their stress consequences on droplets far from equilibrium. While early work in this field focused on dilute surfactant systems with surface active impurities, in application, surfactants are often present at concentrations comparable to or above critical micelle concentrations. The Stebe group has focused on this higher concentration regime, with its associated non-linear surface tension coupling to surface and bulk concentrations, to develop a paradigm for the judicious selection of surfactants and their transport kinetics for desired system response. Thus, surfactants can be selected to alter surface velocity profiles, with implications in interphase mass transfer across droplet interfaces, to control droplet translation rates and pressure drops or forces needed to drive their motion, and to determine drop break up modes ranging from tip streaming to the emission of blobs from drops in extensional flows.