Effect of surfactants on immiscible liquid-liquid dispersion formation in a stirred vessel*

In this study, we model the mixing of two immiscible fluids, oil and water, in the presence of surfactant, in a stirred vessel equipped with a pitched blade impeller. Fully-three dimensional direct numerical simulations are employed in order to capture the complex interfacial deformation, rupture, and droplet coalescence, the change in topology, as well as the surfactant distribution along the interface. We demonstrate the development of a large vortex that reaches the impeller blades which is then followed by interfacial breakup into a multitude of droplets. The addition of surfactants modifies the local value of the surface tension along the interface and also induces Marangoni stress. We show how these surfactant-induced stresses alter the interfacial dynamics and give rise to profound changes in the drop size distribution depending on the choice of physico-chemical parameters such as the surfactant elasticity.