(646i) Field and Flow-Induced Dynamics of Droplets Covered with Ionic Surfactants

The control and stability of emulsion flows and microstructure are key ingredients in the rational design of products and processes in environmental applications, oil industry, food processing, cosmetics, and pharmaceuticals. Emulsion stability is typically driven at the droplet scale and is governed by interfacial phenomena. For example, surface active particles or molecules are known to chemically stabilize the interface against coalescence and may be used to control droplet size heterogeneities. The distribution of surfactants, although confined at the interface, affect the mechanical response of the emulsion at the macroscopic scale. In general, surfactants contain charged (ionic) head and tail groups which interact with polar substances, such as water, leading to often significant changes in interfacial tension. In this work, we conduct numerical experiments on a physical system of droplets covered with ionic surfactants dispersed in a non-ionic liquid medium (e.g., deionized water). Our goal is to develop a fundamental understanding of the effect of surface charge transport on the dynamics of surfactant-covered droplets in locally linear flows. The methodology combines the level-set method to capture the interface implicitly, and the closest point method to solve the surfactant transport equation. Our results show that drop deformation depends on a balance among flow strength, surface charge magnitude, surfactant concentration and interface mobility (i.e. advection, diffusion, and electromigration), where the surface charge density is coupled to the surfactant concentration by a linear relation. The charge balance model predicts that the imposed flow advects charges to regions of high curvature, while diffusion and electromigration tend to smear out charge distribution along the droplet surface. We show that in the limit when electric effects are dominant over viscous-flow distorting forces, the droplet shape changes dramatically as the surface charge accumulates at the droplet tips.