Surfactant Effects on Viscous Interactions of Drops
The term “surfactant” is short for "surface-active agents," like soap. Surfactants reside at the interface between two liquids, or a liquid and a gas, and modify the interfacial tension between the phases. The presence of surfactant affects the interactions between drops in an immiscible medium, including whether or not the drops coalesce. These interactions are important in emulsion stability, sedimentation and creaming, polymer blending, rheology, liquid-liquid extraction, and geophysical flows. Thus, understanding how surfactants work is important to many industries, including food, pesticides, paints, ore flotation, and detergents.
The primary objective of this research is to investigate how surfactants affect droplet interactions and emulsion behavior in several viscous flows, including buoyancy-driven motion, thermocapillary migration, and linear flows, such as shear flow. Current work concerns raindrop growth where drop inertia can also be significant. Analytical and semi-analytical methods will be used for two spherical drops in the presence of surfactant, while boundary-integral methods will be employed for two moderately deformable drops. Two cases will be considered for binary spherical-drop interactions: small deviation in surfactant surface coverage and arbitrary surface coverage. For the case of nearly uniform surfactant coverage, the only effect of the surfactant is on the tangential stress difference and so the problem remains linear. However, the solution for arbitrary coverage will be semianalytical, requiring solution of the non-linear convective-diffusion equation for the surfactant concentration. Although spherical drops may come into contact and coalesce even in the absence of attractive molecular forces, deformation is important because it slows down the rate of film drainage between the drops and inhibits coalescence. In general, the goal is to calculate collision efficiencies by a trajectory analysis for use in population dynamics simulations to predict the behavior of dilute dispersions.
A bibliography of this group’s publications is attached.
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