Synergizing Surfactants and Electric Fields to Engineer the Mechanics of Fluid-Fluid Interface.

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Many materials are made up of blends of fluids, such as oil and water, that do not mix. The processing and control of these materials affects industries from food and pharmaceutical processing to oil recovery that are vital to our economy. This research will provide tools to guide the use of electric fields to manipulate these fluid systems including emulsions, blends, droplets, foams, and many soft materials. The use of electric fields has the potential to be more energy efficient and to allow for more intricate control of the properties of these materials than mechanical stirring. A critical step is understanding how electric fields interact with certain additives called surfactants. Surfactants are compounds that adsorb at the interface between two fluids and are ubiquitous in industrially relevant systems. The researchers will combine experimental studies, computational work, and molecular design of additives to improve existing processes and potentially develop new approaches to material processing.Electric fields are integral to a variety of processes that control multiphase complex fluid systems. Electrocoalescence, jetting, printing, dielectrophoretic manipulation on microfluidic chips, capillary electrophoresis and other processes use electric fields to break, deform, and coalesce fluid interfaces. Electric fields have the advantage of high spatial and temporal control and a quadratic, rather than linear, dependence of power on field strength. These advantages have been exploited in a limited number of processes involving fluid-fluid interfaces; however, broader development has been limited. A lack of understanding of the coupling between applied electric fields and surface active species is at the core of this, hindering the optimization of existing processes and the development of novel low-power electric-field driven replacements of existing mechanical processes. The principal investigators have established synergistic computational and experimental platforms to quantify the influence of electric fields on the electro-hydrodynamic deformation of fluid interfaces, drops, and bubbles. In particular, they have shown that additional time scales associated with electrical transport (e.g. due to charge relaxation) lead to deformation dynamics that are significantly richer than the more familiar scenario of a drop deformed by an imposed fluid flow. In many instances, surface active molecules, or surfactants, accumulate at fluid-fluid interfaces, and their transport dynamics brings further timescales that impact interfacial mechanics. However, fundamental understanding of the interplay of surfactant dynamics and imposed electric fields on the dynamics of fluid interfaces is lacking. Such an understanding is needed to avoid undesirable behavior such as drop breakup in coalescence devices, whose operation at present is guided by empirical observations. The hypothesis that drives this work is that the combined effects of electrohydrodynamics and surfactant transport can be tuned to enable control of drop deformation and break up, which will result in unique techniques to manipulate fluid interfaces in multiphase processes. This hypothesis will be tested using a synergistic experimental and computational approach.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.