Electrohydrodynamics of Surfactant-Laden Fluid-Fluid Interfaces

Electric fields are integral to numerous industrial and scientific processes that involve multiphase complex
fluid systems; for example, electrocoalescence,
electro-emulsication and dielectrophoretic manipulation in micro fluidic devices. In these processes, electric fields deform, break and coalesce fluid interfaces. These systems contain surface-active species, or surfactants, which
simultaneously transport to the interface. To engineer the system response, the deformation, conditions for instability of the interface and the transport of surfactant under electric fields needs to be quantified. In this thesis, we analyze the response of liquid drops, and surfactant transport to interfaces under electric fields. The impact is prediction of drop behavior in devices and development of a unique tool to engineer and manipulate liquid-liquid interfaces.
We first employ boundary integral computations to highlight the role of convection of surface charges in the transition in breakup mode of a weakly conducting drop suspended in another weakly conducting drop, under a uniform electric field. Accumulation of surface charges at the tips of the drop
results in an abrupt change from a breakup mode characterized by bulbous lobes to one distinguished by the formation of conical tips. We next model interaction between multiple conducting drops and disturbances in operating
conditions as temporal fluctuations in the electric field. We use small deformation theory and the boundary integral method to predict the transient deformation and criterion for breakup of the drop under a random electric field. We demonstrate that fluctuations in the external electric field increase the average drop deformation, reduce the time for breakup, and soften the transition from steady state to breakup. We then probe the addition of soluble
surfactants to the drop phase in terms of the effective viscosity of a dilute emulsion of such drops. Small amounts of added surfactant can greatly impact the viscosity of the emulsion for certain regimes of surfactant transport and
depletion. We then proceed to develop an experimental platform to quantify the transport of surfactants to an oil-water interface under electric fields. We show that surfactants that form charge carriers in an oil phase show an enhanced transport under electric fields. Moreover, the field selectively affects surfactant transport in the oil phase. In summary, using theory, computation and experiment, we make significant contributions to underline the ability of
electric fields to manipulate liquid-liquid interfaces.