Electrohydrodynamic interactions of drops

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What do thunderstorms, ink-jet printing, lab-on-a-chip, and crude oil demulsification have in common? Droplets and electric fields. An electric field causes drops to attract, repel, or move around each other in a complex path. Control over these motions is crucial for the optimal performance of the technological processes that use electric fields for droplet manipulation, yet knowledge of many-drop electrohydrodynamics is limited. This project will explore drop interactions in electric field using a combination of theory, simulations, and experiments. Novel computational models will be developed to model drop motions and fluid flow in three-dimensions and account for important physical phenomena such surfactant and charge convection along the droplet interface. The research outcomes will advance fundamental knowledge and impact the various technologies that utilize electric fields to manipulate droplets. The project involves international collaboration that will provide opportunities for training students as globally engaged engineers. The visually appealing nature of the droplet behavior in electric fields will be used in outreach activities to excite students and the general public about fluid dynamics and engineering.The interaction of fluids and electric fields is at the heart of natural phenomena such as disintegration of raindrops in thunderstorms and many applications such as ink-jet printing, microfluidics, crude oil demulsification, and electrosprays. Many of these processes involve droplets and there has been a long-standing interest in understanding drop electrohydrodynamics. While an isolated drop in applied electric fields has been extensively studied, the behavior of many drops is largely unexplored. Even the pair-wise drop interactions have received scant attention and existing models are limited to axisymmetric and two-dimensional geometries. In three dimensions, the electrohydrodynamic interactions can be quite complex and non-trivial. For example, in an applied uniform electric field, instead of chaining along the field direction, drops can initially attract in the direction of the field and move towards each other, but then separate in the transverse direction. To understand the underlying mechanisms, the PI will carry out a theoretical and computational study complemented with experiments that systematically explores the dynamics of a drop pair in an applied uniform electric field. For the first time, the dynamics of two dissimilar drops will be studied at arbitrary separation and orientation of their line-of-centers relative to the applied field direction. New analytical models and accurate numerical simulation using the Boundary Integral Method will be developed to account for charge convection and surfactant redistribution. The project integrates asymptotic theories, numerical simulations and experiments, which will lead to both a much deeper understanding of the underlying fluid dynamics as well as the discovery of new behaviors and engineering opportunities relevant to technologies such as lab-on-a-chip. The proposed research is interdisciplinary, integrating knowledge from the fields of fluid dynamics and applied math, and involves an international collaboration. This will be very beneficial for the education and training of the students associated with the project. The visually appealing nature of the droplet behavior in electric fields will be used in outreach activities to excite students and the general public about fluid dynamics.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.