Electrohydrodynamic interactions of drops: effects of surfactant and drop fluids dissimilarity

The interaction of fluids and electric fields is at the heart of natural phenomena such as disintegration of raindrops in thunderstorms and applications such as crude oil demulsification and electrosprays. 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 [1]. Using a combination of numerical simulations based on a boundary integral formulation and an analytical theory assuming small drop deformations, we study the dynamics of a drop pair in an applied uniform electric field at arbitrary orientation of their line-of-centers relative to the applied field direction. For identical drops covered with insoluble surfactant [2], we find that the surfactant weakens the electrohydrodynamic flow and thus dielectrophoretic interactions play more prominent role in the dynamics of surfactant-covered drops compared to clean drops. If drop conductivity is the same as the suspending fluid, a nondiffusing surfactant can arrest the drops' relative motion thereby effectively preventing coalescence. Drop dissimilarity can also have profound effect on the pair dynamics. While identical drops tend to align with the field, drops with different permittivity and conductivity can move their line-of-centers away from the applied field direction and the drops orient perpendicularly to the field.