(337h) Dynamics of Electrohydrodynamic Emulsions

Application of an electric field across the curved interface of two fluids of low but nonzero conductivities, or "leaky dielectrics," gives rise to sustained electrohydrodynamic (EHD) fluid flow. In a uniform dc electric field the electric and velocity fields around an isolated, neutrally buoyant leaky dielectric drop are fore-aft and azimuthally symmetric about the applied field axis. Consequently, the drop remains stationary. In the presence of neighboring drops, drops interact via the EHD flows of their neighbors, as well as through a dielectrophoretic (DEP) force. We explore the collective dynamics of emulsions with drops undergoing EHD and DEP interactions. The interplay between EHD and DEP results in a rich set of emergent behaviors. We simulate the collective behavior of large numbers of drops; in two dimensions, where drops are confined to a plane; and three dimensions. In monodisperse emulsions, drops in two dimensions cluster or crystallize depending on the relative strengths of EHD and DEP, and form spaced clusters when EHD and DEP balance. In three dimensions, chain formation observed under DEP alone is suppressed by EHD, and lost entirely when EHD dominates. When a second population of drops are introduced, such that the electrical conductivity, permittivity, or viscosity are different from the first population of drops, the interaction between the drops becomes non-reciprocal. Here, in two dimensions drops cluster into active dimers, trimers, and larger clusters that continue to translate and rotate over long timescales.