Tandem locomotion of droplets in a uniform electric field

An isolated charge-neutral drop in a uniform electric field experiences no net force. However, a drop pair can move in response to mutual electrostatic (due to polarization) and hydrodynamic (due to the flow driven by surface electric stresses) interactions. If the droplets are identical, the center of mass of the pair remains fixed. However, if the droplets have different conductivity and/or permittivity, the pair experiences net motion. This droplet cooperative propulsion arises from the electrohydrodynamic flow and vanishes in perfect dielectric systems. We analyze the three-dimensional droplet trajectories using asymptotic theory and numerical simulations using the Boundary Integral Method. We find dynamics that can be quite intricate depending on the initial orientation of the droplets line-of-centers relative to the applied field direction.