Translation and coalescence of freely floating droplets using electrophoresis*

Aqueous droplets could be charged and transported between two DC electrodes in oil (Jung \textit{et al.} J. Colloid Interface Sci. 2008). And in that situation, two oppositely charged droplets coalesce under mild electric field but they do not coalesce under high electric field (Ristenpart \textit{et al.} Nature, 2009). In these previous experiments, the aqueous droplets were totally under oil. In present work, the aqueous droplets are floating on the heavy oil. The floating droplet system has several advantages comparing the submerged droplet system. First, the drag force on the moving droplet is lower than that of the submerged droplet. Second, post process could be simple, because the droplets are exist on the top of the system. Third, droplet coalescence is easy, because they are in same height. In this work, drag force and velocity field of the translating droplet are computed through numerical simulation. And the translating velocity and condition for coalescence of droplets are investigated through the experiments.