Serial coalescence of multiple condensed droplets on superhydrophobic surfaces at the microscale

High contact angles (CA) and low contact angle hysteresis (CAH) are characteristic of structured superhydrophobic surfaces on which droplets are known to be very mobile. When two or more droplets coalesce, the excess surface energy could be converted to kinetic energy for spontaneous droplet motion. Most of the literature focus on the resulting motion normal to the surfaces (jumping), which originates from their low wettability and high CA. On the other hand, low CAH leads to tangential motion of coalesced droplets which has not received much attention. We study the mechanics of such motion for condensate droplets, which often manifests in a gravity-independent serial coalescence of multiple droplets across the surface. This, in turn, leaves a trail of fresh surface for nucleation. At the end of such events the droplet may become stationary or depart by jumping. We further discuss the possible implications for condensation heat transfer since a tangential serial coalescence sequence may clear a much larger area for fresh nucleation compared to normal jumping departure arising from the coalescence of droplets confined at a location.