Emergent collective motion of self-propelled condensate droplets

Recently, there has been much interest in droplet condensation on soft or liquid/liquid-like substrates. The ability of droplets to deform soft and liquid interfaces result in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, inter-droplet attraction). Here, we describe a unique complex collective motion of condensate water droplets that emerges spontaneously when a structured solid substrate is covered with a thin lubricant-oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. Through comprehensive experimental and theoretical study, we show that this self-propulsion (with speeds in the mm s^-1 range) is fuelled by the interfacial energy release upon merging with newly condensed (but much smaller) droplets. The resultant emergent collective motion spans multiple length scales from sub-millimetre to several centimetres, with potentially important heat-transfer and water-harvesting applications.