  Recently, there is much interest in droplet condensation on soft or
liquid/liquid-like substrates. Droplets can deform soft and liquid interfaces
resulting 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 solid substrate is covered with a thin oil film. Droplets
move first in a serpentine, self-avoiding fashion before transitioning to
circular motions. We show that this self-propulsion (with speeds in the 0.1-1
mm/s range) is fuelled by the interfacial energy release upon merging with
newly condensed but much smaller droplets. The resultant collective motion
spans multiple length scales from submillimetre to several centimetres, with
potentially important heat-transfer and water-harvesting applications.
