Capillarity-driven roaming of coalescing condensate droplets on nanostructured superhydrophobic surfaces

Spontaneous jumping of coalescing condensate microdroplets normally from a surface has been shown to exhibit improved heat transfer efficiency over gravity-driven dropwise condensation. However, as such droplets coalesce, they can as well spontaneously propel in-plane and roam across the surface, despite the length scale of the surface structures being multiple orders of magnitude smaller than the droplets. The lack of a symmetry-breaking surface for tangential motion initiation suggests a different mechanism of roaming from coalescence-induced jumping. In this work, we observe such behaviour with high-speed imaging and show that coalescing condensate droplets roam due to the asymmetry of droplet adhesion, as nucleation occurs stochastically within the nanostructures given sufficient subcooling. This asymmetry drives the generation of tangential in-plane momentum, and initiates the roaming coalescence sequence. This conversion from excess surface energy to kinetic energy is found to be more efficient than droplet jumping. Dewetting after coalescence allows propagation and subsequent roaming, delaying condensate flooding of the nanostructures. Condensate roaming coalesces multiple droplets on the surface, resulting in high surface area renewal rates which ultimately improve heat transfer efficiency over even jumping dropwise condensation.