Roughness-enhanced collection of condensed droplets.

Gravity shedding of droplets is limited by droplet pinning, a major limitation for low condensation processes and in particular passive dew harvesting in its use as an alternative source of water. We present experiments showing that, paradoxically, a simple surface treatment increasing roughness (sand-blasting) favors droplet shedding compared to the original substrate, provided that sand-blasting does not increase too much the surface roughness. Sand-blasting ensures the high density of nucleation sites and enhances drops coalescence and growth at a sub-micron scale, thus lowering the lag-time to obtain drop sliding during condensation. Early nucleation indeed overcompensates the delay increase due to roughness. Edges of the substrate, where drops grow faster, also improve water collection, thanks to the early sliding of edge drops that behave as natural wipers. Combining the effects of sand-blasting and edges increases significantly the rate of collection of dew condensation on a substrate at a given time, gains of about 30% can be commonly obtained.