Konrad Rykaczewski, William A. Osborn, Jeff Chinn, Marlon L. Walker, John H. Scott, Wanda Jones, Chonglei Hao, Shuhuai Yao, Zuankai Wang
Surfaces which evince superhydrophobic properties during water condensation have a potential to dramatically enhance energy efficiency in power generation and desalination systems. Although various such surfaces have been reported, their development has been fortuitous, not driven by an understanding of the underlying physical processes. In this work we demonstrate that the growth mechanism of individual water microdroplets on such surfaces is universal and independent of nanoscale surface architecture. The key role of the nanoscale topography is energetic confinement of the base area of condensing drops. The base constriction leads to formation of nearly spherical microdroplets through contact angle increase, which is a necessary condition for droplets to become highly mobile after coalescence. By comparing experimentally observed drop growth with interface free energy calculations we show that the minimum observed confined microdroplet base diameter depends directly on the nanoscale surface roughness and degree of interfacial wetting. We use the new fundamental insight to develop quantitative design guidelines for superhydrophobic surfaces intended for condensation applications.
Droplet coalescence, Environmental Scanning Electron Microscopy, superhydrophobic surfaces