Increased heat transfer through dropwise condensation on a bio-inspired superhydrophobic-hydrophilic surface

Condensation of water vapor is an essential process in power generation, water collection, and thermal management. Because of the high surface energy of the metal surfaces, filmwise condensation of water vapor occurs, forming a static, thermally insulating film. Numerous efforts have been made to create surfaces that promote dropwise condensation; however these result in thermally insulating layers or degrade over time. Nature provides an alternative approach. The Namib beetle (Stenocara gracilipes) has a carapace that collects water by promoting dropwise condensation on raised hydrophilic regions which then roll off and slide along the hydrophobic surface. We designed and fabricated a hybrid superhydrophobic-hydropbiUc surface to mimic, and improve upon, this behavior. Condensation occurs preferentially on the needle surface due to differences in wettability and temperature. As the droplet grows, the liquid on the needle remains in the Cassie state and does not wet the underlying superhydrophobic surface.