B. Mondal, M. Mac Giolla Eain, Q.F. Xu, V. Egan, J. Punch, A.M. Lyons
City University of New York, United States
pp. 380 - 383
Keywords: wetting, Cassie Wenzel, thermal management, dropwise condensation
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 lower hydrophobic surface. We designed and fabricated a hybrid superhydrophobic-hydrophilic surface to mimic, and improve upon, this behavior. Arrays of hydrophilic needles, thermally connected to a heat sink, were forced through a robust superhydrophobic polymer film. 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. Once the droplet reaches a critical volume, gravity overcomes the triple contact line forces and the droplet is released leaving the needle and the superhydrophobic surface available for another cycle of nucleation, growth and release.