Evaporation and Condensation on Two-Tier Superhydrophobic Surfaces

Superhydrophobic surfaces exhibit large contact angle and small hysteresis which promote liquid transport and enhance heat transfer. Here, liquid-vapor phase change behavior is reported on superhydrophobic surfaces with short carbon nanotubes deposited on micromachined posts, a two-tier texture mimicking the surface structure of lotus leaves. Compared to one-tier microtexture which energetically favors the Wenzel state, the two-tier texture with nanoscale roughness favors the Cassie state, the desired superhydrophobic state. During droplet evaporation, the two-tier texture delays the transition from Cassie to Wenzel state. Using two-tier texture with hexadeconethiol coating, continuous dropwise condensation was demonstrated for the first time on engineered superhydrophobic surfaces.