Superhydrophobic surfaces with double roughness in nanoscale promoting continuous dropwise condensation

Micro/nano-structured superhydrophobic surfaces can enhance dropwise condensation via coalescence-induced condensate jumping in well-tailored supersaturation conditions. In this paper we report our energy-based analysis of growth dynamics of dropwise condensates on biomimetic surfaces with two-tier micro/nano-textures, which are superior to flat or solely nanotextured surfaces in controlling nucleation density. To understand the role of condensate state transition, i.e., from partially wetting state (PW) to non-wetting Cassie state, in enhancing condensation heat transfer, we considered adhesion energy, viscous dissipation and contact line dissipation as the main portion of resistant energy that needs to be overcome by the condensate droplets formed in surface cavities. By minimizing the energy barrier of the state transition, we optimized first tier roughness on the hierarchically textured surfaces allowing condensates to grow preferentially in the outof- plane direction. From the molecular kinetic (MKT) point of view, the period of the second tier roughness should be formed in excess of tens of nanometers in order to mitigate contact line dissipation. Our resistant energy study indicates that scaling down surface roughness of each tier to submicron scale or even to nanoscale can significantly facilitate the PW-Cassie transition and expedite self-propelled condensate removal.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016