Energy analysis of condensate growth on superhydrophobic surfaces with hierarchical roughness

Abstract

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 solely nanotextured surfaces in controlling nucleation density. To understand the role of condensate state transition, i.e., from partially wetting state (PW) to suspended Cassie state (S), 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 out-of-plane direction. The nanoroughness of the second tier plays an important role in abating the adhesion energy in the cavities and contact line pinning. From the perspective of molecular kinetic theory (MKT), the hierarchically engineered surface is beneficial to remarkably mitigating contact line dissipation. This study indicates that scaling down surface roughness to submicron scale can facilitate self-propelled condensate removal.