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.
Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .