Influence of groove orientation on dropwise condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays

This work experimentally investigated vapor condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays to study the influence of groove orientation on the condensed droplet dynamics and heat transfer. This study showed that dropwise condensation on hydrophobic surfaces with microgroove arrays strongly depended on the groove orientation owing to the anisotropic wettability. The suction effect of a large droplet spanning several grooves on the liquid filaments filling the microgrooves was demonstrated on a hydrophobic surface with horizontal microgrooves with the phenomenon disappearing on a hydrophobic surface with vertical microgrooves. Furthermore, the horizontal microgrooves resulted in larger droplet departure sizes, significant deformation and retention of falling droplets, and the elimination of the liquid filaments sliding down the surface. The heat flux on the hydrophobic surface with horizontal micro grooves was 10-30% lower than on the plain hydrophobic surface. However, the groove orientation had less influence on the hierarchical superhydrophobic surface with microgroove arrays. At small surface subcooling, the hierarchical surface was rejuvenated through condensed droplet jumping and jumping-induced sweeping, the dominate droplet behavior transformed to coalesced droplet sweeping and the falling of suspended droplets with increasing surface subcooling. The dominate droplet behavior was little influenced by the groove orientation on the hierarchical superhydrophobic surface. Consequently, the heat flux was increased by 35-107% on both hierarchical superhydrophobic surfaces with microgroove arrays compared to the hydrophobic surface with vertical microgrooves.