Forced jumping and coalescence-induced sweeping enhanced the dropwise condensation on hierarchically microgrooved superhydrophobic surface

Abstract

Condensation is of great interest in various heat exchange processes, owing to the elevated heat and mass transfer by phase change. In this work, a hierarchically microgrooved superhydrophobic surface was fabricated by the mechanical broaching and chemical etching methods to enhance the condensation heat transfer. The dynamic behaviors of condensed droplets and condensation heat transfer characteristics were analyzed on such surface. Particularly, there were two droplet jumping modes, the conventional coalescence jumping of small droplets (<100 μm) at small subcooling (ΔT < 5 K) and the forced jumping of large stretched droplets (400-500 μm) in microgrooves at a broad range of subcooling (ΔT < 12 K), simultaneously emerging on the hierarchically microgrooved superhydrophobic surface. The interesting coalescence-induced sweeping behavior independent of gravity is observed at large surface subcooling. The coalescence-induced jumping and sweeping significantly facilitated the renewal of surface. The investigation has showed that a 90% higher heat flux at small subcooling (ΔT < 5 K) and a 66% higher heat flux at large subcooling (5 K < ΔT < 24 K) were reached on the hierarchically microgrooved superhydrophobic surface compared with the plain hydrophobic surface.