Sessile droplet freezing on polished and micro-micro-hierarchical silicon surfaces

Abstract

Droplet freezing on a polished silicon wafer and a micro-micro-hierarchically structured silicon surface, the contact angles of water (at 25.7 °C) on which were 78.77° and 143.82° respectively, were experimentally observed. The surface temperature was maintained at −6.3 °C. Combining high-resolution photography and infrared thermometry, the ice nucleation onset of the droplet was captured. It is found that the hierarchical surface remarkably postpones the ice nucleation compared with the hydrophilic polished silicon because of enhanced free energy barrier for nucleation. The unique micro-micro-scale topology determines the dominance of wetting characteristics on ice nucleation. Besides, the formation of ice shell is initiated randomly at the water-surface-air trijunction base and propagated both circumferentially and vertically upwards during the surface recalescence stage. Subsequently, the internal solidification proceeds from bottom up when the ice shell develops. Moreover, the duration of surface recalescence and internal solidification stages, as well as the shape of solidification front (SF) are evaluated based on the local droplet surface temperatures. The surface recalescence begins simultaneously but ends earlier as closer to the droplet bottom. The temperature at SF edge is higher than that in the middle. Compared with the fully frozen solid cooling stage, the heat transfer rate is much higher during internal solidification.