Droplet impact and LFP on wettability and nanostructured surface

Abstract

The Leidenfrost point (LFP) of a single deionized water droplet on various wettability surfaces was investigated by high-speed visualization. Manipulating both the chemical and geometric properties of a silicon wafer substrate, we prepared test samples with a wide range of wettabilities, from superhydrophilic (<10°) to superhydrophobic (>165°). Nanoscale wire or needle-shaped structures were fabricated by etching process on the silicon substrate. The effects of wettability on LFP were investigated by analyzing how the droplets rebound on the overheated surface. The results show that the more hydrophilic condition, the higher LFP. As the hydrophilic surface causes vigorous nucleate boiling and large viscous dissipation during the droplet contact, higher superheated condition of the substrate is required to trigger the rebounding dynamics and stable film boiling state. Furthermore, the superhydrophilic surface, which was assisted by nanostructures, contains many cavities and produces strong capillary wicking upon droplet contact; hence, explosive transition boiling in the droplet was observed. This study provides a detailed physical understanding of droplet dynamics on overheated nanostructures and its relation on LFP evaluation.