Heat transport for evaporating droplets on superhydrophilic, thin, nanoporous layers

Abstract

Previous studies of droplet spreading on superhydrophilic nanostructured surfaces have demonstrated that enhanced wetting and wickability of such surfaces can produce more rapid and extensive spreading of liquid droplets. There is also ample evidence from prior studies that these types of superhydrophilic nanostructured surfaces can exhibit enhanced vaporization heat transfer performance in for droplet evaporation and pool boiling processes. This study specifically explored the mechanisms of droplet vaporization on superhydrophilic nanoporous thin layers, at a fundamental level, for conditions in which liquid viscous forces and pore capillarity forces dominate, and liquid inertia forces are low. The investigation, summarized here, experimentally explored the droplet evaporation heat transfer performance on superhydrophilic, nanoporous, thin layers on a heated metal substrate. A thermal growth process was used to fabricate a layer of ZnO nanopillars a few microns thick on a copper substrate. The resulting nanoporous layer exhibited ultra-low contact angles and high wickability. The experiments indicate that, on these surfaces, a deposited droplet undergoes an initial rapid spreading phase followed by a slower vaporization phase, which shrinks the droplet until evaporation is complete. Although droplet evaporation without bubble nucleation is the primary focus of this work, the experiments also examined the added effects of bubble nucleation on droplet evaporation at high surface superheats. Based on experimental observations and data, a model of heat transport during the evaporation process was developed. The model predictions are shown to agree well with droplet evaporation time measurements for droplets of different initial sizes and a range of surface temperatures. The experiments and model indicate that the surfaces tested enhance droplet evaporation heat transfer performance by as much as a factor of three compared to an ordinary copper surface at the same conditions. The model is also used to explore the potential for more extreme augmentation of droplet evaporation heat transfer by further enhancement of surface wetting and wickability. The results of this study provide a clearer picture of the interconnection of droplet spreading mechanisms and evaporation heat transfer for these circumstances.