Exploration of the Effects of Nanoscale Surface Morphology Variations on Onset of Bubble Nucleation in Water Droplets Impinging and Boiling on Nanostructured Surfaces

Abstract

Nanostructured hydrophilic surfaces can enhance boiling processes due to the liquid wicking effect of the small surface structures. However, consistently uniform nanoscale interstitial spaces would require high superheat to initiate heterogeneous nucleation in the available small cavity spaces. Experimental studies indicate that surfaces of this type initiate onset of nucleate boiling at relatively low superheat levels, implying that significantly larger interstitial spaces exist, apparently as a consequence of the fabrication process. To explore the correlation between nanostructured surface morphology variations and variation of nucleation behavior with superheat, in this study, a zinc oxide nanostructured coating was fabricated on various copper substrates for wetting and droplet vaporization heat transfer experiments and morphology analysis. Our experiments determined the variation of mean droplet heat flux with superheat, and high-speed videos documented how nucleation features varied with superheat. Image analysis of the electron microscopy images was used to assess the variability of pore size and surface complexity (entropy) over the surface. Our data demonstrates the correlation between surface morphology feature distributions and the variation of nucleate boiling active site density with superheat. Specifically, our results indicate that increased availability of larger-scale surface irregularities with low surface entropy corresponds to enhanced probability of nucleation onset and an increase in active nucleation site density as superheat increases. This information can help guide development of enhanced boiling surfaces by providing insight into the nanosurface feature density distributions that enhance nucleation onset while also providing enhanced wicking and low contact angle over most of the surface.