Mechanism Interaction During Droplet Evaporation on Nanostructured Hydrophilic Surfaces

Abstract

Recent studies of droplet spreading on nanostructured surfaces have demonstrated that the fluid motion and wicking effects impact the morphology of the liquid on the nanostructured surface and the thermophysics of the vaporization process. In the investigation summarized here, models of the spreading mechanism, and mechanisms of heat transport to the interface of a spreading droplet are used to explore the interaction of these mechanisms during the droplet vaporization process on nanostructured hydrophilic surfaces. Exploration of the trends in the model predictions and their comparison with experimental data suggests that the wickability of such surfaces causes an impinging droplet to quickly spread to form a thin liquid film with a somewhat curved interface. This liquid film has a mean thickness in the range of 10–100 microns near the contact line at the outer perimeter of the droplet footprint. If the surface is highly superheated, bubble nucleation and a nucleate boiling mechanism may augment conduction across the liquid film to facilitate evaporation. However, physical arguments and data from droplet evaporation experiments suggest that nucleation in the interstitial spaces of the nanoporous layer may be suppressed as a result of the extremely small size of those spaces. The role of these different mechanisms and the stages of the vaporization process for impinging droplets is discussed in detail. This exploration indicates that the wickability effect on droplet spreading strongly enhances the droplet evaporation heat transfer.

Can Wicking Control Droplet Cooling?

“…A combination of the fast velocity of the bubble released by the nucleation sites and the liquid capillary pumping can prevent the formation of a stable continuous vapor layer. The occurrence of evaporation regime on the NW surface at higher superheat (as shown in Figures S16 and S17) could be explained by the delay of the onset of nucleate boiling on nanostructured hydrophilic surfaces (as reported by Van Carey et al (2018)). Indeed, nanocavities require larger superheat to be activated as shown in Figure a.…”

Nanoscale and Macroscale Effects of Mineral Deposition During Water Evaporation on Nanoporous Surfaces

“…According to Fourier’s law, the temperature drop across the layer was calculated as the layer thickness multiplied by the heat flux, divided by the thermal conductivity where q ″ is the heat flux, k is the thermal conductivity, d T is the temperature drop, and d x is the layer thickness. Temperature drop increases with heat flux, therefore 200 W/cm 2 , previously determined to be the critical heat flux of the nanostructured surface was used to calculate the temperature drop across the ZnO coating and also the mineral deposition layer. The nanoporous layer was approximated as a 5 μm thick solid ZnO layer with thermal conductivity of 23.399 W/mK .…”

“…The nanoporous nature of the surface coating caused the surface to have extremely low contact angles and enhanced rewetting of dry areas. 19,20 Droplet evaporation on the nanostructured surface was found to provide higher heat fluxes than evaporation on a plain copper surface at all superheats with a CHF twice as high as the uncoated copper. 20 Although not the focus of this durability study, similar ZnO nanopillars have also been shown to have desirable antimicrobial characteristics which would prevent biofouling in potential heat exchangers.…”

“…Previous experiments have demonstrated improved droplet spreading on the nanostructured surface and subsequent enhancement of vaporization performance. The nanoporous nature of the surface coating caused the surface to have extremely low contact angles and enhanced rewetting of dry areas. , Droplet evaporation on the nanostructured surface was found to provide higher heat fluxes than evaporation on a plain copper surface at all superheats with a CHF twice as high as the uncoated copper . Although not the focus of this durability study, similar ZnO nanopillars have also been shown to have desirable antimicrobial characteristics which would prevent biofouling in potential heat exchangers. − In order to assess the durability of the ZnO nanostructure, this study focuses on the comparison of clean nanostructured surfaces and the same surfaces after artificially hardened water has been used to replicate accelerated fouling of the surface due to evaporation of hard water.…”