Mechanism Interaction During Droplet Evaporation on Nanostructured Hydrophilic Surfaces

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.