Role of nanoscale roughness in the heat transfer characteristics of thin film evaporation

Thin film evaporation yields high local heat fluxes that contributes significantly to the total heat transfer rate during various two-phase transport processes including pool boiling, flow boiling, and droplet evaporation, among others. Recent studies have shown a strong correlation between the roughness of a surface and its two-phase heat transfer characteristics, but the underlying role of nanoscale surface roughness in thin film evaporation is not fully understood. In the present work, a thin film evaporation model is developed that accounts for the role of the roughness-affected disjoining pressure and flow permeability in determining the film thickness profile and heat transfer rate. Nanoscale surface roughness leads to a flatter evaporating meniscus profile when the effect of disjoining pressure is more pronounced of the two and promotes evaporation, consistent with previous experimental observations. However, our results reveal that surface roughness may also inhibit evaporation and lead to a steeper evaporating meniscus profile when flow permeability has the more pronounced influence on thin film evaporation. It is important to identify the specific surface roughness characteristics that determine whether disjoining pressure or flow permeaiblity has the stronger influence. To this end, a parametric study is performed that analyzes thin film evaporation on V-grooved surfaces of different depths and pitches. While the heat transfer rate increases monotonically with groove depth, there exists an optimal groove pitch that leads to a maximized evaporation rate. Also, when the groove pitch is smaller than a critical value, surface roughness inhibits thin film evaporation.