Evaporation of a sessile drop with pinned or receding contact line on a substrate with different thermophysical properties

Abstract

Effect of substrate thermophysical properties on the evaporation of a sessile drop of water in surrounding moist air is investigated by means of a quasi-steady state diffusion model. The situation of a non-heated solid substrate is invoked and both pinned and de-pinned drops are considered. The used numerical approach allows a full coupling between the three coexisting phases by solving water vapor diffusion equation in the surrounding air and heat conduction equation in all three phases. Results show that, for both pinned and de-pinned drops, a decrease in thermal conductivity of the substrate or an increase of its thickness has a cooling effect on the drop and expands the cold zone close to the liquid–gas interface. Furthermore, the overall heat and mass transfer rates at the liquid–gas interface vary between two limiting values. The maximum evaporation rate is obtained when the drop is on a substrate with very high thermal conductivity or on a very thin substrate. In this case, the drop is mainly supplied with heat from the substrate. The minimum evaporation rate is obtained when the drop is on a perfectly insulating substrate or a very thick substrate. In that case, the needed energy for evaporation is taken mainly from the gas phase. The numerical predictions also highlight different evaporation rate and evaporation flux for pinned or de-pinned drop and they depict a higher evaporation time for the latter.