Nonisothermal evaporation

Abstract

Evaporation of a liquid layer on a substrate is examined without the often-used isothermality assumption -- i.e., temperature variations are accounted for. Qualitative estimates show that nonisothermality makes the evaporation rate depend on the conditions the substrate is maintained at. If it is thermally insulated, evaporative cooling dramatically slows evaporation down; the evaporation rate tends to zero with time and cannot be determined by measuring the external parameters only. If, however, the substrate is maintained at a fixed temperature, the heat flux coming from below sustains evaporation at a finite rate -- deducible from the fluid's characteristics, relative humidity, and the layer's depth (whose importance has not been recognized before). The qualitative predictions are quantified using the diffuse-interface model applied to a liquid evaporating into its own vapor.