Nonisothermal evaporation

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 at which the substrate is maintained. 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. The qualitative predictions are quantified using the diffuse-interface model applied to a liquid evaporating into its own vapor.

Automated summary

This study examines the evaporation of a liquid layer on a substrate without assuming isothermality and considers temperature variations. Qualitative estimates demonstrate that nonisothermality affects the evaporation rate based on the substrate's conditions. If the substrate is thermally insulated, evaporative cooling significantly slows down evaporation to the point that the rate tends to zero with time and cannot be determined solely by measuring external parameters. However, if the substrate is maintained at a fixed temperature, the heat flux from below sustains evaporation at a finite rate, which can be deduced from fluid characteristics, relative humidity, and the layer's depth. The qualitative predictions are quantified using the diffuse-interface model applied to the evaporation of a liquid into its own vapor.