Numerical and Theoretical Analysis of Sessile Droplet Evaporation in a Pure Vapor Environment

The evaporation of sessile droplets is not only a common occurrence in daily life, but it also plays a vital role in many scientific and industrial fields. However, most of the current research is focused on the evaporation of droplets in the air environment, where vapor transport is controlled by the diffusion model, but when the droplet evaporation is in its own pure vapor environment, the above model will no longer apply, and the evaporation will be dominated by kinetic theory. Thus the Hertz-Knudsen model can be applied to describe the evaporation kinetics. However, in most of the studies, it is assumed that the temperature distribution is uniform along the vapor-liquid interface of the droplet, but due to the evaporative cooling effect, this assumption is not correct in actual evaporation. In this paper, theoretical analysis and numerical simulation were combined to study the characteristics of droplet evaporation with multiphysics coupling. In the theoretical model, heat conduction in the droplet and substrate was coupled with vapor transport at the droplet surface. In the numerical simulation, internal thermocapillary flow and heat transfer of the droplet were coupled with vapor transport at the droplet surface. The effects of contact angle, thermocapillary convection, ambient pressure ratio, and substrate superheat on the droplet evaporation characteristics were quantitatively analyzed. It was found that the high substrate superheat or low ambient pressure ratio will enhance the droplet thermocapillary convection as well as evaporation rate. Furthermore, a critical contact angle was found; below this value, the droplet evaporation rate was inversely proportional to the contact angle, but upon this value, the trend was reversed. These findings have important implications for revealing the physical mechanism of kinetics-controlled droplet evaporation in a pure vapor environment.