Transport mechanisms governing the evaporation of a sessile droplet in its pure vapor environment

Droplet evaporation has been drawing the attention of researchers as it is a critical process in many different areas of science and technology. Most numerical and experimental research has focused on the evaporation process of droplets surrounded by air. Hence, the literature on droplet evaporation in a pure vapor environment is still open to progress. The fundamental difference between evaporation into air and pure vapor is the diffusion of vapor and air molecules at the gas phase, which substantially affects the evaporation of droplets surrounded by air. In the absence of air, the phase change at the interface can be defined by Hertz-Knudsen-Schrage equation based on kinetic theory. The current study presents a numerical model of drying water droplets on a solid substrate surrounded by pure vapor. The model accounts for both buoyancy and thermocapillarity-driven convections inside the droplet. When the thermocapillarity effect (Marangoni convection) is omitted, a single Rayleigh cell generated by the buoyancy occurs inside the droplet. This convection, driven by buoyancy, has a minor influence on energy transport. However, when the thermocapillary effect is accounted for, single or multiple Bénard-Marangoni cells occur inside the droplet, resulting in more vigorous convection, enhancing mass and energy transport and reducing the droplet lifetime. During the drying process, the change in the strength and mode of Marangoni convection induce a nonmonotonic variation in evaporation rate with decrasing contact angle.