Evaporation of a sessile water drop on a heated surface with controlled wettability

Abstract

This paper presents an experimental and theoretical study of the evaporation of a sessile water drop to open atmosphere when the temperature difference between the solid substrate and the atmosphere is about 40°C. Using substrates with different wettability we investigate all three modes of droplet evaporation: pinning, partial pinning and depinning. One of the most important results is that at the final stage of the drop life the specific evaporation rate abruptly increases especially for drops with small and moderate contact angle hysteresis. The coupled heat and mass transfer model is considered where the temperature field on the drop surface determines the distribution of vapor concentration on liquid–gas interface. The heat exchange of liquid drop with gas phase strongly affects the temperature distribution on the droplet surface. There is an appreciable increase of temperature close to periphery of the droplet near the contact line. And this leads to increasing of evaporative mass flux near the contact line. We calculate the evaporation rate and conclude that the global evaporation rate is proportional to the contact radius rb while the drop area is proportional to the drop contact radius squared rb2. Thus, the specific evaporation rate (evaporation rate per drop area) is a function of 1/rb and diverges at the end of the evaporation when the drop contact radius tends to zero. The calculated specific evaporation rate is in excellent agreement with the experimental data.