3D unsteady computations of evaporative instabilities in a sessile drop of ethanol on a heated substrate

Droplets are ubiquitous and have been studied for a century; however, their internal flow pattern and related instabilities that occur in the course of evaporation are not fully understood yet. In this paper, we report our investigation results on an ethanol drop evaporating onto a heated substrate under weightlessness conditions and with a pinned contact line. They have been obtained from both experiments and 3D unsteady computations in order to determine what kind of instabilities develop. Our one-sided model demonstrates quantitative agreement with experiments and confirms that experimentally observed instabilities are driven by thermo-capillary stress and not by the gas convection. Post-processed infrared images drawn from computations led us to conclude that the experimentally observed thermo-convective instabilities, which look very similar to hydrothermal waves in the infrared spectrum, are actually nothing else than unsteady Benard-Marangoni instabilities.