Evaporation of vertical and pendant ethanol droplets and internal Marangoni convections

Abstract

Understanding droplet evaporation has broad implications for science and industry. Over the past decades, theoretical models concerning this topic have been extensively studied for better prediction of this process. Although the existing models have shown reliable accuracy both in qualitative and quantitative analysis, most focus on specific physical scenarios and therefore consider only part of the transport mechanisms in the process, e.g., a diffusion-based model. In this study, we numerically investigated the evaporation of vertical and pendant ethanol droplets in an open space and developed a novel comprehensive model for simulation. The model incorporated vapor diffusion, thermal conduction, evaporative cooling, natural convection, and Marangoni stress, showing excellent accuracy in three-dimensional scenes. A sessile droplet evaporating on top of a horizontal substrate was compared. Our analysis started with the temperature distribution surrounding and inside the droplet, from which a complex multi-mechanism coupling behind the concerned issues was implied. We then studied the vapor diffusion and flow features in the gas domain and revealed geometric restriction as an important influencing factor. Next, we focused on the internal flows, demonstrating the dominance of Marangoni convections within the droplet and the characteristics of flow behaviors. The local evaporation flux was identified. Finally, we compared lifetimes in different cases and discussed the effects of droplet volume during evaporation. Overall, a vertical droplet had higher evaporation intensity and internal convection, while pendant and sessile droplets were similar. Our study hopes to provide a reference for ethanol droplet evaporation and possible applications based on such a phenomenon.