Evaporating populations of sessile droplets comprising binary mixtures

Evaporation of a sessile binary mixture droplet is a highly complex phenomenon due to dynamic interplay between thermal and solutal Marangoni flows. The presence of another neighbouring sessile droplet comprising the same binary mixture introduces further complexities via interactions through the substrate, pre-cursor film, and vapour. Here, we propose a lubrication theory-based finite element model for two evaporating sessile drops comprising binary mixture such as ethanol-water deposited on a heated surface. We consider a diffusion-limited evaporation regime for thin droplets in the presence of a pre-cursor film. The liquid components are assumed to be ideally mixed and both components are volatile. The net surface tension is taken as a linear function of both temperature and concentration. Further, we also solve for the vapour concentration for both the volatile components in the presence of air in 2D using the standard diffusion equation. Our solutions demonstrate attraction, coalescence, and repulsion between the droplets as a function of thermal Marangoni, solutal Marangoni, and substrate thermal conductivity. In addition, we also observe the so-called vapor-shielding effect making the evaporation time for two droplets significantly higher than for a single droplet. We also note we also note condensation of water under of higher relative humidity conditions as the droplet cools down below the dew point owed to evaporative cooling.. We also predict condensation of the most volatile component at higher solutal Marangoni numbers.