Evaporation-induced translation of Multiple Binary Droplets

Evaporation of multiple sessile droplets is a complex physical scenario due to their dynamic interaction through the vapour phase. In situations when such evaporating droplets comprise a binary mixture, their interaction becomes more complex due to the inclusion of solutal Marangoni. The physical understanding of such instances is extremely important for many crucial applications ranging from inkjet printing and spray cooling to disease diagnosis and DNA chip manufacturing. We propose a lubrication theory-based finite element model for two evaporating sessile drops comprising a binary mixture on a heated surface. We consider a diffusion-limited evaporation regime for thin droplets in the presence of a pre-cursor film. We consider that the liquid components are ideally mixed and that 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 for two adjacent droplets comprising the same binary mixture compositions demonstrate attraction, coalescence, and repulsion between the droplets as a function of thermal and solutal Marangoni. In addition, we also performed experiments with water-morpholine drops to demonstrate the predictions from our model physically.