Diffuse Interface Simulation of Sessile Evaporating Multicomponent Droplet Populations

The intricate behaviour of evaporating droplet populations is vital across various fields, including inkjet printing, microelectronics cooling, disease detection, and many others. To gain a comprehensive physical understanding of the interaction between sessile evaporating droplets placed on a heated surface, we developed a model based on the diffuse interface method. Using our in-house finite volume code (https://sourceforge.net/projects/tpls), we solve the Cahn-Hilliard equation alongside the equations for energy, mass, and momentum conservation. A diffusion-limited evaporation model based on Raoult’s law is employed for phase change. When droplets evaporate in the vicinity of other droplets, interactions occur through the vapour phase. This phenomenon is frequently referred to as vapour shielding. To investigate, we performed high-resolution 3D simulations of various droplet array patterns. The simulations encompassed a range of pinned droplet arrays, from 4 to 100 droplets. Our simulations reliably predict both the accumulation of vapour and the reduced evaporation rate of droplets within the array. We further examine symmetry breaking of the flow inside the droplets due to thermal Marangoni convection resulting from the asymmetric evaporative cooling. The model is further extended for multiple evaporating droplet arrays composed of binary mixtures (e.g., ethanol-water). For such binary mixture droplets, we observe extremely complex competing solutal and thermal Marangoni convections contributing to the highly asymmetric flow field inside the droplets.