Investigation of selective evaporation in binary mixture droplet using laser interferometry*

Recently, the evaporation of multicomponent droplets on solid substrates has gained significant attention due to its practical applications in various industrial fields, such as spray cooling, inkjet printing, uniform coating, and evaporation-induced self-assembly (EISA). A crucial aspect to understand during the evaporation process of such droplets is selective evaporation, which arises from the different volatility of each component. The selective evaporation induces Marangoni stresses along the droplet surface that can generate a circulating internal flow. Kim and Stone (JFM 2018) used physicochemical effects to directly visualize the selective evaporation of the more volatile liquid components in the evaporating binary mixture droplet. However, there has been no direct observation of the selective evaporation phenomena across the gas-liquid interface of the droplet yet. In this study, using laser interferometry, we measured the time-dependent mole fraction distribution of evaporated vapors of more volatile liquid components. Additionally, we performed side-view shadowgraphy measurements simultaneously to track the change in the droplet radius and contact angle during evaporation. To ensure the accuracy of our findings regarding selective evaporation, we compared and validated the results obtained from interferometry and shadowgraphy. We expect that this direct observation of selective evaporation, i.e., its visualization across the gas-liquid interface using laser interferometry, will provide useful insights into its complex process of the evaporation of a multi-component liquid droplet. These insights can have significant implications for various industrial applications, enabling better understanding and control of evaporation processes in practical settings.