Multiple Marangoni flows in a binary mixture sessile droplet

A multicomponent droplet evaporation problem has widely been investigated due to diverse industrial applications, including ink-jet printing, surface coating and patterning, medical diagnosis, spraying cooling, and fuel combustion. During evaporation, the Marangoni flow caused by selective evaporation should be observed, and it plays an important role in determining the drying process and the final dried deposit pattern. However, most of the works focused on measuring the internal flow field although the Marangoni flow mainly occurred near the liquid-gas interface. In this study, using particle image velocimetry, we tried to measure the interfacial Marangoni flows where the measurement focal plane was set in the vicinity of the apex of the binary mixture droplet. We observed distinctive multiple Marangoni flows in time, such as (i) directional circulating flow, (ii) complicated mixing flow, and (iii) spontaneous bursting flow. Finally, we found that the Reynolds number is different for each case. Based on the flow field measurement result and dimensionless number analysis, we considered a stress balance model that qualitatively described the local surface tension gradient and Marangoni stress.