Sessile Binary Mixture Droplet Evaporation Cycle: Marangoni and Buoyancy Effects in Phase Transition

Evaporation of a binary mixture droplet (BMD) is a fundamental physical phenomenon in nature that is widely applicable in inkjet printing, spray coating, microfluidics in medical diagnosis, surface cooling, and cleaning processes. In this study, the numerical model of sessile BMDs in a pinned state is established to illustrate the impacts of selective evaporation, Marangoni and buoyancy effects, and continuum-compensated flow in droplet evaporation stages, where the flow characteristics and assessment criteria of the dynamic flow development are presented. Initially, Marangoni instability-driven flow (MIF) originates from the competition between thermal and solutal Marangoni effects, followed by chaotic flow with multiple-vortex creation. As evaporation progresses, the droplet is stabilized into an internal axisymmetric vortex. When the majority of ethanol is depleted, the dominated capillary flow drives radial outward flow similar to that of pure water droplets. The transition stages are governed by the new dimensionless number Mg_s/Pe, when Marangoni effects decay rapidly for the chaotic flow to axisymmetric vortex phase transition, and by Gr = 1 as the threshold for the subsequent transition to capillary flow, respectively. In principle, the manipulation of the solutally driven flow allows us to suppress the coffee-ring phenomenon and achieve more uniform deposition.