Evaporating binary droplets: significant impact of natural convection below the millimeter scale*

While the evaporation of pure sessile droplets is well understood, the situation drastically changes when the droplet composition is constituted by a mixture of different liquids, as e.g. in typical applications like inkjet printing.
Due to different volatilities of the components, solutal gradients are induced, leading to solutal Marangoni flow and natural convection.
Remarkably, even for droplets below the millimeter scale, natural convection can dominate the Marangoni flow and decisively determine the direction of the flow inside the droplet.

On the basis of finite element simulations and experiments, we investigate the possible flow types. Finally, we are able to predict the flow in the droplets by phase-diagrams in terms of the Rayleigh and Marangoni numbers.