Interfacial particle accumulation in the drying-teardrop effect

Salts can be found in different forms in almost any evaporating droplet in nature, our homes and in laboratories. The transport processes in such - apparently simple- systems differ strongly from 'sweet' evaporating droplets since the liquid flows in the inverse direction due to Marangoni stresses at the liquid-gas interface. Recently, we studied the inverted flow pattern that takes place in such salty droplets using 3D particle tracking measurements to quantify the full three-dimensional flow. Contrary to what is typically reported, the flow inversion does not prevent the coffee-stain effect; on the contrary, particles accumulate, get trapped at the liquid-gas interface and are consequently advected towards the contact line along the interface. In this work, we make use of confocal imaging to quantify the accumulation process and the growth of the particle interfacial deposits for different salt concentrations along the droplet's interface. The experimental results are contrasted with numerical simulations that capture the solvent evaporation, the evaporation-induced liquid flow and the quasi-equilibrium liquid-gas interface.