Evaporation of sessile droplets: hydrodynamics and salt deposit patterns

In humid environments, the formation of sessile droplets containing pollutants, such as sodium chloride, on certain materials can lead to atmospheric corrosion phenomena. The prediction and understanding of these corrosion phenomena are closely linked to the dynamics of the droplet, and in particular to its evaporation. Ambient parameters such as relative humidity, temperature, substrate material as well as the composition of the droplet, e.g. its pollutant concentration, modify the evaporation dynamics and influence the characteristic time of corrosion. Salt deposits left after evaporation show complex hydrodynamics and indicate potential corrosion sites. Anticipating corrosion zones is crucial in many industrial applications to limit material degradation. This study seeks to better understand the evaporation dynamics of sessile droplets,through parametric studies, with numerical simulations and experimental tests. Pure water droplets are first considered, in order to simplify the studied system and to analyze the different modes. Droplets of saline solution are studied in a second time. During evaporation, salt concentrates inhomogeneously inside the droplet causing surface tension gradients. These stresses along the interface, called Marangoni stresses, give rise to internal flows. Depending on the initial concentration of pollutant, the relative humidity, and the size of the droplet, the morphology of the resulting salt deposits is analyzed. (author)