Ring shells obtained from pure water drops evaporating on a soluble substrate

When a pure water drop evaporates on a soluble substrate, the final ring deposit takes the form of an inclined wall for small and flat drops and of a hollow rim for other geometries. We report here on experiments of evaporation of pure water and salty water drops on salt single crystals and inert substrates, which permit to define precisely the experimental conditions necessary to obtain such structured deposits. Their growth is not a consequence of particular thermal or chemical conditions but of the peculiar geometry of the experiments. Beside the advection of the solute toward the edge by the coffee-stain effect induced by the pinning of the drop, the role of nucleation site played by the salt substrate is a necessary condition for the hollow or inclined rim to appear. These experiments have also shown to which extent the final dryout microstructure is reminiscent of the salt crystallography. Finally, these experiments demonstrate the potentiality of the phenomenon for surface-patterning.

Automated summary

This study investigates the evaporation of pure water and salty water drops on different geometries of substrates, and finds that the formation of structured deposits is determined by the geometry of the experiments rather than specific thermal or chemical conditions. The role of nucleation sites on the salt substrate is a necessary condition for the appearance of hollow or inclined rims. Additionally, the experiments demonstrate the similarity between the final dryout microstructure and salt crystallography.