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

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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.