Crystal Salt Formation — Evaporation Ring Pattern
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Dried salt water on a glass surface — the evaporation rings of the sessile droplet boundary creating concentric rings of sodium chloride crystal growth as the droplet edge retreats.
Description
The evaporation of a salt water droplet on a glass surface creates the ‘coffee ring effect’ for dissolved materials — as the contact line of the droplet is pinned at the original droplet boundary, evaporative flux is highest at the droplet edge, carrying dissolved sodium chloride to the edge where it crystallises. As the droplet shrinks, successive pinning and depinning events create concentric rings of salt crystal growth. This image was made of a glass microscope slide on which a concentrated salt water droplet had completely evaporated, the slide photographed between polarising filters that render the birefringent sodium chloride crystals in vivid colours against a dark background. The concentric evaporation rings are clearly defined, the innermost ring representing the final water position before complete evaporation. At 3:1 macro, individual sodium chloride cube crystals are resolved at the ring boundaries.
