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Multi-scale Patterns formed by Sodium Sulphate in a Drying Droplet of Gelatin

We present a study of patterns, formed in drying drops of aqueous gelatin solution containing sodium sulphate. The patterns are highly complex, consisting of a hierarchical sequence of rings which form concentric bands as well as dendritic crystalline aggregates. When the preparation of the complex fluid is done by mixing an aqueous solution of the salt with an aqueous solution of gelatin prepared separately, another feature is observed in the pattern on the dried out drop. This is a viscous fingering pattern, superposed on the series of rings. We try to explain the origin of these two features from a simple physical approach.