Pattern formation in drying droplets of polyelectrolyte and salt.

We have discovered spectacular patterns formed by drying aqueous drops of sodium poly(styrene sulfonate) containing fixed amounts of sodium chloride. Crystallization of salt molecules takes place at a critical concentration and several factors such as the polyelectrolyte concentration and humidity affect the final patterns formed. The observed patterns include concentric rings, needle-like structures, chains of triangular-shaped crystallites, etc. We describe the observed phenomenology accompanied by a theoretical model for the occurrence of concentric rings.