Periodic Pattern Formation in Evaporating Drops

Solute deposits from evaporating drops with pinned contact line are usually concentrated near the contact line. The stain, or pattern, left on the substrate then consists of a single ring, commonly known as a coffee ring. Here we report on a variation of this phenomenon when periodic patterns emerge. We attribute these to phase transitions in certain solutes as solute concentration increases. Examples may include dissolved to crystalline transition in salt or order-disorder transition in liquid crystals. Activated nature of the phase transitions, along with the newly imposed boundaries between phases, may then invert solute density profile and lead to periodic deposits. Hereby we develop a general theoretical model and report on experimental observations on salt in water.