Pulsating Crystallization in Evaporating Salt Droplets

Evaporation of sessile salt droplets often leads to well characterized solute deposition patterns and the formation of rigid crystalline crusts. While these crusts are generally regarded as static and mechanically stable, we report a novel pulsating regime that emerges during crust formation in sessile droplets of a special salt. The pulsating dynamics is marked by periodic morphological transitions in the crystallizing surface. The resulting dynamics reflect an interplay between evaporation-driven concentration gradients, internal convection, crystal growth kinetics, and interfacial stresses. By controlling substrate temperature, contact angle, and salt concentration, we demonstrate a range of distinct crystallization morphologies throughout the evaporation process. Insights are obtained to unveil the physical mechanism for the pulsation, which can be controlled to enable dynamic crystallization patterning for applications in inkjet printing, diagnostic patterning, and pharmaceutical spray drying.