Emergence of different crystal morphologies using the coffee ring effect

Macroscopic patterns in nature formed during crystal growth e.g. snow crystals have a significant influence on many material properties, such as macroscopic heat conduction, electrical conduction, and mechanical properties, even with the same microscopic crystal structure. Although crystal morphology has been extensively studied in bulk, the formation of patterns induced by re-crystallization during evaporation is still unclear. Here, we find a way to obtain concentric circles, a dendritic pattern, and a lattice pattern by pinning the edge of droplets using the coffee ring effect; only aggregates of crystallites are seen in the absence of pinning. Our systematic study shows that the macroscopic patterns depend both on initial concentration and evaporation rate. In addition, our qualitative analysis suggests that the local concentration of solute at the center of the pattern is related to the macroscopic patterns.