Coupling between crystallization and evaporation dynamics: Periodically nonlinear growth into concentric ringed spherulites

Despite the extensive study of periodic precipitation and rhythmic crystal growth into ringed patterns, the detail of the evolution process remains unclear, and thus the explanation is rather elusive. Herein, we focus on monitoring the detailed growth process and dynamic, elucidating the underlying mechanism, and exploring key factors for the generation of poly(ε-caprolactone) (PCL) concentric ringed spherulites in evaporating droplets. In situ observation exhibits that accompanying the rhythmic evolution of the crystal, the region ahead the growth face changes periodically and the radial growth within each period is non-linear. It shows that having an evaporation-driven convection that carries liquid to the growth face drives the periodic dimple generation and rupture that leads to the rhythmic growth into discrete ringed spherulites. The non-linear growth is attributed to the coupling of evaporation and crystallization. We find that there are two key factors in the drying process that ensure the occurrence of the evaporation-driven flow and then the periodic crystal pattern. The structure formation reveals a complex interplay among solvent extraction, solution flow, solute diffusion, and crystal growth.