Dynamic Crystallization and Phase Transition in Evaporating Colloidal Droplets.

Abstract

Evaporating colloidal droplets are an omnipresent phenomenon in nature and engage in many scientific and commercial technologies. Despite their apparent importance, many of the fundamental aspects remain unknown, particularly the relationships between evaporation kinetics, volume fraction, crystallization, and phase transition. Here, we follow the structural evolution and drying dynamics across the liquid-to-solid transition of evaporating colloidal droplets containing polystyrene nanospheres with both spatial and temporal resolutions through the in situ small-angle X-ray scattering and ex situ electron microscopy techniques. We find the unconventional evaporation-driven heterogeneous crystallization and the sequential stacking of face-centered cubic (fcc), random hexagonal close-packed (rhcp), and random close-packed (rcp) superlattice structures. The crystallization and phase transition processes are further elucidated and coordinated with the real-time volume fraction variation, which constitutes a rich and dynamic picture of the self-assembly process. Starting with the Onsager principle, we provide quantitative analysis to the evaporation kinetics, including concentration gradient, gelation, and cavitation. Our findings impart a new mechanism of dynamic nucleation and crystallization and reveal the intimate link between structural heterogeneity and evaporation kinetics.