Evolution of Interparticle Correlation and Size Stratification in an Evaporating Colloidal Droplet

Evaporation-induced assembly of colloidal particles is a ubiquitous phenomenon that is widely utilized in pharmaceutical and chemical industries. Understanding the temporal evolution of structural correlation during such a nonequilibrium process of assembly is crucial, as it is governed by a combination of thermodynamic and kinetic parameters. Real-time monitoring of a single micrometric evaporating colloidal droplet to track structural evolution is a challenging task owing to several technical constraints. However, coarse-grained particle dynamics simulations can provide detailed insights into the temporal evolution of the structural correlation during such assembly process. In this work, a particle dynamics simulation is presented to elucidate the structural evolution, and the results are further validated using scattering and imaging experiments. The simulation captures two key stages in the structural evolution: (i) the formation and thickening of a correlated particle shell driven by capillary confinement and (ii) small-on-top size stratification for polydisperse colloids. The present work illustrates the journey of a single dilute colloidal droplet transitioning to a jammed nanostructured microcapsule, a characteristic of the spray-drying process. Simulation results corroborate well with small-angle X-ray scattering results obtained from spray-dried nanostructured microgranules and a single drying colloidal droplet. These results are relevant in tuning the synthesis of nanoporous microcapsules for drug delivery and encapsulating drugs within a protective coating using the one-step rapid evaporative assembly process.