Dynamics and Morphological Outcomes in Thin-Film Spherical Crystallization of Glycine from Microfluidic Emulsions: Experimental Studies and Modeling

We present detailed experimental studies, supported by a theoretical model, of dynamics and morphological outcomes in thin-film spherical crystallization of glycine from microfluidic emulsions. Specifically, the effects of droplet size, shrinkage rate, and temperature are studied with the aim of understanding and delineating, from a processing standpoint, crystallization conditions that ensure spherulitic crystal growth in individual droplets, ultimately yielding compactly packed spherical crystal agglomerates (SAs). Our experiments reveal the existence, under all processing conditions, of a critical concentration (supersaturation) beyond which droplet shrinkage due to evaporation is dramatically arrested. The morphological outcome of the crystallization then depends on whether a nucleation event in a droplet occurs before (yielding loosely packed or faceted single crystals) or after (compactly packed SAs) this critical concentration is reached. We analyze our observations within the framework of a simple physical model based on classical nucleation theory and the theory of nonstationary Poisson processes, which accurately captures the overall trends. Experiments were conducted in a temperature range of 45-85 degrees C, droplet diameter range of 50-160 mu m, and film thickness range of 0.5-1.5 mm. We found that smaller droplets and faster shrinkage rates (in thinner films) favor the formation of compact SAs at a given temperature, and lower temperatures generally favor compact SA formation at a fixed drop size and shrinkage rate. This work builds on our recent demonstration of spherical crystallization in microfluidic emulsions and provides valuable guidelines for the design of spherical crystallization processes using this method.