Controlled spherulitic crystal growth from salt mixtures

Spherulites are spherical crystals that are polycrystalline assemblies of radially organized crystallites. Despite their wide prevalence and relevance to fields ranging from geology to medicine, the dynamics of spherulitic crystallization and the conditions required for such growth remain ill-understood. Here, we reveal the conditions for controlled spherulitic growth of sodium sulfate crystals from evaporating aqueous solutions mixtures of sulfate salts at room temperature. We reveal that divalent metal ions in the salt solutions induce spherulitic growth of sodium sulfate through non-classical nucleation and self-assembly of (nearly)-oriented nanocrystals. A key result is the very high viscosity (~ 111 Pa ⋅ s) of the highly supersaturated solutions at the onset of spherulite precipitation. This allows for slow dynamics that facilitates the formation of a large number of mesoscopic prenucleation clusters, that subsequently show diffusion-limited growth and assemble into the spherulitic shapes. The spherulites are found to be metastable structures that form in out-of-equilibrium conditions. As the supersaturation decreases during growth, Na₂SO₄ spherulites can also evolve into other shapes depending on the evaporation rate. These findings shed light on the conditions that govern spherulite formation and provide practical strategies for tuning their morphology.