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We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

The hard sphere disorder-order transition serves as the paradigm for crystallization. However, measurements of the crystallization kinetics for colloidal hard spheres in the coexistence regime are incomplete for early times and are affected by sedimentation. We use time resolved Bragg light scattering to characterize crystal nucleation and growth in a microgravity environment on the space shuttle. In contrast to the classical picture of the nucleation and growth of isolated crystallites, we find substantial coarsening of growing crystallites. We also observe dendritic growth and face-centered cubic as the stable structure.

Recent observations of the disorder−order transition for colloidal hard spheres under microgravity revealed dendritic crystallites roughly 1−2 mm in size for samples in the coexistence region of the phase diagram. Order of magnitude estimates rationalize the absence of large or dendritic crystals under normal gravity and their stability to annealing in microgravity. A linear stability analysis of the Ackerson and Schätzel model for crystallization of hard spheres establishes the domain of instability for diffusion-limited growth at small supersaturations. The relationship between hard sphere and molecular crystal growth is established and exploited to relate the predicted linear instability to the well-developed dendrites observed.