Crystallization kinetics and morphology in phase separating and sedimenting mixtures of colloidal spheres and rods

The crystallization of sedimentating silica spheres in the presence of silica-coated boehmite rods in low-salt dimethylformamide is studied by means of confocal scanning laser microscopy. As expected, addition of rods gives rise to a net attraction due to the depletion effect. Upon increasing rod volume fractions, below a predicted equilibrium binodal, crystalline ordering of the spheres takes place faster but gives cause for more grain boundaries. Addition of rods at volume fractions in the theoretically predicted two-phase region gives rise to aggregation and glasslike sediments. We explain these results on the basis of the different gravitational lengths and sedimentation rate of both species: higher rod concentrations drive the system quicker into the two-phase region of the predicted phase-diagram.