Crystal nucleation in sedimenting colloidal suspensions

Homogeneous crystal nucleation from a metastable hard-sphere colloidal liquid has been extensively studied in simulations and experiments. A 12 order of magnitude difference between simulated and experimental nucleation rates is observed, the origin of which remains a puzzle. Here, we experimentally study crystal nucleation at the single particle level in suspensions of hard-sphere-like colloids under the influence of sedimentation. We find that sedimentation significantly enhances the nucleation rate, but contrary to what was previously thought, this is not due to simple density fluctuations, as the nucleation barriers become independent of the local density in a sedimentating fluid. Instead, we find an enhancement of the local dynamics, which suggests that local convective cells are responsible for shear-enhanced crystallization at low volume fractions.