  We study crystal nucleation under the influence of sedimentation in a model
of colloidal hard spheres via Brownian Dynamics simulations. We introduce two
external fields acting on the colloidal fluid: a uniform gravitational field
(body force), and a surface field imposed by pinning a layer of equilibrium
particles (rough wall). We show that crystal nucleation is suppressed in
proximity of the wall due to the slowing down of the dynamics, and that the
spatial range of this effect is governed by the static length scale of bond
orientational order. For distances from the wall larger than this length scale,
the nucleation rate is greatly enhanced by the process of sedimentation, since
it leads to a higher volume fraction, or a higher degree of supercooling, near
the bottom. The nucleation stage is similar to the homogeneous case, with
nuclei being on average spherical and having crystalline planes randomly
oriented in space. The growth stage is instead greatly affected by the symmetry
breaking introduced by the gravitation field, with a slowing down of the
attachment rate due to density gradients, which in turn cause nuclei to grow
faster laterally. Our findings suggest that the increase of crystal nucleation
in higher density regions might be the cause of the large discrepancy in the
crystal nucleation rate of hard spheres between experiments and simulations, on
noting that the gravitational effects in previous experiments are not
negligible.
