  The discrepancy in nucleation rate densities between simulated and
experimental hard spheres remains staggering and unexplained. Suggestively,
more strongly sedimenting colloidal suspensions of hard spheres nucleate much
faster than weakly sedimenting systems. In this work we consider firstly the
effect of sedimentation on the structure of colloidal hard spheres, by tuning
the density mismatch between solvent and colloidal particles. In particular we
investigate the effect on the degree of five fold symmetry present. Secondly we
study the size of density fluctuations in these experimental systems in
comparison to simulations. The density fluctuations are measured by assigning
each particle a local density, which is related to the number of particles
within a distance of 3.25 particle diameters. The standard deviation of these
local densities gives an indication of the fluctuations present in the system.
Five fold symmetry is suppressed by a factor of two when sedimentation is
induced in our system. Density fluctuations are also increased by a factor of
two in experiments compared to simulations. The change in five fold symmetry
makes a difference to the expected nucleation rates, but we demonstrate that it
is ultimately too small to resolve the discrepancy between experiment and
simulation, while the fluctuations are shown to be an artefact of 3d particle
tracking.
