  From a reanalysis of the published literature, the low-shear viscosity of
suspensions of hard spheres is shown to have a dynamic crossover in its
concentration dependence, from a stretched exponential at lower concentrations
to a power law at elevated concentrations. The crossover is sharp, with no
transition region in which neither form applies, and occurs at a volume
fraction (ca. 0.41) and relative viscosity (ca. 11) well below the sphere
volume fraction and relative viscosity (0.494, 49, respectively) of the lower
phase boundary of the hard sphere melting transition. For soft spheres --
taking many-arm star polymers as a model -- with increasing sphere hardness
$\eta(\phi)$ shows a crossover from random-coil polymer behavior toward the
behavior shown by true hard spheres.
