  We propose a simple and general model accounting for the dependence of the
viscosity of a hard sphere suspension at arbitrary volume fractions. The model
constitutes a continuum-medium description based on a recursive-differential
method that assumes a hierarchy of relaxation times. Geometrical information of
the system is introduced through an effective volume fraction that approaches
the usual filling fraction at low concentrations and becomes one at maximum
packing. The agreement of our expression for the viscosity with experiments at
low- and high-shear rates and in the high-frequency limit is remarkable for all
volume fractions.
