  We propose an improved effective-medium theory to obtain the concentration
dependence of the viscosity of particle suspensions at arbitrary volume
fractions. Our methodology can be applied, in principle, to any particle shape
as long as the intrinsic viscosity is known in the dilute limit and the
particles are not too elongated. The procedure allows to construct a
continuum-medium model in which correlations between the particles are
introduced through an effective volume fraction. We have tested the procedure
using spheres, ellipsoids, cylinders, dumbells, and other complex shapes. In
the case of hard spherical particles, our expression improves considerably
previous models like the widely used Krieger-Dougherty relation. The final
expressions obtained for the viscosity scale with the effective volume fraction
and show remarkable agreement with experiments and numerical simulations at a
large variety of situations.
