  We report on measurements of self-diffusion coefficients in discrete
numerical simulations of steady, homogeneous, collisional shearing flows of
nearly identical, frictional, inelastic spheres. We focus on a range of
relatively high solid volume fractions that are important in those terrestrial
gravitational shearing flows that are dominated by collisional interactions.
Diffusion over this range of solid fraction has not been well characterized in
previous studies. We first compare the measured values with an empirical
scaling based on shear rate previously proposed in the literature, and
highlight the presence of anisotropy and the solid fraction dependence. We then
compare the numerical measurements with those predicted by the kinetic theory
for shearing flows of inelastic spheres and offer an explanation for why the
measured and predicted values differ.
