  We study the transport properties of particles draining from a silo using
imaging and direct particle tracking. The particle displacements show a
universal transition from super-diffusion to normal diffusion, as a function of
the distance fallen, independent of the flow speed. In the super-diffusive (but
sub-ballistic) regime, which occurs before a particle falls through its
diameter, the displacements have fat-tailed and anisotropic distributions. In
the diffusive regime, we observe very slow cage breaking and Peclet numbers of
order 100, contrary to the only previous microscopic model (based on diffusing
voids). Overall, our experiments show that diffusion and mixing are dominated
by geometry, consistent with fluctuating contact networks but not thermal
collisions, as in normal fluids.
