  Granular flows during a shear-induced mixing process are studied using
Discrete Element Methods. The aim is to understand the underlying elementary
mechanisms of transition from unmixed to mixed phases for a granular material
featuring a broad distribution of particles, which we investigate
systematically by varying the strain rate and system size. Here the strain rate
varies over four orders of magnitude and the system size varies from ten
thousand to more than a million granules. A strain rate-dependent transition
from quasistatic to purely inertial flow is observed. At the macroscopic scale,
the contact stresses drop due to the formation of shear-induced instabilities
that serves as an onset of granular flows and initiates mixing between the
granules. The stress-drop displays a profound system size dependence. At the
granular scale, mixing dynamics are correlated with the formation of shear
bands, which result in significantly different timescales of mixing, especially
for those regions that are close to the system walls and the bulk. Overall, our
results reveal that although the transient dynamics display a generic behavior
these have a significant finite-size effect. In contrast, macroscopic behaviors
at steady states have negligible system size dependence.
