  The bulk dynamics of dense granular materials arise through a combination of
particle-scale and mesoscale effects. Theoretical and numerical studies have
shown that collective effects are created by particle-scale anisotropic
structures such as grain connectivity (fabric), force transmission, and
frictional mobilization, all of which influence bulk properties like bulk
friction and the stress tensor through the Stress-Force-Fabric (SFF)
relationship. To date, establishing the relevance of these effects to
laboratory systems has remained elusive due to the challenge of measuring both
normal and frictional contact forces at the particle scale. In this study, we
perform experiments on a sheared photoelastic granular system in an quasi-2D
annular (Couette) cell. During these experiments, we measure particle
locations, contacts, and normal and frictional forces vectors during loading.
We reconstruct the angular distributions of the contact and force vectors, and
extract the corresponding emergent anisotropies for each of these metrics.
Finally, we show that the SFF relation quantitatively predicts the relationship
between particle scale anisotropies, the stress tensor components, and the bulk
friction coefficient, capturing even transient behaviors. As such, this method
shows promise for application to other dense particulate systems where fabric
anisotropy can provide a useful measure of bulk friction.
