Particle scale anisotropy controls bulk properties in sheared granular materials

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, 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
a quasi-2D annular cell. During these experiments, we measure particle
locations, contacts, and normal and frictional force 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 for the first time in an experimental
system 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 — closing
the gap between experimental measurements and prior theoretical and
numerical models. These datasets contain the particle position
information, force network information, as well as the data from the three
data-containing figures in the paper relating bulk stress information to
the particle scale anisotropies.