  We investigate the emergence of long-range correlations in granular shear
flow. By increasing the density of a simulated granular flow we observe a
spontaneous transition from a dilute regime, where interactions are dominated
by binary collisions, to a dense regime characterized by large force networks
and collective motions. With increasing density, interacting grains tend to
form networks of simultaneous contacts due to the dissipative nature of
collisions. We quantify the size of these networks by measuring correlations
between grain forces and find that there are dramatic changes in the statistics
of contact forces as the size of the networks increases.
