Quantifying heterogeneity in frictional contacts between photoelastic particles

The properties of granular materials arise largely from the interparticle friction and collective behavior. Due to this, it is important to understand the interactions between individual particles within the system. Numerical simulations commonly assume that a constant coefficient of friction is sufficient to describe the frictional behavior of inter-particle contacts; however this assumption has been poorly validated in the photoelastic materials commonly used in granular experiments. Furthermore, other factors, such as multicontact effects or a breakdown of Coulomb friction for soft particles, may play an important role in governing their interactions. We report on experiments in an apparatus developed to image a linear series of quasi-2-dimensional photoelastic particles confined to a small gap as they undergo particle-scale slip events. The setup allows for several possible contributing variables to be tested including lateral confining pressure, surface roughness, and loading rate. Using PeGS photoelastic analysis, we are able to measure the effects of sliding friction over an ensemble of slip events. By varying the conditions under which loading occurs, we evaluate the extent to which modeling the interaction as Coulomb failure holds under a constant coefficient of friction.