Shear thickening in granular suspensions: inter-particle friction and dynamically correlated clusters

We consider the shear rheology of concentrated suspensions of non-Brownian frictional particles. The key result of our study is the emergence of a pronounced shear-thickening regime, where frictionless particles would normally undergo shear-thinning. We clarify that shear thickening in our simulations is due to enhanced energy dissipation via frictional inter-particle forces. Moreover, we evidence the formation of dynamically correlated particle-clusters of size $\xi$, which contribute to shear thickening via an increase in \emph{viscous} dissipation. A scaling argument gives $\eta\sim \xi^2$, which is in very good agreement with the data.