  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.
