A general(ized) local rheology for wet granular materials

We study the rheology of dry and wet granular materials in the steady quasistatic regime using the discrete element method in a split-bottom ring shear cell with focus on the macroscopic friction. The aim of our study is to understand the local rheology of bulk flow at various positions in the shear band, where the system is in critical state. Wedevelop a general(ized) rheology, in which the macroscopic friction is factorized into a product of four functions, on top of the classical mu(I) rheology, each of which depends on exactly one dimensionless control parameter, quantifying the relative importance of different micro-mechanical machanisms. These four control parameters relate the time scales of shear rate t(gamma), particle stiffness t(k), gravity tg and cohesion t(c), respectively, with the governing time scale of confining pressure t(p). While t(gamma) is large and thus of little importance for most of the slow flow data studied, it increases the friction in critical state, where the shear rate is high and decreases friction by relaxation (creep) where the shear rate is low. tg and tk are comparable to tp in the bulk, but become more or less dominant relative to tp at the extremes of low pressure at the free surface and high pressure deep inside the bulk, respectively. The effect of wet cohesion on the flow rheology is quantified by tc decreasing with increasing cohesion. Furthermore, the proposed rheological model predicts well the shear thinning behavior both in the bulk and near the free surface; shear thinning rate becomes less near the free surface with increasing cohesion.