Rheology of wet granular materials in shear flow: Experiments and discrete simulations

The behaviour of wet granular media in shear flow is characterized by the dependence of apparent friction μ^∗ and solid fraction Φ_S on the reduced pressure P^∗ and the inertia number I. Reduced pressure, P^∗=σ₂₂a²/F₀, compares the applied normal stress σ₂₂ on grains of diameter a to the tensile strength of contact F₀ (proportional to the surface tension Γ of the liquid and the beads diameter). A specifically modified rotational rheometer is used to characterize the response of model wet granular material (beads with diameter of submillimetric range) to applied shear rate γ under controlled normal stress σ₂₂. Discrete Element Method (DEM) simulations in 3D are carried out in parallel and numerical results are compared with experimental ones. As P^∗ is reduced, an increase of the apparent friction coefficient μ^∗=σ₁₂/σ₂₂, measured in the critical state and in slow flows with inertial effects is observed. While the agreement between experiments and simulations is good for dry materials as well as for wet materials in the quasistatic limit (I→0), some differences appear at finite I, for which some possible origins are discussed.