The flow of a very concentrated slurry in a parallel-plate device: Influence of gravity

Abstract

We investigate, both experimentally and theoretically, the flow and structure of a slurry when sheared between two horizontal plates. The slurry, otherwise called “wet granular material,” is made of non-Brownian particles immersed in a viscous fluid. The particles are heavier than the immersion fluid, in contrast to the so-called “suspensions,” corresponding to density-matched fluid and particles. Consequently, gravity influences the structure and flow profiles of the sheared material. Experiments are carried out in a plane Couette device with a model slurry composed of quasimonodispersed spherical polymethylmetacrylate particles in oil, at high average solid concentration (about 59%). Optical observation reveals a typical two-phase configuration, with a fluidized layer in contact with the upper plate and on top of an amorphous solid phase. We provide data on velocity profiles, wall slip, average shear stress, and average normal stress, versus the angular velocity of the upper plate. To interpret the data, we propose a model for the ideal case of infinite horizontal flat plates (plane Couette flow). The model, of mean-field type, is based on local constitutive equations for the tangential (τ) and normal (N) components of the stress tensor and on material expressions relating the material viscometric coefficients (the shear viscosity η and the normal stress viscosity ψ) with the local concentration (Φ) and the local shear rate. One-, two-, and three-phase configurations are predicted, with nonlinear flow and concentration profiles. We conclude that model equations correctly describe the experimental data, provided that appropriate forms are chosen for the divergences of η and ψ near the packing concentration (Φm), namely, a (Φm−Φ)−1 singularity.