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

We investigate, both experimentally and theoretically, the fow and structureof a slurry when sheared between 2 horizontal plates. The slurry, otherwisecalled a "wet granular material", is made of non-Brownian particles immersed ina viscous fluid. The particles are heavier than the fluid, consequently,gravity influences the structure and flow profiles of the sheared material.Experiments are carried out in a plane Couette device, with a model slurrycomposed of approximately monodisperse spherical PMMA particles in oil, at highaverage solid concentration (about 58%). Optical observation reveals a typical2-phase configuration, with a fluidized layer in contact with the upper plateand on top of an amorphous solid phase. We provide data on velocity profiles,wall-slip and shear stress versus the average shear rate. To interpret thedata, we propose a model for the ideal case of infinite horizontal flat plates.The model, of mean field type, is based on local constitutive equations for thetangential and normal components of the stress tensor and on expressionsrelating the material viscometric coefficients (the shear viscosity eta and thenormal viscosity psi) with the local concentration (phi) and the local shearrate. 1-,2- and 3-phase configurations are predicted, with non linear flow andconcentration profiles. We conclude that the model equations correctly describethe experimental data, provided that appropriate forms are chosen for thedivergence of eta and psi near the packing concentration (phi_max), namely a(phi_max-phi)^-1 singularity.Comment: 26 pages, 12 figures ; submitted to Physics of Fluid