Apparent slip and viscoplasticity of concentrated suspensions

The apparent slip flows of incompressible and viscoplastic (Herschel-Bulkley) fluids in plane Couette, capillary, and rectangular slit dies under fully developed, isothermal, and creeping flow conditions were analyzed assuming that the apparent slip layer consists solely of the binder and its thickness is independent of the flow rate. Both the drag-induced (plane Couette) and pressure-induced (capillary and slit) flows generate the same dependencies of the wall-slip velocity on the wall shear stress. Navier's slip coefficient, which relates the wall-slip velocity to the shear stress, is similar for all three flows and is a function of the thickness of the apparent slip layer and the shear viscosity of the binder. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the suspension at the wall and the yield stress. The analysis of the slip data of various concentrated suspensions of rigid particles reveals that, as a first approximation, the apparent slip layer thickness is related to the particle diameter and the ratio of the volume loading level over the maximum packing fraction of the particles. (C) 2005 The Society of Rheology.