PARTICLE STRESS AND VISCOUS COMPACTION DURING SHEAR OF DENSE SUSPENSIONS

This study describes the transition between the quasi-static and the v iscous regimes of shearing of thin layers of spheres in a viscous flui d at high solid loadings. Experiments were conducted in a Couette-type shear cell in two complementary modes: (a) constant particle normal s tress, variable solid fraction and (b) constant solid fraction, variab le particle normal stress. During steady shearing under the constraint of constant solid fraction, transition from a strain rate independent stress to one with a linearly dependent behavior was found to occur w ith a local minimum in the stresses with respect to strain rate; corre spondingly, the solid fraction assumed a maximum with respect to strai n rate under conditions of constant normal stress. These are the first observations of such a phenomenon, which we call viscous compaction. At sufficiently high strain rates, the mixture exhibited a linear Newt onian-like scaling between strain rate and both shear and normal stres ses. These measurements of normal stress are the first since those of Bagnold.