Transition from quasi-static to rate dependent shearing of concentrated suspensions

This study describes the transition between the quasi-static and the viscous regimes of shearing of thin layers of spheres (10 particle diameters) in a viscous fluid at high solid fractions (>49%). Experiments were conducted in two modes: a) constant normal stress, variable solid fraction, b) constant solids fraction, variable normal stress. At sufficiently high strain rates, mixtures exhibited a linear Newtonian-like scaling between stresses (normal and shear) and strain rate. The presence of a measurable normal stress in this regime disproves earlier claims of its absence. The onset of viscous effects is shown to allow the mixture to pack more efficiently, although at sufficiently elevated strain rates, viscous dilatation takes over. During transition from the strain-rate independent regime, it is shown that instabilities may occur in conventional viscometers as a result of the instrument configuration rather than any inherent mixture property.