The effects of concentration and composition upon the rheological properties of concentrated monomodal, bimodal, and trimodal suspensions of neutrally buoyant rigid spheres in Newtonian fluids were experimentally determined. Isothermal flow curves were calculated from torque measurements made with a Gilinson-Dauwalter-Merrill concentric cylinder viscometer with a stationary inner cylinder, a diameter ratio of 1.08, and a gap of 1.5 mm at as many as ten distinct shear rates between 0.06 and 100. inverse seconds. Median sphere diameters were 26, 61, 125, 183, and 221 microns. End effects were determined by a two bob technique, and smooth and artificially roughened cylinders were used to determine wall effects.
Monomodal suspensions were found to be power law fluids with strictly Newtonian flow properties at concentrations of 35 volume percent of solids or less, but exhibiting dilatancy at concentrations above 40 volume percent of solids. Relative viscosities and a value of the Mooney self-crowding factor λii, are reported for monomodal suspensions with concentrations between 20 and 40 volume percent of solids, and the power law parameters are reported for monomodal suspensions with concentrations between 40 and 52 volume percent of solids.
Bimodal suspensions were found to be power law fluids with strictly Newtonian flow properties in the range of total concentrations up to the onset of dilatancy at total concentrations near 60 volume percent of solids for compositions at which the relative viscosity was minimized. The Mooney bimodal crowding factors, λij, were determined at nine diameter ratios between 0.12 and 0.68.
Trimodal suspensions with size ratios 0.14, 0.33, 1, at a total concentration of 45 volume percent of solids were found to be Newtonian fluids. The relative viscosities of these trimodal suspensions were found to be greater at all compositions than the minimum relative viscosity of an equally concentrated bimodal suspension of spheres with a size ratio of 0.14.
The experimental results were discussed in terms of previous investigations and of the principal theoretical results appearing in the literature. Additional work was recommended in order to define the limits of Newtonian flow and the power law parameters for bimodal and trimodal suspensions as a function of size ratio, composition, and concentration. The approximate location of the anticipated minimum in relative viscosity as a function of trimodal composition was discussed.
