I 11. Laminar-Flow Properties of Flocculated
Suspensions
The turbulent heat, mass, and momentum transport characteristics of suspensions of particles with near-colloidal dimensions have previously been shown t o be related to the non-Newtonian laminar-flow properties of the suspension. However the laminar-flow properties were not studied systematically. The present study showed that the principal factors affecting the magnitude of the laminar-flow properties of flocculated suspensions were the concentration and particle diameter of the solid phase. The range of variables included concentrations from 0.02 t o 0.23 volume fraction solids and particle sizes from 0.35 to 13 p. Materials tested included thorium oxide, kaolin, titanium oxide, aluminum oxide, graphite, magnesium, and uranium dioxide.
A t high rates of shear the data were fitted satisfactorily with the Bingham plastic model. With the use of this model, the yield stress was directly proportional to the cube of the volume fraction solids and inversely proportional to the first or second power of the particle diameter, depending on the particle shape. The logarithm of the ratio of the coefficient of rigidity of the suspension t o the viscosity of the suspending medium was directly proportional to the volume fraction solids over the complete range of concentrations studied. Although specific electrolytes (such as oxalate or pyrophosphate) deflocculated the suspensions even a t low concentrations, the suspensions remained flocculated both in the presence of up t o 0.1 M of 1:l electrolyte and over a pH range of 4 to 12. This i s consistent with present theories of the stability of colloidal and nearcolloidal particles.
Although the concentration-dependence relationships of the yield stress and coefficient of rigidity have been proposed previously and have been shown to apply to specific systems, this is the first time they hove been shown to apply to a broad class of materials. I n addition it is the first time that the laminar-flow properties used in the particle-size and concentration correlations have been determined over a shear-rate range which permits application of the results i n correlations of turbulent heat, mass, and momentum transfer data obtained i n systems of commercial interest.