The measurement of the absolute viscosity of fluids exhibiting anomalous flow properties and a study of the phenomenon of thixotropy

Abstract

The theory of the measurement of the absolute viscosity (the' ratio of shearing stress to rate of shear) of instantaneously thixotropic fluids from flow along a tube and from flow between concentric cylinders makes assumptions requiring critical examination. An instrument using flow along a tube is suitable only for instantaneously thixotropic fluids: an instrument using flow between concentric cylinders is also suitable for normally thixotropic fluids. The design of these instruments is particularly related, "both to the conditions necessary for the validity of the given theory and to the characteristics of the fluids used. The usually accepted treatment of the tube flow is in error if the fluid has a yield value. The fluids used, ball-mill dispersions of solid particles in liquid media, have static and dynamic yield values that are different, and this necessitates a special measuring procedure in both instruments. The relation between the rate of shear and the shearing stress determined by either instrument is the same within the limits of experimental error: this error is small so that the validity of the methods of measurement is demonstrated. The dynamic yield values of these fluids are independent of temperature, but the absolute viscosity at any given shearing stress (including the limiting viscosity for high rates of shear measured in another instrument of the concentric cylinder type) varies as the viscosity of the medium. At low shearing stresses the absolute viscosity can be represented as a function of shearing stress by a simple empirical equation. As the concentration of particles is reduced, the limiting viscosity approaches the viscosity of the medium more rapidly than predicted by Einstein's equation.