Flow turbidity in colloidal kaolinite dispersions

Abstract

The optical transmission and turbidity of a colloidal dispersion of non-spherical particles depends on the number of particles per unit volume and the average extinction cross-section per particle. Laminar shear flow of the dispersion causes preferential orientation of the non-spherical particles and changes the average optical extinction cross-section per particle. The result is a velocity gradient dependence of the optical turbidity.

This paper describes apparatus designed to make accurate flow turbidity measurements with the velocity gradient direction either parallel or at right-angles to the incident light direction.

Results are presented for colloidal kaolinite clay particles dispersed in water and in a range of more viscous water + glycerol mixtures. A semi-empirical expression is developed which relates the turbidity to the velocity gradient by means of a time T and dimensionless parameter p. Values of T derived from the experimental results suggest that T is a relaxation time of an individual kaolinite particle averaged over the distribution of particle sizes in any particular clay fraction. The parameter p is found to be dependent on the polydispersity of the fraction, decreasing with increasing fraction breadth. The linear dependence of T on continuous phase viscosity indicates that the clay particles interact hydrodynamically with the continuous phase. The cubic dependence of T on the equivalent Stokes diameter (ESD) suggests that ESD is proportional to the face diameter of the plate-like kaolinite particles. The use of flow turbidity in colloidal particle size measurement is discussed, and the technique compared with that of flow birefringence.