Optical extinction of randomly orientated and shear flow orientated colloidal kaolinite particles
The optical transmission of a colloidal kaolinite dispersion subject to laminar shear flow is shown experimentally to depend on the magnitude of the velocity gradient, the particle size, the measurement wavelength and the colloid concentration. With the light direction parallel to that of the velocity gradient, the transmitted intensity change on shear was found to either increase or decrease compared with the dispersion at rest. The sign of the change for a given dispersion depends on the particle size in the dispersion and the measurement wavelength employed.
The anomalous diffraction theory of optical scattering cross-section is applied to the case of the plate-like kaolinite particles. It is shown to account for the magnitude and the particle size dependence of the turbidity of dispersions at rest, where the particles are randomly orientated with respect to the incident light. It also explains why the transmitted intensity on shear increases or decreases depending on the particle size and wavelength, when the light is incident mostly on the particle faces.