TABLE OF CONTENTS Applications of optical methods to study dilute colloidal dispersions subject to flow were pioneered by Mason and coworkers. These authors used simple turbidity measurements to follow the orientation dynamics of ellipsoidal particles during transient shear flow experiments [175,176]. In addition, the superposition of shear and electric fields were studied. The goal of this work was to verify the predictions of theories predicting the orientation distributions of prolate and oblate particles, such as that discussed in section 7.2.1.2. This simple technique clearly demonstrated the phenomena of particle rotations within Jeffery orbits, as well as the effects of Brownian motion and particle size distributions. The method employed a parallel plate flow cell with the light sent down the velocity gradient axis.
Although turbidity measurements are attractive because of their simplicity, they suffer from an important limitation: they cannot distinguish between changes in the integrated light intensity from changes in the degree of orientation and changes in the average angle of orientation relative to a laboratory frame. Both effects will occur during the transient shear flow of nonspherical particles. Dichroism measurements, however, when reported with simultaneous determinations of the average orientation angle of the imaginary refractive index tensor, can eliminate mis ambiguity. The first reports of dichroism measurements on dilute colloidal particles subject to transient shear flow were made by Frattini and Fuller [35,177]. This work used photoelastic, polarization modulated dichroism to study the orientation of colloidal bentonite.
The influence of non-Newtonian rheology of a suspending...
