Executive Summary
This report summarizes research supported by I.F.P.R.I. at Stanford University on the use of high speed optical polarimetry measurements to characterize the dynamics and structure of fine particle suspensions. The work is divided along two lines:
1 . Characterization of single particle properties. The optical experiments employed in the laboratory of the principal investigator provide sensitive measurements of anisot- ropy. In dilute suspensions, this would refer to orientation of nonspherical particles by external fields (flow, electric, or magnetic). These measurements can be used to determine a number of physical characteristics of colloidal particles, such as size and shape distributions, and dipole moments, This report summarizes a new technique whereby the first direct measurements of dipole moments for colloidal particles can be obtained. The method combines optical measurements of the average orientation angle of parti- cles as a function of time following the simultaneous inception of a shearing flow and an electric field. In the limit of very small times following the application of flow, the convection diffusion equation governing the particle dynamics can be solved exactly and the experiment yields a determination of the induced electric dipole moment of the particles in suspension. Results are reported here on measurements made on iron oxide particles suspended in a glycerin/water mixture. It is demonstrated that this method yields measurements of the anisotropy in the particle polarizability without resorting to approximate models of particle orientation and in a manner that is insensitive to the presence of brownian motion.
2. Field induced structure in dense suspensions. The application of external fields onto dense suspensions of colloidal spheres can induce structural anisotropy. Optical experiments in the laboratory of the principal investigator have been used to measure anisot- ropy in the particle pair distribution function as a result of applying either electric or flow fields, Two problems have been investigated over the past two years: (1) electric field induced anisotropy and chain formation, and (2) the connection between structural anisotropy and shear thickening behavior in dense suspensions. The findings of this research have recently been published in two papers and preprints of the published work are included in an appendix to the annual report.