Rheo-optical properties of Co-γ-$Fe_2O_3$ suspension in magnetic and hydroydynamic fields

Abstract

This thesis addresses the problem of experimentally characterizing the steady state orientation in a dilute suspension of rigid, spheroidal particles induced by combined external fields (hydrodynamic and magnetic fields). Conservative linear dichroism and orientation angle of Co-γ-$Fe_2O_3$ suspension was measured and compared with the theoretical results for the model of rigid spheroidal particles dissolved at low concentration in a dielectric Newtonian fluid and subjected to shear and magnetic fields which has been developed elsewhere [1]. Co-γ-$Fe_2O_3$ particles are taken to be rigid spheroids of a certain aspect ratio and with permanent magnetic moment. A theoretical interpretation was applied to explain the experimental results of for Co-γ-$Fe_2O_3$ suspensions. The orientational anisotropy which results from imposition of the external fields can be fully described by two quantities: the average orientation direction of the major axis of the particles (orientation angle), and the degree of particle alignment about that direction (birefringence, dichroism). The experimental technique is based on flow and magnetic fields induced dichroism. The degree of particle alignment and the orientation direction are inferred directly from the observed magnitude and extinction direction of the dichroism using an appropriate light scattering theory. The Rayleigh approximations to the light scattering theory are considered in this work. The optical method employs polarization modulation scheme and represents a novel use for the photo-elastic modulator. This optical method provides a simultaneous measurements of flow and magnetic fields induced dichroism and its extinction angle. The first and second harmonics of the detector signal, which are dependent functions of the dichroism and extinction angle, are measured simultaneously.