Measurement of magnetically induced linear optical birefringence and dichroism in colloidal dispersions

Abstract

Linear birefringence and linear dichroism are a result of preferential orientation of non-spherical molecules or colloidal particles in liquids or dispersions. A novel apparatus is described for measuring simultaneously the birefringence and dichroism induced in liquids or colloidal dispersions by a magnetic field. The apparatus uses phase modulation and a phase-sensitive detector, the modulator being a phase plate rotated at a steady speed in the light path. Theory shows that a birefringent dichroic dispersion produces in-quadrature photovoltaic signals as measured by the photocurrent of a photomultiplier. These signals are separately measurable using phase-sensitive detection. Results are given for the nominally transparent liquid benzene and several dilute but turbid, colloidal, mineral aqueous dispersions. The latter show clearly the existence of both permanent and anisotropic induced magnetic moments in the colloidal particles, with complete particle alignment being reached at magnetic fields of about 1 T.