Experimental size determination of spheroidal particles via the T-matrix method

Abstract

Light scattering (LS) properties have been used for determining the size and shape in colloidal suspensions of hematite particles in water. The system has been approximated by a suspension of randomly oriented, monodisperse, spheroidal particles with an equivalent-volume-diameter d eq and an axial ratio, or eccentricity, ε= a/ b ( b=revolution axis), in the single-scattering mode. Relative intensity measurements have been taken at several angles, and the results have been compared with theoretical models obtained by combining Waterman’s Extended Boundary Condition Method (EBCM) together with Mishchenko’s analytical averaging scheme for randomly oriented particles. Both full and depolarized LS have been measured and fitted to theoretical curves. While both methods give good agreement on particle size, shape is not always characterized unequivocally with full LS. The problem is overcome by using depolarized LS, which, for spherical particles, is zero; fit between experimental depolarized LS and theoretical predictions give the correct value of eccentricity, as well as a more accurate value of d eq. This is an experimental validation that depolarized LS can be useful for determining size and shape of real particle systems.