Rheooptical determination of aspect ratio and polydispersity of nonspherical particles

A rheooptical method proposed rapidly determines the aspect ratio and polyd ispersity of small axisymmetric, nonspherical particles. The time evolution of the average orientation angle of optically isotropic, nonabsorbing part icles upon inception of a shear flow tc ns monitored by a polarization modu lation method by using dilute suspensions of the particles. Since the orien tation angle is a geometric propel ty of the particles, no arbitrary assump tions on the scattering mechanism had so be made to analyze the data. The p eriod of the dampen oscillatory response of the orientation angle is relate d to the average aspect ratio. In the limit of strong flows the damping onl y depends on dispersion in particle shape. An analytical expression to rela te the polydispersity of the equivalent hydrodynamic aspect ratio to the da mping function is presented as well as the applicability of the technique b y studying the hydrodynamic aspect ratio and associated polydispersity in a sample containing ellipsoidal hematite particles.