Relaxation optic phenomena in polydisperse suspensions and determination of particle sizes using transmitted light parameters

We discuss the physical mechanisms of orientational optic effects measured for polarized or unpolarized light transmitted through a densely packed or dilute particulate medium. The main attention is paid to particle size effe cts for transient linear dichroism, linear birefringence, and change in tra nsmittance of dilute suspensions after an orienting field is switched off. A general integral equation is derived for relaxation of the transmitted li ght parameters for a polydisperse dilute suspension with axially symmetrica l particles of arbitrary size and axial orientation distribution. Theoretic ally predicted non-exponential decay of the turbidimetric electrooptic effe ct is illustrated by an experimental example. We have also derived equation s that define the average rotational diffusion coefficients and the average particle sizes calculated from the initial slope and the area of the relax ation curve, respectively. The dependence of relaxation curve on particle p arameters (size, shape, refractive index, polydispersity) has been studied by using the T-matrix method and an isotropic spheroid model. For dichroism and birefringence, we have found that the dependence of the optical contri bution to the integral equation kernel on the equivolume fraction size r ca n be described by power functions with exponents that are close to the Rayl eigh exponents if r < lambda/pi and decrease abruptly at r > lambda/pi. The se exponents define the moments of particle size distribution to which the average diffusion constant and the average particle size correspond. (C) 19 99 Elsevier Science B.V. All rights reserved.