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 effects for transient linear dichroism, linear birefringence, and change in transmittance of dilute suspensions after an orienting field is switched off. A general integral equation is derived for relaxation of the transmitted light parameters for a polydisperse dilute suspension with axially symmetrical particles of arbitrary size and axial orientation distribution. Theoretically predicted non-exponential decay of the turbidimetric electrooptic effect is illustrated by an experimental example. We have also derived equations that define the average rotational diffusion coefficients and the average particle sizes calculated from the initial slope and the area of the relaxation curve, respectively. The dependence of relaxation curve on particle parameters (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 contribution to the integral equation kernel on the equivolume fraction size r can be described by power functions with exponents that are close to the Rayleigh exponents if r < lambda/pi and decrease abruptly at r > lambda/pi. These exponents define the moments of particle size distribution to which the average diffusion constant and the average particle size correspond. (C) 1999 Elsevier Science B.V. All rights reserved.