Depolarized light scattering from prolate anisotropic particles: the influence of the particle shape on the field autocorrelation function

TL;DR

This paper presents a detailed theoretical analysis of depolarized dynamic light scattering from anisotropic particles, showing how particle shape influences the scattering function and how this can be used to determine particle geometry.

Contribution

The study introduces a formal multipole expansion approach to relate particle shape and aspect ratio to scattering measurements, including translational-rotational coupling effects.

Findings

The scattering function depends strongly on particle shape and aspect ratio.

Short-time behavior reveals deviations from simple $Q^2$ proportionality.

Particle shape details can be inferred from scattering data.

Abstract

We provide a theoretical analysis for the intermediate scattering function typically measured in depolarized dynamic light scattering experiments. We calculate the field autocorrelation function $g_1^{\rm VH}(Q,t)$ in dependence on the wave vector $Q$ and the time $t$ explicitly in a vertical-horizontal scattering geometry for differently shaped solids of revolution. The shape of prolate cylinders, spherocylinders, spindles, and double cones with variable aspect ratio is expanded in rotational invariants $f_{lm}(r)$. By Fourier transform of these expansion coefficients, a formal multipole expansion of the scattering function is obtained, which is used to calculate the weighting coefficients appearing in the depolarized scattering function. In addition to translational and rotational diffusion, especially the translational-rotational coupling of shape-anisotropic objects is considered. From the short-time behavior of the intermediate scattering function, the first cumulants $\Gamma(Q)$ are calculated. In a depolarized scattering experiment, they deviate from the simple proportionality to $Q^2$. The coefficients $f_{lm}(Q)$ strongly depend on the geometry and aspect ratio of the particles. The time dependence, in addition, is governed by the translational and rotational diffusion tensors, which are calculated by means of bead models for differently shaped particles in dependence on their aspect ratio. Therefore, our analysis shows how details of the particle shape---beyond their aspect ratio---can be determined by a precise scattering experiment. This is of high relevance in understanding smart materials which involve suspensions of anisotropic colloidal particles.