Abstract

In quasielastic light scattering the decay of the autocorrelation function of the scattered light is due to both translational and rotational diffusion. The clusters formed by the kinetic aggregation of colloids have a random structure, which is scale invariant or fractal. This results in a large contribution from rotational diffusion when the size of the cluster becomes comparable to or greater than 1/q, where q is the scattering vector. We measure the scattering from gold colloid clusters, which are aggregated in two different ways, producing clusters with fractal dimensions of 1.8 and 2.1. Both aggregation mechanisms can be simulated by computer, generating the coordinates of the individual particles comprising the clusters. We use these to calculate the total decay of the autocorrelation function, and explicitly include the contributions of rotational diffusion in terms of the moments of the angular anisotropy to the scattering. We find that, when the cluster size becomes larger than 1/q, the higher-order moments contribute significantly to the total scattering. We discuss the effects of this on the autocorrelation function, as both the cluster size and scattering vectors are varied, the shape of the predicted values of the autocorrelation function is compared with experimental results.

© 1986 Optical Society of America