Effect of rotational diffusion on quasielastic light scattering from fractal colloid aggregates

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Abstract
We examine the contribution of rotational diffusion to quasielastic light scattering (QELS) from fractal colloid aggregates, both theoretically and experimentally. Rotational diffusion makes a substantial contribution to QELS when the size of the clusters is large compared with the inverse of the scattering wave vector, due to the anisotropy of the clusters at all length scales smaller than the cluster size. We evaluate the rotational contributions to QELS by performing a multipole expansion of the light scattered from computer-simulated clusters. Experimentally, rotational contributions are observed through measurement of the wave-vector dependence of the first cumulant. We find excellent agreement between cumulants calculated through our multipole-expansion technique and those obtained in our experimental measurements.