This paper concerns the dynamic light scattering by suspensions of polydisperse hard spherical particles. It is concluded that for relatively high volume fractions and fairly narrow distributions, the light scattering correlation function should, to a good degree of approximation, be composed of two independent modes with well-separated decay times. The faster decaying mode describes collective stochastic compression–dilation motions and is present even for a monodisperse system. The slower decaying mode describes the exchange of different species. The relative mode amplitudes are calculated in the Percus–Yevick approximation for hard spheres. The two decay times of fast and slow mode are associated with diffusion coefficients D+ and D−, respectively. In the case of scattering power polydispersity D+ and D− are rigorously identified with collective and self-diffusion, respectively. In the case of size polydispersity D+ and D− can be associated with ‘‘average’’ collective and self-diffusion coefficients. The considerations are limited to zero scattering wave vector and volume fractions above 0.15. Comparison with experimental results from other papers are in favor of the theory.

We have performed photon correlation spectroscopy measurements on silica particles dispersed in cyclohexane. The diffusion coefficient D of the particles has been measured as a function of their volume fraction φ and could be described by D=D0(1+kDφ), where D0 is the diffusion coefficient at infinite dilution and the constant kD has a value of 1.3±0.2. This value is in agreement with the theoretical values of kD for hard spheres of Burgers, Batchelor, and Felderhof. These results have been compared with time averaged light scattering and sedimentation measurements. The time averaged light scattering experiments confirmed the hard sphere character of the particles earlier found by van Helden. The sedimentation constant has been measured as a function of the volume fraction of the particles resulting in a virial constant ks=6±1, which agrees with the theoretical values for hard spheres.

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Photon correlation spectroscopy experiments on silica particles dispersed in cyclohexane over a large concentration range and near the optical matching point have been explained by the occurrence of two different relaxation modes: one mode due to the collective diffusion of the particles and the other mode due to the self-diffusion. This is in agreement with the theoretical considerations of Weissman and Pusey. In the diluted region, information has been obtained about the behavior of the self-diffusion coefficient at zero wave vector (K) as a function of the particle volume fraction (φ). We found that the following relation exists: Ds(K = 0) = D0(1 + k′sφ), where D0 is the diffusion coefficient at infinite dilution and the constant ks′ has a value of −2.7±0.5. The concentration dependent behavior of the collective and self-diffusion coefficient has further been compared with the results of sedimentation and viscosity measurements. At high particle concentrations we observed that the relation between collective diffusion and sedimentation by means of the generalized Einstein relation has been confirmed qualitatively. The self-diffusion coefficient appeared to be inversely proportional to the viscosity of the solution. The sedimentation experiments have been found to be in agreement with the theories of Reed and Anderson over the whole concentration range, while at high particle volume fractions the theory of Lekkerkerker seemed to reasonably fit. Finally, we observed at φ≳0.40 that numerically, the values of the collective and self-friction coefficient are nearly the same.

Theories on liquid structure properties are applied to concentrated colloidal dispersions in nonpolar media. Light scattering experiments on microemulsions (W/O) with benzene and toluene as the continuous phases are characterized in terms of a hard sphere repulsion and a perturbative attraction. The origin of the attraction and the influence of polydispersity on particle size are discussed. Light scattering of crosslinked PMMA-latex particles in benzene as a function of scattering angle showed structure formation at higher latex concentrations. The results were discussed in terms of the structure factor S(K), susceptibility x(K) and the radial distribution function g ( v ) of the particle centres. The latex behaves as an " expanded " liquid structure. This structure formation was also reflected in the diffusion coefficient measured by quasi-elastic light scattering.