Long-Time Dynamics of Concentrated Colloidal Suspensions

Abstract

The relaxation of density fluctuations in charge-stabilized colloidal suspensions is characterized by the dynamic structure factor S(k, t), which can be measured by dynamic light scattering. In concentrated suspensions S(k, t) decays non-exponentially, due to memory effects. A suitable measure of the overall decay of S(k,t) is the reduced memory function △(k), which can be determined experimentally from the time-integral of S(k, t) and the first cumulant. Formally exact results for △(k) can be obtained from the many-body Smohichowski equation, but for its evaluation one has to introduce approximations. Earlier calculations of △(K), based on a particular form of the mode-coupling approximation, were found to be in disagreement with experimental results. In particular, it was predicted for monodisperse suspensions that △(k → 0) = 0, whereas experimental data extrapolate to finite values of △(0). By extending the previous theory to polydisperse suspensions it will be shown that small amounts of polydispersity give rise to finite values of the non-exponentia.lity factor △ _M (0), of the measured dynamic structure factor.S _M ,(k, t). These effects arise from incoherent one-particle contributions to S _M (k, t) in the polydisperse case, which dominate S _M ,(k, t) for strongly correlated suspensions in the long-wavelength limit. Details can be found in G Nägele, P Baur and It Klein, Physica A231, 49–61, 1996, and P Baur G Nägele and R Klein, Phys. Rev. E53, 6224–6237, 1996