NONEXPONENTIAL RELAXATION OF DENSITY-FLUCTUATIONS IN STRONGLY INTERACTING COLLOIDAL SUSPENSIONS

The relaxation of density fluctuations is characterized by the experim entally accessible dynamic structure factor S(k, t). Whereas the short -time behaviour of this quantity is well understood, its long-time cha racteristics are more difficult to determine since memory effects beco me important giving rise to non-exponential relaxation. Formally, exac t results for these memory effects can be derived on the basis of the many-body Smoluchowski equation, but for their evaluation one has to i ntroduce approximations. Previous results, obtained by a mode-coupling approximation, were used to calculate a mean relaxation time <(tau)ov er bar>(k) of S(k, t) and a reduced memory function Delta(k), which ch aracterizes the deviation of S(k, t) from a simple exponential decay i n time. A comparison with experimental results showed only partial agr eement. Whereas theory predicts Delta(k --> 0) = 0, the experiments fo und that Delta(k) approaches a finite value in this limit. It will be shown that this discrepancy is due to polydispersity effects. We have improved the mode-coupling approximation and have extended the theory to polydisperse systems. It is remarkable that fairly small amounts of polydispersity give indeed rise to a finite value of Delta(k --> 0) f or the dynamic structure factor of the polydisperse system in contrast to a vanishing Delta(k --> 0) for monodisperse suspensions.