Non-exponential relaxation of density fluctuations in strongly interacting colloidal suspensions

✍ Scribed by Gerhard Nägale; Peter Baur; Rudolf Klein

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 531 KB
• Volume: 231
• Category: Article
• ISSN: 0378-4371
• DOI: 10.1016/0378-4371(95)00462-9

✦ Synopsis

The relaxation of density fluctuations is characterized by the experimentally accessible dynamic structure factor S(k, t). Whereas the short-time behaviour of this quantity is well understood, its long-time characteristics are more difficult to determine since memory effects become important giving rise to non-exponential relaxation. Formally, exact results for these memory effects can be derived on the basis of the many-body Smoluchowski equation, but for their evaluation one has to introduce approximations. Previous results, obtained by a modecoupling approximation, were used to calculate a mean relaxation time ~(k) of S(k, t) and a reduced memory function A(k), which characterizes the deviation of S(k, t) from a simple exponential decay in time. A comparison with experimental results showed only partial agreement. Whereas theory predicts A(k ~ 0) = 0, the experiments found that A(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 A(k ~ 0) for the dynamic structure factor of the polydisperse system in contrast to a vanishing A(k ~ 0) for monodisperse suspensions.