Dynamics of concentrated colloidal suspensions: diffusion, aggregation and gelation

We show that diffusing wave spectroscopy (DWS) permits the characterization of dynamic properties of concentrated colloidal suspensions on a large range of time– and length–scales ranging from a few angstroms to tens of nanometres. We focus in particular on the study of the aggregation and sol–gel transition in concentrated colloidal suspensions. We present a new technique to overcome the problem of nonergodicity in DWS of solid–like systems. Using this technique we obtain quantitative information about the microscopic dynamics all the way from an aggregating suspension to the final gel, thereby covering the whole sol–gel transition. At the gel point a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. We combine DWS with small–angle neutron scattering and demonstrate that we can obtain an in situ characterization of the changes in the microstructure and the local particle dynamics as the system undergoes a sol–gel transition.