Aggregation and gel formation in biopolymer solutions

The application of light scattering to concentrated colloidal suspensions h as often been considered too complicated due to strong multiple scattering. Here we show that diffusing wave spectroscopy (DWS) permits the characteri zation of dynamic and static properties of such systems on a large range of time and length scales. In particular we focus on the aggregation and sol- gel transition in different colloidal systems such as latex suspensions or milk. Using DWS we obtain quantitative information about the microscopic dy namics 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 i s observed. As biopolymer solutions and gels represent one of the most inte resting class of gelling systems we have performed a systematic study using solutions of casein micelles which we destabilized and investigated during the sol-gel transition using DWS and rheological measurements. We can find the same behavior as found in concentrated suspensions of monodisperse lat ex spheres that undergo a sol-gel transition. The changes observed in the m icroscopic dynamics can be clearly linked to the formation of a macroscopic gel with drastically modified viscoelastic properties.