Simultaneous light and small-angle neutron scattering on aggregating concentrated colloidal suspensions

A new sample environment has been developed in order to perform light and small-angle neutron scattering (SANS) simultaneously on colloidal systems. The combination of SANS and diffusing wave spectroscopy (DWS) is of particular use in the high-concentration regime. DWS provides information on the local dynamic properties of the individual particles, whereas SANS gives access to the structural properties on similar length scales. The combination of both methods thus allows one to obtain structural and dynamic information over a very large range of length and time scales. Using this new setup, the onset of aggregation and the sol-gel transition in concentrated destabilized polystyrene sphere suspensions have been investigated. At the gel point, a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. However, while the DWS measurements indicate that dramatic changes in the local dynamics occur over a long period, the SANS pattern quickly reaches its final appearance. The SANS experiments thus indicate that a fluid-like structure is arrested in the course of the gel formation. The data are found to be in good qualitative agreement with computer simulations.