Dynamics in Concentrated Colloidal Suspensions

Abstract

This thesis reports results from a study of particle dynamics in colloidal glasses in the absence and presence of a gravitational field. It also investigates the reentrant glass transition. All the experimental results are obtained after a real space analysis using a confocal scanning laser microscopy as experimental technique that significantly widens the time window over which colloidal systems can be studied. A fluorescent recovery of bleached regions in concentrated suspensions of fluorescent colloidal hard spheres was followed in real space. This method provided data for mean squared particle displacements up to time scales that are three orders of magnitude beyond those available by present experimental techniques like dynamic or static light scattering. It was shown that, above the (hard sphere) glass transition density, particles move over distances on the order of their own diameter on time scales of 106 to 108 Brownian times. Moreover, the mean squared displacement, , showed power-law behavior over seven time (?) decades: ??^ (0.30?0.05). This behavior is different from earlier observations by dynamic light scattering. It was argued that these differences are caused by gravity effects. Further on, the influence of gravity on the long-time behavior of the mean squared displacement in glasses of colloidal hard spheres was studied. For the first time, a significant influence of gravity on the mean squared displacements of the particles was presented. In particular, the systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) showed anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. It was shown that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explained the observation that colloidal hard sphere systems which are a glass on earth rapidly crystallize in space. A quantitative analysis of the structure and dynamics of concentrated suspensions of colloids in which the magnitude of the short range attractive potential was increased by adding non-adsorbing polymers was done. These systems undergo a reentrant glass transition upon increasing polymer concentration. The melting of the glass is accompanied by significant changes in the displacement distribution and its moments. However, no significant variations were detected in the shapes of the displacement distributions. Moreover, structural correlation functions and the magnitude of local density fluctuations did not vary significantly between the glass states and the fluid. The influence of gravity on long-time diffusion in glasses was also studied using a new experimental setup that allowed a real space determination of particle displacements in directions parallel and perpendicular to the gravity field. Dispersions of particles were prepared in different solvent combinations that provide different gravitational lengths. It was shown that the direction of the external gravitational field couples to the particle dynamics and influences the values of mean square particle displacements.