Origins of heterogeneous dynamics in colloidal gels

Heterogeneous dynamics of collodial depletion gels arises directly from structural features in some specific cases identified in this thesis. Our experimental system consists of sterically stabilized, fluorescent poly(methyl methacrylate) spheres dispersed in a density and refractive index matched solvent of cyclohexyl bromide and decalin. Volume fraction is fixed (phi c = 0.20) at a midrange value far from low volume fractions modeled by fractal gels or high volume fractions near the attractive glass transition. The depletion interaction is induced with non-adsorbing polystyrene.; For the first study, the range of inter-particle attraction was fixed (Rg/a = 0.043) while the strength of pair attraction was varied (c/c* = 0.15--1.54). With increasing attraction, a maximum in structural heterogeneity is found locally by examining contact number distributions and, on a longer range, using number density fluctuations. The maximum in structural heterogeneity correlates with a plateau of dynamic arrest dividing our attractive system into a cluster phase and a network phase.; We further analyze the relationship between local structure and dynamics by selecting a test case from each of the two regimes identified: weak cluster and strong network gels. Contact number distinguishes a number of dynamic features in the cluster gel regime. Mean squared displacement decreases with increasing contact number. Particles with a lower contact number took longer to reach the time scale on which they achieve their maximum heterogeneity and had a larger magnitude of non-Gaussian character in the cluster sample. Contact number showed no effect in dividing the dynamics of network gels. All structural classifications of particles studied in either regime produce heterogeneous dynamics. The nature of that heterogeneity is different between cluster and network gels.; We apply an electric field to a sample equivalent to one of the strongest gels formed in the cluster regime. We observe and quantify both formation and breakage of inter-particle bonds in a preliminary analysis to identify characteristics for automatic event recognition. Particle pairs that had a breaking bond elongated significantly in the one frame where the event was identified. Particles forming a bond compressed evenly but significantly over the four frames tracked. Images are acquired at 0.1 s per frame. More displacements of particles involved in breakage or healing events tend to be in the mid-range of displacements that is unfavorable to a quiescent sample and might be analogous to an activated complex in a transition state.