Dynamical heterogeneities in sols and supercooled liquids

This thesis explores the nature and causes of heterogeneous dynamics in supercooled liquids approaching the glass transition and sols approaching the gel phase. In Chapter 1, we motivate the questions being addressed by giving an overview of the glass transition and gelation process, including the anomalous diffusive behavior and structural relaxation exhibited by liquids approaching the transition points. Evidence of spatially heterogeneous dynamics is presented and a connection is made between the glass transition, colloidal gelation and chemical gelation.;The dynamics of a sol undergoing chemical gelation are studied in Chapter 2, An analysis of the static properties of the sol, especially the size distribution of molecules in the suspension, shows the model to be well-described by percolation theory. Molecular dynamics simulations provide insight into the diffusive behavior of the molecules. The motion of the center of mass of the clusters is shown to be collective in high density systems near the gelation point.;A structural signature of collective dynamics is demonstrated in Chapter 3 using a normal mode analysis. The dynamic behavior of a 2D model of a liquid is obtained with an isoconfigurational ensemble of molecular dynamics trajectories. A normal mode analysis performed on the initial configuration shows a high degree of correlation between the regions of high-amplitude vibrational motion and the spatial distribution of regions with a high probability of irreversible reorganization. This result demonstrates that the low-frequency quenched normal modes can be used to predict the spatial structure and length scale of irreversible reorganization, providing a long sought-after link between the local structure and the dynamics.