Soft glassy rheology and structure of colloidal gels

In this dissertation, we compare rheology of thermoreversible colloidal silica gels with predictions of the soft glassy rheology (SGR) model. We characterize the structure, configuration and local bond rigidity of fractal colloidal gels. A method for preparation of anisotropic particles is also presented. The principal findings are summarized below.; We compare rheological properties of dense thermoreversible silica gels (&phis;=20%) with predictions of the SGR model. Linear rheology and the steady-state rheological responses of the silica gels are in agreement with the SGR model. However, we find significant disagreement between the SGR model prediction of the step strain response and the experimental determination of the damping function, particularly for step strains greater than ten times the yield strain. The noise temperature extracted from the linear rheological data is 1.05+/-0.01, indicating that colloidal gels are not glasses.; We directly visualize the microstructure of dilute fractal colloidal gels (&phis;=0.5%) to characterize their local configuration and bond rigidity. Two classes of fractal gels are studied. Radial distribution function, contact number distribution and common-neighbor analysis are calculated. We identify the backbone of gel networks and calculate the backbone fractal dimension and the relative number of particles in the backbone. We establish a simple measure of local bond rigidity based on the prevalence of load bearing triangle structures in the fractal gel network. We find significant difference in these measures between the two classes of fractal gels.; We report a method to prepare anisotropic particles with one hemisphere fluorescent, based on the recently reported gel trapping technique of Paunov and Cayre (Advanced Materials, 16 778 2004). We immobilize carboxylic modified polystyrene particles at the surface of a polydimethylsiloxane film and utilize an amine-carboxylic covalent coupling reaction to dye one hemisphere of the particles. We collect the half-trapped particles by either degrading the film with isopropanol and NaOCH3 or removing the trapped spheres by adhering them to water-soluble tape. The anisotropy of the particles is confirmed with Scanning Electron Microscopy and confocal microscopy.