Bond dynamics, microstructure, and rheology of colloidal gels.

摘要

This thesis introduces a new model colloidal gel for measuring particle interactions, microstructure, and rheology. The results from these measurements are analyzed to quantitatively predict the elastic modulus from the particle bond stiffness and microstructure.;Many experimental model gels typically have a refractive index and density matched solvent that allow for confocal microscopy and rheometry experiments for studying the gel microstructure and bulk rheological properties of industrial relevance. In this work, a model gel is created by suspending fluorescent poly(12-hydroxysteric acid) (PHSA) stabilized poly(methyl methacrylate) (PMMA) particles in cyclohexane and cyclohexyl bromide with polystyrene depletant to induce an interparticle attraction. The solvent mixture is optimized in this depletion gel to provide the refractive index contrast needed for optical trapping without greatly diminishing the confocal imaging resolution. Optical trapping provides the ability to relate gel rheology to the dynamics of individual bonds and interparticle forces. Therefore, this model gel is unique because particle-level interactions, microstructure, and bulk rheology data can all be obtained from one set of materials.;Optical tweezer experiments are used to directly measure the rupture forces between particles in this model system. Because the rupturing is controlled by thermally activated kinetics, there is a force distribution that describes the rupturing at each depletant concentration. A model is also developed that predicts the rupture force distributions based on a known interaction potential and the load rate being applied to the particle bond. Using a combined Derjaguin-Landau-Verwey-Overbeek (DLVO) and Asakura and Oosawa depletion potential, the model predictions agree with the experimental results, confirming that this potential describes the particle interactions.;The confirmed interaction potential is then used to calculate the bond rigidity and combined with structural information from confocal microscopy and measured elastic moduli of depletion gels. The results from these three experiments can be input into a model that assumes the network propagates elasticity over the characteristic cluster length scale in the gel structure and the volume fraction of clusters in the gel to calculate the elastic modulus. The cluster volume fraction and bulk particle volume fraction are related to the volume fraction of particles in a cluster, which can be used to map these samples onto the adhesive hard sphere phase diagram. The results suggest that the arrest line is an extension of the attractive glass line into the metastable binodal region, and these depletion gels are forming through the mechanism of arrested phase separation.;Finally, a layer-by-layer (LBL) synthesis is developed to produce silica nanoshells for a second model system that is compatible with optical trapping active microrheology experiments. The nanoshells are synthesized by depositing silica onto a cationic polystyrene template using a modified Stober synthesis and then calcining the particles to remove the polystyrene core. The nanoshells are robust enough to withstand additional functionalization as demonstrated by grafting octadecyl chains to the surface to make them organophilic or by adding a layer of fluorescent silica to aid in visualization. Spectrophotometry and conductivity measurements are used to study the rates at which water, ethanol, and aqueous sucrose solution (60% w/w) permeate the nanoshells. The different filling rates can be utilized to density-match the particles in the 60% sucrose solution. This neutrally buoyant suspension can then be used in active microrheology experiments to measure the suspension viscosity. (Abstract shortened by UMI.).