Structural evolution and rheology of colloidal gels by dynamic simulation

Abstract

In this dissertation, we utilize currently large-scale dynamic simulations to examine how colloidal gels evolve over long periods of time, with a goal of increasing our understanding of the stability of gelled products, our main motivation. A major theme in this work is the connection between time-dependent hierarchies of gel structure and time-dependent rheology. First, we demonstrate that structural evolution continues long after gelation in quiescent gels. This evolution is characterized by a growing structural length scale, a feature ubiquitous in phase separating systems. A combination of measurements of particle microstructure, mean-squared displacements, and intermediate scattering functions suggests that domain growth occurs primarily via diffusion of particles along the surface of the particulate network. Second, we examine the consequences of structural evolution on the linear viscoelasticity. In the limit of large frequencies, viscoelastic moduli increase asymptotically with the square root of the frequency, similar to the moduli of dispersions without hydrodynamic interactions. The gels stiffen with age over all frequencies. We observe that the age-dependent moduli are proportional to the growing structural length scale and provide a scaling argument to account for this. Finally, we perturb the gels with step shear stresses to investigate the phenomenon of delayed yield observed in experiments. The computational model recreates experimentally observed phenomena: failure times that decrease rapidly with increasing stress, fluidization under large stresses, and strain stagnation under weak stresses. We observe that all yielding gels yield at a critical strain, approximately 2-3%, but only above a critical stress. However, at intermediate stresses gels not only yield but also re-solidify. Less than 0.1% of the bonds in a colloidal gel need break by the critical strain for macroscopic failure to progress. The re-solidification transition is accompanied by formation of new bonds between broken clusters from the original gel driven together by convection, and the resultant structure is distinct from those of quiescently aged gels.