Microrheology of Gelling Suspensions (University of Delaware) and Microstructural Recovery of Yielded Colloidal Gels (University of Michigan)

Publication Reference: 
Author Last Name: 
Michael J. Solomon and Eric M. Furst
Report Type: 
Research Area: 
Wet Systems
Publication Year: 
Publication Month: 
United States

Executive Summary – University of Delaware

The goal of this work is to determine whether there is a connection between gelation and the attractive glass state of colloidal suspensions. If gelation is the result of an arrested phase separation, then the gel line may exist as an extension of the attractive glass line down into the two-phase region of the colloidal phase diagram. Likewise, if the local microstructure of a gel is similar to a glass, then the high density regions in a gel will have the same microrheology as a glass. The potential similarities between the microstructure of the high density regions of a gel and the attractive driven glass are potentially missed if gels were only studied using bulk rheology techniques, where measurements would be an average over the entire structure. Thus, the development of microrheology and simultaneous direct confocal imaging is critical for understanding the origin and properties of colloidal gels.

Executive summary - University of Michigan

The goal of our work is to determine a simple correlation between microstructure and strain-dependent elasticity in colloidal gels by visualizing the evolution of cluster structure in high strain-rate flows. We control the initial gel microstructure by inducing different levels of isotropic depletion attraction between particles suspended in refractive index matched solvents. Contrary to previous ideas from mode coupling and micromechanical treatments, our studies show that bond breakage occurs mainly due to the erosion of rigid clusters that persist far beyond the yield strain. This rigidity contributes to gel elasticity even when the sample is fully fluidized; the origin of the elasticity is the slow Brownian relaxation of rigid, hydrodynamically interacting clusters. We find a power-law scaling of the elastic modulus with the stress-bearing volume fraction that is valid over a range of volume fractions and gelation conditions. These results provide a conceptual framework to quantitatively connect the flow-induced microstructure of soft materials to their nonlinear rheology, and imply that the modification of particle shape and surface roughness may allow us to design gels that have improved mechanical properties at even lower volume fractions.