Section I. Project results: University of Delaware
Our primary goal is to trace whether there is a connection between gelation and the attractive driven glass. If gelation is the result of an arrested phase separation, then the gel line may be an extension of the attractive driven 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 may have the same microrheology as a glass. Observations of the potential similarities between the microstructure of the high density regions of a gel and the attractive driven glass could potentially be 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 is critical for understanding the origin and properties of colloidal gels.
Our work this year focused on characterizing interaction potentials in the model colloidal suspensions used for microrheology experiments. Model suspensions are finely tuned to enable simultaneous optical micromanipulation using laser tweezers and direct confocal imaging of the suspension microstructure. At the same time, gravitational sedimentation is minimized through density matching. One of the key problems has been understanding charging and the electrostatic repulsion that arises between particles in the brominated solvents used in these suspensions. The first half of our work focused on characterizing these suspensions. This has led us to begin developing replacement model systems in order to better control the colloidal interactions in these studies.
Section II: Project results: University of Michigan
Microstructural recovery of yielded colloidal gels
Introduction and Background
Colloidal gels are dynamically arrested, space-spanning networks of particles formed when the pair potential between colloids is strongly attractive at small separations. The structure, dynamics, and rheology of gels are strongly dependent upon the colloidal volume fraction. At low volume fractions, fractal clusters have been observed. At higher volume fractions, glassy states with slow dynamics are formed when short-range attractions modify the cage structure. At intermediate volume fractions, gels form with different morphologies depending on the interparticle potential: cluster-like networks form with strong attractions and contain large voids and corresponding aggregates of particles, while string-like networks form with under stronger attractions and consist of long, conjoined branches of particles1. Gelation is thought to involve the competing processes between kinetic arrest, percolation, and equilibrium phase separation2,3.
Gels and glasses undergo yielding when a sufficiently large strain is applied, resulting in fluidization and bond rupture. This process is thought to be characterized by the formation of large voids and aggregates. The mechanism and causation of yielding has been examined extensively using light scattering and neutron scattering4-6. Alternatively, investigation of microstructural changes using optical microscopy techniques potentially allows identification of the exact spatial location of the particles in the gel network as they undergo deformation. Unfortunately, typical deformation rates for yielding, particularly those most relevant to industrial processes, do not allow tracking of individual particle trajectories. As a result, the analysis of orientations and trajectories of colloidal particles during flow has been limited to low shear rates.