The microstructure of gelled colloidal systems is a key determinant of their rheological
response. In this project, we addressed three aspects of this critical problem. In the first
part, we identified high contact number, stress bearing configurations in colloidal gels
that had been subjected to non-linear step strain, and determined that these stress bearing
clusters are predictive of the non-linear elasticity of the gel. The idea, borne out by
analysis of the experiments, is that the hydrodynamic interaction of a fluid of jammed,
stress bearing clusters is the principal determinant of the post-yield rheology of colloidal
gels. In fact, the abundance and size of these stress-bearing clusters could likely be
identified from a measurement of the rheological response. In the second part, in
collaboration with the Furst group at the University of Delaware, we developed a model
colloidal system in which force, structure, and rheology could all be measured, so that the
relationship between these three properties could be better understood, especially with
respect to modeling the important rheological quantity of the yield stress. This model
system needed to balance a number of constraints related to refractive index matching
and density matching. We found a particular condition that satisfied all the constraints,
and therefore opens this area to fundamental research. In a final project, we observed
that we could produce colloids with controllable roughness. We studied the effect of this
variable on a number of rheological properties of colloidal suspensions. We found that
the shear thickening response was most affected by colloid roughness.