Microstructure in Gelling Systems

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Author Last Name: 
Michael J. Solomon
Report Type: 
Research Area: 
Wet Systems
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Publication Month: 
United States

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.