During the period 9/1/07 – 8/31/10, $123,000 of IFPRI support was allocated evenly between the University of Michigan and the University of Delaware for support of experimental studies of the microstructure of gelling. In this report we discuss results that have identified: (i) how microstructure in colloidal gels of monodisperse particles is modified by application of stresses that lead to rupture and yielding; (ii) how microstructure in systems of polydisperse particles is affected by sedimentation and delayed collapse.
In many suspensions, slurries, and complex fluid formulations of industrial interest, the colloidal scale interparticle forces are attractive. These attractions set up a complex microstructure with slow dynamics that determine properties such as yield stress, stability, and shear-rate dependent viscosity. In particular industrial situations, these properties may be more or less desirable. In every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as a function of time, is of paramount interest in process and product development. Because industrial materials are comprised of colloids with heterogeneous interparticle forces, polydisperse size distribution, and nonuniform shape, the effects of these parameters on the microstructural origin of yield stress and stability should be assessed. Current capabilities for prediction of rheology from underlying microstructure is largely limited to linear properties in systems with well-characterized interparticle forces and monodisperse particle sizes.