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; however, in every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as 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 non-uniform shape, the effect 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.
In this project we are developing tools for characterization of the microstructural evolution in gelled colloidal systems that can be applied to non-linear rheological and stability phenomena, such as stress-induced yielding and gravitationally-induced collapse. These microstructural characterization tools can be applied to develop a quantitative link between microstructure and bulk suspension properties, such as the gel modulus, yield stress and delayed sedimentation. This project will expand our current knowledge of attractive systems by explicitly considering the effects of size polydispersity on the gel microstructure and bulk rheology, thus enabling us to bridge the results of model systems to relevant industrial materials. Microstructure will be quantified by direct visualization with confocal and optical microscopy during flow and sedimentation. Concomitant measurements of non-linear rheological properties such as the yield stress on identical systems will also be conducted. Interactions and forces will be probed by laser tweezers microrheology. Systematic exploration of polydispersity effects will be accomplished by mixing fractions of monodisperse colloids in known amounts to achieve a suspension with well-characterized polydispersity.