Rheology and Interparticle Interaction Studies in Complex Media
Bhosale, Prasad S.
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Dense colloidal dispersions are rheologically complex and exhibit properties like yield stress, creep, and elasticity due to aggregation and other interparticle interactions. The overall rheological performance of the system is due to the multiscale properties and mechanics: the nanoscale interactions between particles determine the microscopic texture, i.e. aggregate compactness, structure and strength. At microscale the micromechanics and microstructure evolution in aggregates under stress define the macroscale rheological performance. The interparticle forces and microstructure also evolve over time (aging interparticle bonds and aggregate structures), leading to time/frequency dependent rheological behavior. The systems of specific interest here are dense nuclear waste slurries stored at the US Department of Energy sites at Hanford, WA and Savannah, GA since 1943. Rheological modifiers are currently being investigated for effective transport of these dense waste slurries in the vitrification process developed for more permanent radioactive waste disposal. The overall objective of the study is to fundamentally understand the performance of rheology modifiers in environments similar to those of nuclear waste slurries. We are particularly focused on the effect of rheology modifiers on interparticle interactions and its implication towards bulk rheological performance. These interactions were investigated through various macroscale (rheology and electroacoustics) and microscale (atomic force microscope) colloidal interaction measurement techniques. The implications of these interactions towards rheological performance are studied using the simple yield stress model defined by Russel et al. Here the model has been extended to steric, and bridging forces between particles. It was identified that polyelectrolytes, specifically poly (acrylic acid), is an effective rheology modifier for high pH and high salt content environments of nuclear waste slurries. The correct choice of molecular weight of the PAA was especially important to its effectiveness: too low a value provides insufficient steric stabilization, while too high a value induces bridging. The effect of poly (acrylic acid) adsorption on the electrokinetic behavior of alumina dispersions under high pH conditions was also investigated as a function of polymer concentration and molecular weight as well as the presence, concentration and ion type of background electrolyte. The interparticle interaction forces were directly measured in nuclear waste simulant using atomic force microscopy. A link between the dynamics of polymer bridging and disruption and bulk rheology was established for a model dense colloidal suspension of silica particles flocculated by polyethylene oxide (PEO). In the finial study, as a side project, we have expanded acoustic spectroscopy and electroacoustic techniques to characterize particles dispersed in viscoelastic polymer gel media (another complex system of interest).
- Chemical engineering