This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces on the moderate to high shear rheological properties and shear stability of wet dispersions that span the colloidal to particulate range. The current emphasis is on particle shape and its effect on the shear thickening transition, as well as the dynamics of the shear thickening transition. Ongoing research into the influence of polymer stabilization, and in particular on the role of adsorbed polymer in modifying the hydrodynamic and interparticle forces acting between particles under flow will not be summarized here, but in the next year’s report.
In PART I of this report, the transient shear rheology (i.e. frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but limits to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude. This work provides crucial evidence for the dynamics of the shear thickening transition, and also suggests that dynamic oscillatory measurements can be used to study shear thickening in addition to steady shear.
PART II provides experimental evidence for the effects of varying particle shape on the shear rheology and shear thickening in concentrated colloidal dispersions. A series of 2:1, 5:1 and 8:1 aspect CaCO3 dispersions were prepared, characterized, and examined rheologically. Prelimary SANS measurements corroborates the supposition that alignment can reduce the low shear viscosity and severity of shear thinning at higher particle loadings. In comparison to previous work on charged silica dispersions, these dispersions shear thicken at lower stresses, but still shear thicken at higher stress than would be expected for hard sphere dispersions at the same loading and average particle size. Ongoing experimentation and modeling is designed to fully elucidate the influence of particle shape on the low and high shear suspension rheology.