Electric field effects on the rheology of shear thickening colloidal dispersions

The main goal of this dissertation is to investigate the effects of electric fields applied orthogonal to the vorticity-flow plane on the shear thickening phenomenon in concentrated colloidal dispersions.; Two well characterized stable silica dispersions in different media are investigated. The first is a charge stabilized dispersion where electric fields are found to delay the onset of shear thickening resulting in the E-FiRST or E&barbelow;lectric Field R&barbelow;esponsive S&barbelow;hear T&barbelow;hickening effect. The effect is attributed to the suppression of hydrocluster formation by field induced forces that orient particle pairs away from the compressive axis. A micromechanical theory is proposed, which successfully connects the electric field induced interparticle force to the shift in the critical stress for shear thickening. The second dispersion is a novel nonconducting silica-in-silicone dispersion where increasing the applied field strength results in an increase in the yield stress and apparent critical stress. The increment in critical stress is attributed to the overwhelming presence of the yield stress, which highlights an important limitation of the E-FiRST effect where field induced changes in the critical stress can be overshadowed by a large yield stress.; The influence of medium viscosity on the two aforementioned dispersions is also investigated. This is achieved by variation of (a) the sample temperature and (b) the medium molecular weight of the silica-silicone system. Both tests validate the theoretical prediction that in the absence of particle interactions, the critical stress is independent of medium viscosity. The variation of medium molecular weight also illustrates phenomena associated with polymer adsorption where the onset of shear thickening is delayed due to added interparticle repulsion.; Finally, the rheology of shear thickening dispersions is measured under uniaxial extensional flow. These colloidal dispersions are found to thicken under uniaxial extension as well. The results of this work are expected to aid in the design of dispersions and devices that harness and control the shear thickening phenomenon for technological applications.