| Summary: | Dense granular suspensions are mixtures of non-Brownian particles and liquid. As the packing fraction of the particles approaches the jamming point, dense suspensions can increase their viscosity dramatically beyond a certain shear rate, i.e., they exhibit strong and possibly discontinuous shear thickening. In this thesis, we experimentally investigate how viscous and frictional interactions between particles affect the shear thickening behavior. First, combining oscillatory rheological measurements with in-situ high speed imaging, we observe dilation beyond a critical strain and the end of shear thickening as the maximum confining stress is reached and the contact line moves. Modeling the shear flow by a local constitutive relation, we quantify the contributions from different sources of stress to the measured overall viscosity. With increasing viscosity of the suspending liquid, eta0, the viscous contributions can become sufficiently large so that they compete with effects from frustrated dilation and soften the discontinuous nature of shear thickening in dense granular suspensions. Second, we found that shear thickening becomes weaker with eta0 and eventually disappears for highly viscous solvent. In this regime, the suspensions show a Newtonian-like behavior with constant viscosity under shear. The crossover from granular to Newtonian regimes reflects the competition between friction and viscous interactions. Third, we experimentally investigate the temporal stress fluctuations of dense suspensions during steady state shear. For low viscosity suspending liquids, we show that, in the shear thickening regime, shear and normal stresses are highly coupled and exhibit significant fluctuations with time. As shear rate increases, the stress distributions evolve from Gamma to Gaussian distributions. By contrast, for highly viscous solvents, stress fluctuations are greatly reduced and only show Gaussian distributions at different shear rates. We find that the fluctuation behaviors are associated with spatial correlations in the suspension. Combining the fluctuation analysis in different regimes, we quantitatively show how the spatial correlation length scale &xgr; varies within the shear thickening regime for different solvent viscosities. Finally, we link the global rheological properties of dense suspensions to their local correlations.
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