| dc.description.abstract | Suspensions are ubiquitous in nature and industry, ranging from magma, blood, and mud to cement, paint, and molten chocolate. Despite the Newtonian nature of the suspending fluid and the expectation of linearity in the absence of inertia, these suspensions display a diverse array of non-linear rheological behaviours. The origin of these behaviours is puzzling, particularly in the case of non-Brownian particles, because of the absence of a time scale in the system other than the inverse of the shear rate. By combining experimental rheology, atomic force microscopy to determine the interaction forces between particles and phenomenological modelling, we show that the global rheology of non-Brownian suspensions can be predicted from the non-hydrodynamic interactions between two particles.
The first part of the thesis deals with suspensions that display an increase in shear viscosity μ with the increase in shear rate, commonly referred to as shear thickening. For particle volume fractions φ above random loose packing, the increase in shear viscosity μ becomes discontinuous at a critical shear rate. A common feature of previous studies on dense suspension rheology is that the sample is pre-sheared at a constant shear rate ̇γ for a long time, typically over an hour, ostensibly to remove the preparation and loading history of the sample. However, no reasons have been offered for such a lengthy pre-shear. By performing shear stress sweeps on a freshly loaded sample, we show the transition from continuous shear thickening to discontinuous shear thickening to the emergence of a non-monotonic ‘S-shaped’ curve with the increasing number of sweeps. We thus show that strain is a key variable in determining the rheology of the suspension. Finally, for a given φ, the steady-state profile exhibits Newtonian behaviour at low stresses, followed by a decrease in ̇γ as shear forces overcome repulsions and, ultimately a substantial increase in ̇γ at higher shear stress. The system has two critical stress scales; the lower stress scale arises from the repulsive forces between the particles and determines the transition from Newtonian to shear thickening. The higher shear stress scale is the yield stress of dry grains. We propose a phenomenological model that captures the rheological changes with strain using the mean coordination number z (number of contacts per particle) as a microstructural variable. The model predicts the shear strain-dependent rheology by tracking the coordination number, which saturates to its steady state value in the contact-dominated regime via a first-order process. Particle imaging velocimetry is performed on the top surface and shows the change in velocity profile from Newtonian prediction to shear localization, substantiating the above claims.
The second part of the thesis deals with suspensions that display a decrease in μ with the increase in shear rate, commonly referred to as shear thinning. We synthesize two types of particles using a batch polymerization process that exhibit shear thinning, with the emergence of yield stress at low ̇γ. The yield stress is a monotonic function of φ and disappears as the volume fraction decreases (φ ≤ 0.3). The system has a critical shear rate at which shear forces overcome the attractions between the particles and determines the transition from yielding to power law behaviour. The strength of attractive forces determines the nature of yielding, resulting in non-monotonic yielding observed in ̇γ sweeps for strongly attractive suspensions. The proposed model predicts the qualitative features of the rheology for both suspensions.
Finally, we select a particle-fluid combination that exhibits Newtonian behaviour for a range of volume fractions up to φ = 0.55. The absence of any non-hydrodynamic interactions between the particles is confirmed using atomic force microscopy. It emphasizes our argument that examining particle-particle interactions within the suspending fluid enables the prediction of the rheology of dense non-Brownian suspensions. | en_US |
