On the rheology of dense pastes of soft particles
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Many concentrated paste-like materials are composed of deformable particles randomly packed into a dense suspension. Examples of the constituent soft particles include polyelectrolyte microgels, emulsion droplets, polymer coated colloids, and star polymers. These materials share in common many properties such as yield stress, shear thinning, non-zero normal stresses, wall-slip, shear-banding, memory and aging (similar to that in structural, spin and polymer glasses). Their unique properties make soft particle pastes (SPPs) scientifically interesting and extremely useful in industrial applications (as rheological modifiers). In this dissertation particle simulations, theoretical models and experiments are used to study the flow dynamics and rheological behavior of SPPs near confining surfaces - wall-slip and shear flow, and in the bulk - elasticity at small stresses and the non-linear shear rheology. In the study of slip near smooth surfaces, rheological measurements are shown that identify the influence of the chemical nature of the shearing boundary on slip at the shearing boundary. A modified elastohydrodynamic model is presented that incorporates attractive and repulsive short range interactions between the paste particles and captures the corresponding suppression and promotion of slip at the wall. Further, fluorescence microscopy and particle tracking velocimetry is used to visualize slip and flow of pastes near smooth boundaries and study the sensitivity of the bulk flow profile to the nature of the shearing surface. In the study of elastic properties of pastes, SPPs are modeled as three-dimensional systems of randomly packed elastic spheres. Simulations are performed wherein the packing is subject to small deformations to compute the high- and low-frequency shear moduli. The simulation results are compared with the data from experiments on microgel pastes. This model is extended to study paste dynamics under simple shear with added pairwise elastohydrodynamic lubrication interactions between the densely packed soft particles. The shear and normal stress differences generated during simple hear flow are calculated that compare well with the experimental data. In addition, the pair distribution function of the initial isotropic configuration, the elastically deformed and the steady sheared configurations is investigated. A semi-empirical analysis of the microstructure and its evolution due to shearing is presented.