Theoretical characterization of colloid-polymer nano-composites

Abstract

The goal of the thesis is to observe microscopic mechanisms in colloid dispersions that cause shear thinning and shear thickening. We develop the necessary tools to model these phenomena. We present a non-equilibrium molecular dynamics study of the Couette flow of rigid spherical nanoparticles in a simple Lennard-Jones fluid. We evaluate the viscosity of the dispersion as a function of shear rate and nanoparticle volume fraction. We observe shear thinning behavior at low shear rates; as the shear rate increases, the shear forces overcome the Brownian forces, resulting in more frequent and more violent collisions between the nanoparticles. This in turn results in more energy dissipation and increased shear stress. We show that in order to stay in the shear-thinning regime the nanoparticles have to order themselves into layers longitudinal to the flow to minimize the collisions after Brownian forces become negligible. As the nanoparticle volume fraction increases, there is less room to form the ordered layers; consequently, as the shear rate increases, the nanoparticles collide more, which results in turn in shear thickening. Most interestingly, we show that at intermediate volume fractions the system exhibits metastability, with successions of ordered and disordered states along the same trajectory, and these states correspond to shear thinning and shear thickening respectively. Two-dimensional pair correlation functions are evaluated for further analyses of microscopic states of the non-Newtonian regimes. At low shear rates shear thinning is related to the deformation of the microscopic structure dominated by Brownian forces. Our results suggest that layering may lead to shear thinning in the right conditions but it's not necessary. Results also support the order-disorder transition but it's also not necessary. The leading reason for shear thickening is the increase in the frequency and intensity of the frictional interactions of dispersed particles. Layer formation is a mechanism that prevents the emergence of shear thickening.