Structure and Rheology of Anisotropic Colloids PDF Download

Author: Vincent Labalette
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Languages : en
Pages : 0

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Book Description
Colloidal clays are hydrous magnesium phyllosilicates (sometimes aluminum), usually bearing a negative structural charge coming from isomorphic substitution compensated by the presence of cations in the basal spacing or on the surface of the colloid. These nanoparticles have a platelet shape with an aspect ratio going from 20 to 100. When immersed in water, clays hydrate and swell, leading to the release of the cations. The hydroxyl group presents on the edge of the particles are sensitive to the pH (titrable sites) resulting in an amphoteric edge charge. At low pH the rim is positively charged and becomes neutral or negative at pH 11. Therefore, suspensions of colloidal clays have both charge and shape anisotropies. Thanks to these features, clay dispersions exhibit interesting optical properties (ochreous clays), mechanical properties (tile manufacturing, surface coating) and even cleaning properties (grease-removing). Although studied for decades, the behavior of colloidal clays remains controversial. In this manuscript, we propose a coarse-grained model to simulate particles with both structural and charge anisotropy. This model allows studying the behavior of colloidal suspensions at equilibrium and under shear flow. Contrary to the Monte-Carlo method usually employed to model the equilibrium behavior of anisotropic particles, the model presented in this thesis takes into account hydrodynamic interactions, allowing the dynamics of the system to be studied. The particles are coarse-grained as clusters of spheres bound by springs or constrained to a rigid body motion thanks to solid mechanics equations. The dynamics of the particles are computed using the Accelerated Stokesian Dynamics code (ASD), and the electrostatic interactions are computed in a pairwise additive fashion with a Yukawa potential. The implementation of this coarse-grained model in the ASD method allows studying the microstructure of anisotropic particles presenting similar features than Laponite, a 2:1 synthetic smectite clay widely studied experimentally and numerically in the literature. Several studies are presented here while varying the volume fraction and the range of electrostatic interactions. The dynamics of formation of the observed structures (Wigner glass, gel, overlapping coin, etc.) and their structural evolution behavior are then discussed. Finally, the rheological response of the different structures to a start-up shear has been studied, highlighting the importance of the ratio between the electrostatic and the hydrodynamic forces. For initially percolated systems, it has been shown that the stress response on the applied strain depends on the initial microstructure at short times, and exhibits shear-thinning and final viscous response independent of the initial structure.