(181c) Homogeneous Shear Simulations of Liquid-Solid Suspensions of Attractive Microparticles

Examples of microparticle suspensions occur frequently in both industry and nature. For example, concrete reigns as the most abundant man made material in existence. Yet, the rheology of its key ingredient, cement ­- a microparticle suspension -­ , remains poorly understood. Indeed, the simulation of microparticle suspensions, much more simple than cements, present many technical challenges that must be overcome. Such challenges include the need to resolve a broad range of relevant length and time scales. Additionally, many physically relevant model systems are thermodynamically unstable, and particles in this size are expected to make contact. In this talk, we present a first principles approach to studying the rheology of microparticle suspensions: including micro-scale interparticle property measurements, meso-scale simulations of homogeneously sheared suspensions, and comparison with macro-scale rheometer experiments.

The systems of interest in this study are suspensions of Portland cement and fly ash, with solid volume fractions ranging from 0.3 to 0.55. Suspensions of these particles are insensitive to Brownian motion and fluid inertia, exhibiting Peclet and Reynolds number of O(10⁵) and O(10^-2), respectively. We make use of coupled soft-sphere discrete element method (DEM) and Fast Lubrication Dynamics (FLD) simulations to probe the rheology of model monodisperse suspensions of cementitious microparticles. The DEM model used to simulate the particle dynamics consists of a linear spring dash-pot model with a short-ranged van der Waals potential well, extending beyond the surface of the particle. The hydrodynamics are modeled using an extension to Stokesian dynamics, shown to be accurate at moderate to dense volume fractions. Here, the hydrodynamics are dominated by two-particle lubrication forces. Lastly, the parameters used in the simulations have been extracted from atomic force microscopy measurements of particle properties in an aqueous environment, e.g. interparticle friction, Hamaker constants, and radius of typical surface asperities.

Along with volume fraction and particle properties, the shear rate of these systems was also varied between 10s^-1 and 100s^-1. Among the important findings made in this study was the abrupt formation of a physical gel, i.e. percolated particle network, during the simulation. The constitutive law behavior that we observe in all simulations is well-described by the Bingham plastic model, which is consistent with gelation and the existence of a finite yield stress. Stress and strain rate data were compared with vane rheometer experiments. The stress behavior of fly ash suspensions is found to be well predicted by the simulations up to a multiplicative constant. Stress predictions for Portland cement suspensions are found to be less accurate. Finally, the effects of walls, polydisperisity, and non-sphericity are also discussed.