Multiscale modeling of clay-water systems

Abstract

The engineering properties of soils are highly affected by clay content and clay-water interactions. However, existing macro-scale continuum models have no length scale to describe the evolution of the clay microstructure and its role in affecting macroscopic properties. This research proposes a bottom-up multiscale modeling approach to understand the physics underlying macroscopic clay behavior. Atomistic models are developed to understand clay-water interactions using the ClayFF force field. We simulate water adsorption between clay layers for a reference mineral, Wyoming montmorillonite (Na-smectite) using molecular dynamics simulations. The elastic properties of the system are found to vary with the hydration state (amount of interlayer water) and can be well-approximated using cross-anisotropic symmetries. The simulated adsorption isotherm and stiffness are compared with available experimental measurements. We develop a perturbation approach to investigate the free energy associated with relative positions of two clay platelets in water. The free energy functions for edge-edge and faceface particle associations are used to calibrate the Gay-Berne (GB) potential that represents each platelet as a single-site ellipsoidal body. A coarse-graining upscaling approach then uses the GB potentials and molecular dynamics to represent the meso-scale aggregation of clay platelets (at submicron length scale). Results from meso-scale simulations obtain the equilibrium/ jamming configurations for mono-disperse clay platelets. The results show aggregation for a range of clay platelets dimensions and pressures with mean stack size ranging from 3-8 platelets. The particle assemblies become more ordered and exhibit more pronounced elastic anisotropy at higher confining pressures. The results are in good agreement with previously measured nanoindentation moduli over a wide range of clay packing densities. The current research represents an important step forward towards multiscale modeling of soils and can be used to study any system composed of platy constituents.