Rheology and Structure of Model Smectite Clay: Insights From Molecular Dynamics

The low frictional strength of smectite minerals, such as montmorillonite, is thought to play a critical role in controlling the rheology and the stability of clay-rich faults. In this study, we perform molecular dynamics simulations on a model clay system. Clay platelets are simplified as oblate ellipsoids interacting via the Gay-Berne potential. We study the rheology and structural development during shear in this model system, which is sheared at constant strain rates for 10 strains after compression and equilibrium. We find that the system exhibits velocity-strengthening behavior over a range of normal stresses from 1.68 to 56.18 MPa and a range of strain rates from 6.93 x 105 to 6.93 x 108/s. The relationship between shear stress and strain rate follows the Herschel-Bulkley model. Shear localization is observed at lower strain rates despite the velocity-strengthening friction, while homogeneous shear is realized at higher strain rates. The structure change due to shear is analyzed from various aspects: the porosity, particle orientation, velocity profile, and the parallel radial distribution function. We find that particle rearrangement and compaction dominate at the early stage of shear when the shear stress increases. The shear band starts to form in the later stage as the shear stress decreases and relaxes to a steady-state value. The structural development at low strain rates is similar to previous experimental observations. The stacking structure is reduced during shear and restores logarithmically with time in the rest period. Smectite clay minerals have relatively low frictional strength compared to other geomaterials. The strength and stability of clay-rich faults depend on the shear behavior of clay minerals. In this study, we perform a series of simulations to investigate the shear behavior and structure of a simple clay model. The complex shape and interactions of natural clay are simplified as disc-shaped ellipsoid and a generalized potential. The system is sheared at constant strain rates for 10 strains after compression and relaxation. The simulations focus on the effect of normal stress and strain rate on shear stress and clay structure. We find that the shear stress increases with strain rate for all conditions tested. Two shear scenarios are observed with shear localization in a narrow region at low strain rate while shear is homogeneous at high strain rate. The structural development at low strain rates is similar to previous experimental observations. We observe that the stacking structure of clay platelets weakens during shear. In the rest period after shear, the stacking structure restores logarithmically with resting time and can be associated with the healing of clay-rich fault. A model clay system shows velocity-strengthening behavior following the Herschel-Bulkley model Despite the velocity-strengthening friction, shear band is formed at low strain rates The stacking structure of clay platelets is reduced during shear and undergoes logarithmic healing with time in the system at rest