Methodology for Estimation of Nanoscale Hardness via Atomistic Simulations

Compression of Na-Montmorillonite Swelling Clay Interlayer Is Influenced by Fluid Polarity: A Steered Molecular Dynamics Study

The compressive responses of an interlayer of the dry sodium-montmorillonite (Na-MMT) swelling clay as well as the clay intercalated with organic fluids of a wide range of dielectric constants from 110 (formamide) to 20 (acetone) are quantitatively evaluated using steered molecular dynamics simulations. Representative dry clay and clay with fluid (clay-fluid) molecular models are constructed, and the stress-strain relationships upon compression of these models are studied using constant force steered molecular dynamics (SMD) simulations. Our results show that the polarity of the fluids and the amount of the fluid molecules in the clay interlayer play a significant role in the interlayer spacing, interlayer volume, interlayer strain, interlayer modulus, nonbonded interactions, and conformation of the fluid molecules upon externally applied stresses. The clay interlayer responses upon compression are essential for the development of multiscale modeling of swelling clays and prediction of the reliable compressive behavior, which are critical for the accurate analysis and economical design of the infrastructures in swelling clay areas and the densification of clays for ceramics manufacturing.

Interparticle Interactions in Colloidal Systems: Toward a Comprehensive Mesoscale Model

Intermolecular interactions control the collective behavior of colloidal clusters. The overwhelming majority of literature focuses on cohesive attributes of intermolecular forces as they govern the jamming process. However, the overlooked sliding component plays a critical role in the slow relaxation dynamics of colloidal aggregates and the emergence of discontinuous shear thickening in dense suspensions. Here, we use crystalline calcium-silicate-hydrate (C-S-H) as a model system to explore synergistic cohesive and sliding interactions. We use the free energy perturbation approach to reconstruct potential of mean force profile between two finite-sized nanolayers in an aqueous environment. We show that sliding free energy barriers decay exponentially as the separation distance increases. The characteristic length scale of the decay is related to the interface corrugation. We introduce a simple yet effective mechanism to capture the sliding behavior of colloids with surface roughness. Moreover, we develop a global free energy landscape model by juxtaposing cohesive and sliding interactions. This model enables us to measure the height of energy barriers, which is essential to elucidate deformation mechanism and dynamics of colloidal aggregates. For cohesive colloids, our approach predicts a sublinear relation between applied normal and shear stress at the onset of sliding that is in contrast to Amontons' laws of friction. We demonstrate that the sublinear trend is due to the adhesion and nature of soft contact at the nanoscale. The proposed framework provides a new route to draw a more realistic picture of intermolecular interactions in nanoparticulate systems such as geomaterials, cementitious systems, complex colloidal assemblies, and dense suspensions.