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NEMD modeling of nanoscale hydrodynamics of clay-water system at elevated temperature

The engineering problems involving clay under non-isothermal conditions (e.g., geothermal energy harvest, landfill cover system, and nuclear waste disposal) are multiscale and multiphysics by nature. The nanoscale hydrodynamics of clay at elevated temperature is essential in developing a physics-based multiscale model for clay under non-isothermal conditions. The nonequilibrium molecular dynamics (NEMD) is a useful tool to study the nanoscale hydrodyndamics of clay. This article presents an NEMD modeling of hydrodynamics of clay nanopores at elevated temperatures. Water flow confined in pyrophyllite and montmorillonite clay nanopores is investigated. The nonequilibrium state is maintained by uniformly exerting an external force on each water molecule. The NEMD simulations have provided a molecular-scale perspective of temperature effect on clay-water density, water flow velocity, shear viscosity, clay-water slip length, hydraulic conductivity, and clay-water friction coefficient. The numerical results have shown a strong temperature dependence of fluid flow velocity, shear viscosity, clay-water slip length, and hydraulic conductivity at the nanoscale. We have validated the applicability of cubic law in determining hydraulic conductivity at the nanopore scale under at elevated temperature. It is found from our numerical results that slip clay-water boundary condition is an essential factor in properly determining nanoscale fluid flow velocity. By numerical examples, we also study the impact of nanopore size and clay layer thickness on the hydrodynamics of the clay-water system.