Molecular forces of water flow in clay nanopores

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Clays have complex multiscale pore structures and water flow in clay nanopores plays an important role in various engineering applications, such as adsorption of pollutants and barriers of contaminated lands. Therefore, the micro-physical mechanism is necessary to be considered from bottom up to better control the nanoscale water flow process and the computational simulation has become the main method to study the process. In this paper, non-equilibrium molecular dynamics (NEMD) simulations are performed to investigate the hydrodynamics properties and the molecular forces of water flow in montmorillonite nanopores. The velocity profiles of water flow are parabolic but slight offsets are observed near the surfaces. The simulated dynamic viscosity of water is 0.72±0.04 cP and the average hydraulic conductivities are (1.61±0.01), (3.05±0.07), and (5.21±0.33) ×10^-11 m/s for pore size h = 3.5, 5.0, and 6.5 nm, respectively. During the NEMD simulations, the time-fluctuation of the force of clay matrix F^sc on the water molecules in clay nanopores is found to satisfy the Gaussian distribution and its time-average value basically equals the applied force F^d to maintain a dynamic mechanical equilibrium. The mechanical-equilibrium water flow can be equivalent as a simply supported beam with four distributed forces acting on it. The force of clay f^s, similar to the pedestal counter force, concentrates on the water molecules in the adsorbed layer while the force of cations f^c is mainly exerted on the diffuse layer. The integral of driving force f^d, the force of clay f^s and the force of cations f^c along the z-axis induce the shear strain of the pore water while the integral of the force of water molecules f^w can be regarded as the internal viscous force to resist the shear deformation.

9th International Congress on Environmental Geotechnics (ICEG2023)

Advances in Numerical Modeling