Abstract: Nanopores are prevalent within various clay morphologies, and water flow in clay nanopores is significant for various engineering applications. In this study, we performed non-equilibrium molecular dynamics (NEMD) simulations to reveal the molecular force mechanisms of water flow in clay nanopores. The water dynamic viscosity, slip length, and average flow velocity were obtained to verify the NEMD models. Since the water confined in the nanopores maintained a dynamic mechanical equilibrium state, each water lamina can be regarded as a simply supported beam. The applied driving force, the force from clay crystal layers, the force from compensating sodium ions, and the force from other water laminae were further calculated to investigate the force mechanisms. The van der Waals barrier above the surface and hydraulic gradient lead to distribution differences in water oxygen atoms, which contribute to a net van der Waals resistance component of the force from clay crystal layers. Meanwhile, the water molecules tend to rotate to generate the electrostatic resistance component of the force from clay crystal layers and balance the increasing hydraulic gradient. Due to the velocity difference, the water molecules in the slower lamina have a higher tendency to lag and generate a net electrostatic resistance force as well as a net van der Waals driving force on the water molecules in the faster lamina, which together make up the viscous force.

Key words: Molecular dynamics; Hydrodynamics; Clay; Nanopore; Molecular force; Boundary effect; Viscous force

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