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Non-Newtonian shear viscosity of confined water in forsterite nanoslits: insights from molecular dynamics simulations

ABSTRACT The confined water flow through forsterite nanoslits is vital to the security of CO2 geological sequestration in deep saline aquifers. Molecular dynamics simulations for confined water in the forsterite nanoslits are performed in this work. We investigate the effects of nanoslit height, hydraulic pressure drop, temperature and hydrostatic pressure on the water flow characteristics. Firstly, equilibrium molecular dynamics simulations are conducted to select a minimum nanoslit height for the subsequent non-equilibrium molecular dynamics (NEMD) simulations, to weaken the ultra-dominant nano-confinement effect on water flow. NEMD simulations reveal that the velocity profiles transfer from parabolic to plug-like shape with the increase in hydraulic pressure drop. The distance-dependent shear viscosity is determined by analyzing velocity profiles. The results indicate that the confined water gradually deviates from the Newtonian to non-Newtonian fluid, resulting from the more condensed water molecule layers introduced by the large hydraulic pressure drop. Additionally, we demonstrate that the water flow is enhanced with the temperature increasing. The enhancement in water flow is attributed to the shear viscosity reduction, resulting from the looser hydrogen-bonding network. Comparatively, the impact of variations in the hydrostatic pressure on the velocity profiles and shear viscosity is insignificant. This work provides atomistic insights into the non-Newtonian transport behaviour of high velocity water flow under nano-confinement or extreme environmental conditions.