Capillary filling of confined water in nanopores: Coupling the increased viscosity and slippage

Understanding the capillary filling of confined water is crucial for a broad range of science and engineering problems. In this study, Lucas-Washburn (LW) theory is revisited by further considering the effective viscosity and slippage to accurately model the capillary filling behaviors in nanopores, where the effective viscosity is investigated by considering the energy barrier induced by pore wall while the true slip is modelled with the wettability by considering the mobility of first-liquid-layer molecules. The proposed model is successfully validated against the published experimental and molecular simulation data. The results show: (1) The effective viscosity of confined water can be several times higher than that of bulk water for a hydrophilic nanopores with a diameter smaller than 10 nm, and the magnitude will decrease with an increasing of contact angle and pore size; (2) the notable interaction potentials induced by Van der Waals' component and hydrophobic component are out to a distance of 2-3 molecular layers from a surface while the decaying of electrostatic component strongly depends on the magnitude of Debye length, which is related to the ionic concentration of solution; (3) the increase of contact angle will mitigate the anomalous degree of capillary filling in nanopores because it will not only strengthen the hydrophobic repulsive interaction but also make the slippage effect more remarkable; (4) compared with the nanoslits, the more noticeable anomalous characteristic will be observed in nanotubes at the same condition, knowing the wettability and geometrical factors of nanoporous media may help us estimate the capillary filling more accurately.