Recognizing and controlling water friction in nanoconfinement from the first water layer

Water friction in nanoconfinement is of great importance in water lubrication and membrane-based applications, yet remains fraught with doubts despite great efforts. Our molecular dynamics simulations demonstrate that the first water layer adjacent to the surface plays an important role in interfacial friction. Applying a uniform strain to the surface (changing the lattice constant) can induce a significant change in friction, and is quite different scenarios for the hydrophilic and hydrophobic cases. Specifically, in the hydrophilic case, there is a maximum friction when the lattice constant approaches the preferential oxygen-oxygen distance of the first water layer (a constant value), and the further it deviates the smaller the friction. The maximum friction corresponds to the most ordered first water layer. While in the hydrophobic case, the friction increases monotonically with the increasing lattice constant, which hardly changes the first water layer structure but only increases the difficulty of water molecular jump (meaning jump from one equilibrium position to another). Starting from the molecular jump in the first water layer, a theoretical dependence of the friction on the molecular activation barrier and the shear velocity is established, which provides a reasonable explanation for the friction behavior. Moreover, the water transport behavior in nanochannels supports the finding of the friction dependence on the lattice constant, suggesting great potential for improving and controlling water transport. Our results not only provide a novel understanding of nanoconfined water friction, but are instructive for friction control and water transport.