  The rate of water flow through hydrophobic nanocapillaries is greatly
enhanced as compared to that expected from macroscopic hydrodynamics. This
phenomenon is usually described in terms of a relatively large slip length,
which is in turn defined by such microscopic properties as the friction between
water and capillary surfaces, and the viscosity of water. We show that the
viscosity of water and, therefore, its flow rate are profoundly affected by the
layered structure of confined water if the capillary size becomes less than 2
nm. To this end we study the structure and dynamics of water confined between
two parallel graphene layers using equilibrium molecular dynamics simulations.
We find that the shear viscosity is not only greatly enhanced for subnanometer
capillaries, but also exhibits large oscillations that originate from
commensurability between the capillary size and the size of water molecules.
Such oscillating behavior of viscosity and, consequently, the slip length
should be taken into account in designing and studying graphene-based and
similar membranes for desalination and filtration.
