What Is the Viscosity of Liquid Water Confined in a Hydrophobic Nanotube? Estimation Using a Novel Approach

Computation of the shear viscosity (η) of liquid water under nanoconfinement is a daunting task, albeit crucial in different areas related to nanofluidics. The viscosity of bulk water is computed using the Green–Kubo formula or Stokes–Einstein (SE) relation. However, these relations are less useful in the case of water confined in a cylindrical nanotube since the stress tensors are asymmetric and the SE relation breaks down. Using a novel method, based on the translational Jump-corrected Confined Stokes–Einstein (JCSE) relation that takes into account the breakdown of the SE relation, we compute the viscosity of water inside some hydrophobic carbon nanotubes (CNTs) in a wide range of temperatures including the supercooled regime. The viscosity of water in narrow CNT(10,10) is seen to be surprisingly higher compared to bulk water at the same thermodynamic condition. This contradicts the usual notion and the past theoretical observations suggesting significantly low viscosity of water inside hydrophobic CNTs.