Commensurability Effects in Viscosity of Nanoconfined Water

TL;DR

This study reveals that the viscosity of nanoconfined water exhibits oscillations due to commensurability effects when confined within subnanometer capillaries, significantly impacting flow dynamics in nanofluidic systems.

Contribution

It demonstrates how the layered structure of water causes viscosity oscillations in subnanometer confinement, highlighting the importance of microscopic effects in nanofluidic flow behavior.

Findings

Viscosity is greatly enhanced in subnanometer capillaries.

Viscosity exhibits large oscillations due to commensurability effects.

Flow properties are significantly affected by molecular-scale interactions.

Abstract

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.