Unidirectional Oriented Water Wire in Short Nanotube

TL;DR

This study demonstrates that a unidirectional water wire can form intrinsically in short nanotubes without external fields, potentially enabling sustained water flow in biological and nanofluidic systems.

Contribution

It reveals the intrinsic formation of unidirectional water wires in short nanotubes through molecular dynamics and first-principles calculations, challenging the need for external fields.

Findings

Unidirectional water wire forms with only 21 water molecules.

Flip mechanism of water chain is stepwise, not concerted.

Unidirectional flow may persist in biological membranes without external fields.

Abstract

The orientation of water molecules is the key factor for the fast transport of water in small nanotubes. It has been accepted that the bidirectional water burst in short nanotubes can be transformed into unidirectional transport when the orientation of water molecules is maintained in long nanotubes under the external field. In this work, based on molecular dynamics simulations and first-principles calculations, we showed without external field, it only needs 21 water molecules to maintain the unidirectional single file water intrinsically in carbon nanotube at seconds. Detailed analysis indicates that the surprising result comes from the step by step process for the flip of water chain, which is different with the perceived concerted mechanism. Considering the thickness of cell membrane (normally 5-10 nm) is larger than the length threshold of the unidirectional water wire, this study suggests it may not need the external field to maintain the unidirectional flow in the water channel at the macroscopic timescale.