Dipole alignment of water molecules flowing through a carbon nanotube

TL;DR

This study uses molecular dynamics simulations to demonstrate that water molecules flowing through a carbon nanotube align their dipoles along the flow direction, with alignment increasing with flow velocity and saturating at high speeds.

Contribution

It reveals the microscopic mechanism of water dipole alignment in nanotubes and models this behavior using Langevin theory, providing new insights into nanoscale water flow.

Findings

Water molecules align their dipoles along the flow direction.

Alignment increases with flow velocity and saturates.

Preferential entry of inward-pointing dipoles causes alignment.

Abstract

The fast flow rate of water through nanochannels has promising applications in desalination, energy conversion, and nanomedicine. We have used molecular dynamics simulations to show that the water molecules passing through a wide single-walled carbon nanotube (CNT) cavity get aligned by flow to have a net dipole moment along the flow direction. With increasing flow velocity, the net dipole moment first increases and eventually saturates to a constant value. This behavior is similar to the Langevin theory of paraelectricity with the flow velocity acting as an effective aligning field. We show conclusively that the microscopic origin of this behavior is the preferential entry of water molecules with their dipole vectors pointing inward along the CNT axis.