The role of carbon-nanotube vibrations in the enhanced transport of confined water*

In the last several decades, the transport of water confined in carbon nanotubes (CNTs) generated significant research interest due to an enhanced flow rate and related potential applications. Very large, classical-molecular dynamics (MD) simulations have been performed to understand the fast water flow in CNTs, but significant issues still remain open. By using density-functional-theory (DFT) MD simulations, here we initially reveal that, in the absence of any externally applied pressure, the vibrations of the CNT's carbon atoms induce directed diffusion of water confined in medium-diameter CNTs. We found that the longitudinal vibrations of the CNT induce collective diffusion of water molecules in the direction of the CNT's axis, while the in-plane CNT vibrations further enhance the diffusion of water by breaking the H-bonds between water molecules. The results provide additional insights on the mechanism of the fast water flow in CNTs.