Quantum-Mechanically Enhanced Water Flow in Subnanometer Carbon Nanotubes

Water flow in carbon nanotubes (CNTs) starkly contradicts classical fluid mechanics, with permeabilities that can exceed no-slip Haagen-Poiseuille predictions by 2-5 orders of magnitude. Semiclassical molecular dynamics accounts for enhanced flow rates that are attributed to curvature-dependent lattice mismatch. However, the steeper permeability enhancement observed experimentally at about nanometer-size radii remains poorly understood, and suggests emergence of puzzling non-classical mechanisms. Here, we address water-CNT friction from a quantum mechanical perspective, in terms of water-energy loss upon phonon excitation. We find that combined weak water- phonon coupling and selection rules hinder water-CNT scattering, providing effective protection to water super flow, whereas comparison with a semiclassical theory evidences a friction increase that can exceed the quantum mechanical prediction by more than 2 orders of magnitude. Quasi-frictionless flow up to subnanometer CNTs opens new pathways toward minimally invasive trans-membrane cellular injections, single-water fluidics, and efficient water filtration.