  Water-flow in carbon nanotubes (CNT's) starkly contradicts classical fluid
mechanics, with permeabilities that can exceed no-slip Haagen-Poiseuille
predictions by two to five orders of magnitude. Semi-classical molecular
dynamics accounts for enhanced flow-rates, that are attributed to
curvature-dependent lattice mismatch. However, the steeper
permeability-enhancement observed experimentally at $\sim$nm-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 superflow, whereas comparison with a
semiclassical theory evidences a friction increase that can exceed the
quantum-mechanical prediction by more than two orders of magnitude.
Quasi-frictionless flow up to sub-nm CNT's opens new pathways towards
minimally-invasive trans-membrane cellular injections, single-water fluidics
and efficient water filtration.
