Anomalous Water Flow in Sub-Nanometer Carbon Nanoconfinement

Sub-nanometer confinement fundamentally transforms fluid transport phenomena by introducing pronounced molecular-scale interactions that challenge the validity of classical hydrodynamic models. Utilizing advanced molecular dynamics simulations, this study systematically analyzes the transport properties of Lennard-Jones fluids and polar water within carbon nanotubes and graphene-based nanochannels. In these ultra-confined regimes, where molecular discreteness dominates and continuum constructs such as viscosity and velocity profiles become ambiguous, this study observes that water exhibits flow velocities up to threefold higher than Lennard-Jones fluids, quantitatively consistent with recent experimental and computational observations in sub-nanometer confinement. This anomalous transport originates from the disruption of hydrogen-bond networks, intensified interfacial Pauli exclusion forces, and a reduction in effective viscosity. Curvilinear geometries, typified by carbon nanotubes, induce enhanced molecular ordering and facilitate accelerated mass flux, while planar nanochannels impose steric constraints that suppress transport efficiency. The intrinsically lower molecular mass of water further augments its dynamic response under nanoscale confinement. Notably, despite the breakdown of classical assumptions, continuum-like velocity profiles emerge in specific regimes, signifying the onset of pseudo-continuum behavior at the molecular scale. These results elucidate the fundamental physical origins of confinement-induced transport anomalies and establish a predictive theoretical framework for extending continuum fluid concepts into the atomistic regime.