Asymmetric response of confined water transport in carbon nanotubes to entrance and exit effects

Water transport in carbon nanotubes (CNTs) is highly sensitive to sub-nanoscale structural perturbations, yet the axial-position dependence of a fixed-size fracture has not been systematically quantified. In this work, we employ molecular dynamics simulations to systematically map the spatial sensitivity of water flux in (6,6) single-walled CNTs by introducing an original mobile fracture probe comprising a fixed-gap fracture translated along the tube axis. For both 3 and 5 nm CNTs, the flux remains nearly unchanged when the fracture is located in the mid-section, forming a pronounced plateau, but decreases as the fracture approaches the tube ends. In particular, the reduction is strongly asymmetric in that fractures near the inlet suppress flux much more severely than those near the outlet. Hydrogen-bond statistics, non-bonded energy profiles, and density-based potential of mean force consistently suggest that inlet-side fractures destabilize the confined water chain and induce a structural disruption that extends into the tube, whereas outlet-side effects remain more localized. These results indicate that CNT exhibits substantial tolerance to minor defects in the interior while being limited by an inlet-dominated bottleneck, highlighting the importance of preserving entrance integrity in robust, high-throughput nanofluidic devices and CNT-based membranes.