Revisiting the Origin of Enhanced Water Flow in Carbon Nanotubes with Deep Potential Molecular Dynamics.

Carbon nanotubes (CNTs) enable ultrafast water flow, but the underlying mechanism remains debated. Here, using Deep Potential molecular dynamics simulations with quantum mechanical accuracy, we revisit enhanced water transport in CNTs by analyzing interfacial friction and confined water properties. It is found that the interfacial friction coefficient is primarily governed by the free-energy barrier amplitude. The viscosity of confined water shows discontinuous changes in channels below 1.62 nm in diameter but varies nearly continuously in wider ones. For larger channels, viscosity can be approximated by a weighted average of interfacial and bulk values, allowing accurate prediction of flow enhancement within a continuum fluid mechanics framework. Across all channel sizes, variations in structural order parameters and average hydrogen-bond number correlate with viscosity changes, indicating that altered water structure drives viscosity modification. This work reveals the coupling between confined water structure, viscosity, and nanofluidic transport, providing insights for designing advanced nanofluidic devices.