Molecular Dynamics Study of Water Flow in Silica Nanochannels: Surface Wettability Governs the Transition from Reduction to Enhancement

We present a molecular dynamics study investigating water flow in silica nanochannels, systematically exploring how surface wettability, channel size, and pressure gradient modulate transport. At the nanoscale, fluid flow fundamentally deviates from continuum predictions due to the dominance of surface effects, fluid-wall interactions, and molecular ordering. Our simulations reveal a distinct wettability-governed transition in flow behavior. Hydrophilic silica surfaces consistently exhibit significant flow reduction, characterized by negative or near-zero slip lengths. Apparent permeability decreases up to 70% for untreated channels and a more pronounced 90% for hydroxyl-modified channels, directly attributed to strong fluid-wall attractions and interfacial water structuring.

In contrast, hydrophobic graphene-coated channels demonstrate remarkable flow enhancement, driven by substantial positive slip lengths that increase nonlinearly with the applied pressure gradient. This leads to exceptional permeability enhancements, reaching up to 500% in 4 nm graphene channels. We find that while all channels experience increased absolute flow with larger size, the magnitude of this relative enhancement for hydrophobic surfaces is critically size-dependent, diminishing significantly in wider pores. Conversely, permeability reduction on hydrophilic surfaces remains relatively constant across varying channel sizes. Furthermore, while flow rates generally increase with pressure gradient, our results show a tendency towards non-linear response and velocity convergence at higher driving forces, indicating the limits of the linear transport regime. These findings explain the molecular mechanisms underlying flow modulation in confined environments and help resolve prior discrepancies in the literature, offering crucial insights for designing advanced nanofluidic devices, tailored filtration membranes, and more efficient energy technologies.