Dimensionality transition and enhanced diffusion of water channels between periodically modulated surfaces: Insights from molecular dynamics

Water nanoconfinement leads to diverse transport phenomena depending on the geometry of the void space, such as 2D confinement between surfaces and 1D confinement inside nanotubes. We employ molecular dynamics to investigate water transport between periodically modulated surfaces. Our results reveal a spontaneous dimensionality transition from 2D aqueous sheets to 1D isolated channels, driven by the surface topography. By progressively increasing the inter-surface separation while keeping a fixed water density, we observe a dimensionality transition in the water network, where the system evolves from a 2D layer to 1D channels self-organized along surface grooves. Our results demonstrate that the self-diffusion coefficient (D) in these 1D nanochannels is strongly sensitive to the nanotube diameter and the surface-water interaction potential. Reducing the nanotube diameter from 10 to 5 nm triggers an increase in Dz by up to tenfold between confinement geometries. Notably, in the 5 nm systems, the diffusion coefficient exceeds bulk water values by up to 2.5. This enhanced diffusion is discussed in terms of the cooperative and collective molecular motion emerging from extreme geometric constraints. We also report a symmetry reduction observed in the confinement limit in the interstitial voids of the bulk bundle material, where water clusters into specific groove regions to minimize surface area. These findings provide insights into the statistical mechanics of fluids in complex periodically modulated environments and the structural transition from disconnected to percolating water networks in periodically modulated nanoporous environments.