Structure and flow of low-dimensional water

When water flows through 1D or 2D channels, its behaviour deviates substantially from the well-established principles of hydrodynamics. This is because reducing the dimensionality of any interacting physical system amplifies interaction effects that are beyond the reach of traditional hydrodynamic equations. In low-dimensional water, hydrogen bonds can become stable enough to arrange water molecules into an ordered state, causing water to behave not only like a liquid but also like a solid in certain respects. In this Review, we explore the relationship between the molecular ordering of water and its ability to flow in low-dimensional channels, using viscosities of bulk water, vapour, and ice as benchmarks. We also provide a brief overview of the key theoretical approaches available for such analyses and discuss ionic transport, which is heavily influenced by the molecular structure of water. The dynamic interaction between low-dimensional water transport and ion-coupled structural features lies at the heart of recent advances in the design and investigation of angstrom-scale biomimetic and neuromorphic channels. Water’s structure and viscosity change markedly under reduced dimensionality. This Review explores how viscosity depends on the dimensionality of confinement (1D or 2D) and examines the interplay between geometric and ionic constraints in shaping transport properties within angstrom-scale water channels.