Recent progress in machine learning has made it possible to overcome some limitations of ab-initio and force field methods 16, 19, 20, 21. Molecular dynamics (MD) 7, 8, 9, 10, 11, 12 and ab-initio simulations 12, 13, 14, 15, 16, 17 suggest that 2D water can transit into various structured states with distinctive molecular arrangements 18. 2D water locked between two graphene sheets has been recently found in such a structured state 6. The same happens in 2D water: The hydrogen bonds may become stable enough to bring water molecules into an ordered state making water behave like a solid in some aspects. In general, reducing dimensionality of any interacting physical system (for instance, squeezing it into a plane) amplifies interaction effects. In particular, the hydrogen bonds are believed to be responsible for the rather high melting and boiling points of water, as well as for the expansion upon freezing. What makes water so special as compared with most other liquids are the relatively strong hydrogen bonds 5. It may nevertheless be possible to find some evidence of unconventional phenomena in two-dimensional (2D) water-a-few-angstrom thick water monolayer squeezed between two solid planes 4. The most controversial claims regarding non-orthodoxal properties of bulk water have been debunked by subsequent comprehensive experiments 1, 2, 3. Despite (or due to) its simple chemical composition, the structure of water often causes scientific controversies such as polywater 1, memory effect 2, chain-like formation of water molecules 3, to mention a few. Water is the most important substance for life on Earth and has remained in the scientific focus for centuries, if not for millennia. The viscosity parameter values depend strongly on whether graphene or hexoganal boron nitride layers are used to confine 2D water that offers an interesting opportunity to obtain various nanofluids out of the same water molecules just by using alternate materials to fabricate the 2D channels. We demonstrate that the very ability of two-dimensional water to flow in short channels is governed by the second (dilatational) viscosity coefficient, leading to flow compression and velocity saturation in the high-pressure limit.
In this paper, we propose and explicitly solve a non-linear hydrodynamic equation describing two-dimensional water flow with viscosity parameters deduced from molecular dynamic simulations. As a consequence, the transport of such two-dimensional water combines hydrodynamic and nanofluidic features, intimately linked with each other. A water monolayer squeezed between two solid planes experiences strong out-of-plane confinement effects while expanding freely within the plane.
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