Fluidity of water in subnanometric confinement Di Jin 1 , Haichang Lu 2 , Paramita Manna 1 , Jacob Klein 1 1 Weizmann Institute of Science, Israel, 2 Beihang University, PR China
The fluidity of water within nano-confinement plays a crucial role in various processes, influencing shear stress in soft-matter tribology and nanofluidic applications. It is closely linked to the no-slip boundary condition, a fundamental principle in theoretical fluid dynamics that assumes a fluid is stationary in the immediate vicinity of a solid boundary. In a groundbreaking study conducted two decades ago 1,2 , the viscosity of water within sub- nanometer confinement between hydrophilic mica surfaces was measured using highly purified water and the surface force balance (SFB) technique. The results demonstrated that the viscosity of nano-confined water remains within a factor of three of its bulk value. In this study, we revisit the problem with the same experimental setup but introduce the high-speed imaging technique, enabling us to observe the dynamic process of the mica surfaces coming into contact under van der Waals attraction. By incorporating the hydrodynamic stress into the force balance equation, we can estimate the viscosity of water at an angstrom resolution within the last 2 nm of the confinement. Surprisingly, our findings reveal a substantial increase in water viscosity within the final ca. 5 angstroms. Using classical molecular dynamics simulations, we successfully validated the observed increase in water viscosity within a theoretical system that closely resembled the experimental conditions. To gain further insight into the mechanism behind this increased viscosity within subnanometric confinement, we conducted ab initio molecular dynamics simulations. Our analysis revealed that the formation of hydrogen bonds between water molecules and the oxygen atoms on the mica surfaces is the primary factor contributing to the near immobility of the last layer of water. Our results are consistent with the previous observation of water's nearly bulk-like fluidity within nanometric confinement, as the increase in viscosity occurs exclusively within the last layer of water on the surface, with viscosity promptly reducing to near-bulk value as the distance from the surface increases. This finding also offers insights into the widely recognized no-slip boundary condition, which has been classically demonstrated through the immobility of tracer dye on a solid surface in a flow. Our study effectively reconciles these seemingly contradictory phenomena, shedding light on the unique behavior of boundary water. References 1. Raviv, U., Laurat, P.& Klein, J. (2001). Fluidity of water confined to subnanometre films. Nature, 413(6851), 51–54. https:// doi.org/10.1038/35092523 2. Raviv, U.& Klein, J. (2002). Fluidity of bound hydration layers. Science, 297(5586), 1540–1543. https://doi.org/10.1126/ science.1074481
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