General mechanical characteristics of hydration water layer

The hydration water layer (HWL), a ubiquitous form of water on the hydrophilic surfaces, exhibits anomalous characteristics different from bulk water and plays an important role in interfacial interactions. Despite extensive studies on the mechanical properties of HWL, one still lacks holistic understanding of its general characteristics including energy dissipation, which is critical to understanding of the viscoelasticity of HWL as well as identification of nanoscale dissipation processes. First, we derive the general stress tensor of the ubiquitous HWL, based on the empirical hydration force, by combining the elasticity and hydrodynamics theories. The tapping and shear components of the tensor describe the elastic and damping properties of HWL, respectively, in good agreement with experiments. In particular, unified understanding of HWL dynamics provides the otherwise unavailable intrinsic parameters of HWL, which offer additional but unexplored aspects to the supercooled liquidity of the confined HWL. Second, we address the energy dissipation process of nanoconfined HWL between two atomically flat hydrophilic solid surfaces (area of ~120 nm2) by small amplitude-modulation, noncontact atomic force microscopy. Based on the viscoelastic hydration-force model, the average dissipation energy is ~1 eV at the tapping amplitude (~0.1 nm) of the tip. In particular, we determine the accurate HWL thickness of ~6 layers of water molecules, as similarly observed on biological surfaces. Such a long-range interaction of HWL should be considered in the nanoscale phenomena such as friction, collision and self-assembly. Our results may allow deeper insight on systems where HWL is critical, including hydrophilic interfacial interactions and biological celluar dynamics.