Energy dissipation of nanoconfined hydration layer: Long-range hydration on the hydrophilic solid surface

Abstract

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 energy dissipation, which is critical to characterization of viscoelastic materials as well as identification of nanoscale dissipation processes. Here we address energy dissipation 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.

Probing nonlinear rheology layer-by-layer in interfacial hydration water

“…The QTF, when operated in the shear mode (28,35) of dynamic force microscope, allows accurate noncontact measurements at precisely controlled tip-sample separation due to its high stiffness (∼ 2 × 10 4 N/m) and high quality factor (∼ 10 4 ), oscillating at various amplitudes (0.3 ∼ 14.6 nm). As detailed in ref.…”

“…As detailed in ref. 28, the tip was strongly epoxied to QTF and made flattened (diameter of the flat area is 60 ± 5 nm) while the cleaned mica was tightly fixed on the piezoelectric transducer using an adhesive glue. The silica tip and mica are atomically flat with the root-meansquared roughness (28) of 0.014 and 0.039 nm (or the corresponding peak-topeak roughness of about 0.07 and 0.11 nm) on a scanned area of 100 × 100 nm, respectively.…”

“…show dramatic variations with A 0 (or equivalently, shear rate), used for our analysis of nonlinear rheology of HWL. Notice that, above y 0 = 2 nm, the capillary effects dominate while the HWL effects disappear (28), and the nonlinear rheological properties due to capillarity are detailed in the ref. 27 (as shown in figure 3A of ref.…”

“…In ref. 28, it is shown in detail that the HWL obtains energy from shear deformation under the shear stress (σ 21 ), which is then dissipated in proportion to _ γ 2 0 (or v 2 tip ) at low shear rate and under 2.0-nm thickness. However, if elastic turbulence occurs in the HWL, a portion of the acquired energy by shear stress should be constantly transferred to the turbulent kinetic energy, which is the important characteristics that accompanies turbulence (31), and thus the net dissipated energy of the HWL becomes reduced.…”

Shear-stress function approach of hydration layer based on the Green-Kubo formula

“…In particular, we find that the spatial part of the SC function can have negative values when the thickness of HWL is larger than the decay length of the hydration force, while the spatially independent part simply decays as e~'lx (r is the relaxation time). Notice that the negative SC means that the shear stresses on the HWLs of two nearby hydrophilic surfaces act in the opposite direction, which results from the weakly bound HWL that exists between the two strongly bound HWLs [18,22], Moreover, we suggest that the SC of HWL is closely related to the physical origin of the exponentially decaying hydration force because the SC function exhibits the empirically known exponential hydration-force form [12,14].…”