Viscous Water Meniscus under Nanoconfinement

A dramatic transition in the mechanical properties of water is observed at the nanometer scale. For a water meniscus formed between two hydrophilic surfaces in the attractive region, with $\le 1\mathrm{nm}$ interfacial separation, the measured viscosity is 7 orders of magnitude greater than that of bulk water at room temperature. Grand canonical Monte Carlo simulations reveal enhancement in the tetrahedral structure and in the number of hydrogen bonds to the surfaces as a source for the high viscosity; this results from a cooperative effect of hydrogen bonding of water molecules to both hydrophilic surfaces.

Figure 1

Schematic: experimental system. The scanning electron microscope image shows the parabolic Au tip with radius of curvature of 500 nm. (a)–(e) Normal force ($F_N$, solid curves and left axis) and friction force ($F_{\mu }$, dashed curves and right axis) profiles as a function of relative displacement between the tip and the sample at the indicated RH values for the following interface combinations: (a) ${\mathrm{CH}}_3$ tip on ${\mathrm{CH}}_3$ surface; (b) COOH tip on ${\mathrm{CH}}_3$ surface; and (c)–(e) COOH tip on COOH surface. In each of the above panels, the thin pink and blue curves are individual $F_N$ profiles during approach and retract, respectively; the thick red (approach) and blue (retract) curves are averaged data. Only averaged data are shown for $F_{\mu }$ profiles (dashed brown curves for approach and dashed green curves for retract). A, B, andC represent positions for water nucleation, capillary condensation, and film contact, respectively.

Figure 2

Snapshots of the gap regions for $\mathrm{COOH}/\mathrm{COOH}$ (left column) and ${\mathrm{CH}}_3/\mathrm{COOH}$ (right column) systems taken from the MC simulations during approach ($D=1.1$, 0.7, and 0.3 nm). The oxygen atoms in water or COOH groups are blue or red, respectively. The dashed white lines represent hydrogen bonds. The thick blue lines depict the boundaries of the periodically replicated system.

Figure 3

The number of water molecules within the simulation cell ($N_A$, bottom row), the number of hydrogen bonds to each acid surface ($N_{\mathrm{HB}}$, middle row), and the tetrahedral order parameter of water molecules ($q$, top row) as a function of interfacial separation for the $\mathrm{COOH}/\mathrm{COOH}$ (left column) and ${\mathrm{CH}}_3/\mathrm{COOH}$ (right column) systems. The solid and dashed curves depict the approach and retract data, respectively.