Formation, Manipulation, and Elasticity Measurement of a Nanometric Column of Water Molecules

Nanometer-sized columns of condensed water molecules are formed by an atomic-resolution force microscope operated in ambient conditions. An unusual stepwise decrease of the force gradient associated with the ultrathin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights ($\approx 0.5\mathrm{N}/\mathrm{m}$) and plateau lengths ($\approx 1\mathrm{nm}$). Such “quantized” elasticity is indicative of an atomic-scale stick slip at the tip-water interface. A thermodynamic-instability-induced rupture of the water meniscus (5 nm long and 2.6 nm wide) is also found. This work opens a high-resolution study of the structure and interface dynamics of a nanometric aqueous column.

Figure 1

Experimental schematics of a high sensitivity AFM setup operating in ambient conditions. The inset shows an atomic-resolution AFM image of a dry and clean mica substrate, obtained at a modulation-amplitude change of 5% (scan area: $1.5\mathrm{nm}\times 1\mathrm{nm}$) [6].

Figure 2

Measured values of the force gradient of the water meniscus as a function of its elongation at a given RH of (a) 2%, (b) 15%, (c) 31%, and (d) 45%. The approach and retraction speed is $0.15\mathrm{nm}/\mathrm{s}$. The black solid line in (b) and (c) is a guide to the eye.

Figure 3

Reproducibility of the repeated short-ranged approach-retraction cycles of the force-gradient measurement, obtained at 13% RH. (a) The tip makes a deeper initial approach followed by repetitive cycles of retraction and approach until the water bridge is intentionally ruptured. (b) Close-up plot of the repeated cycles, with an arbitrary vertical offset for an eye guide. The approach and retraction speed of the tip is $0.15\mathrm{nm}/\mathrm{s}$.