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

Abstract

Nanometer-sized columns of condensed water molecules are created by an atomic-resolution force microscope operated in ambient conditions. Unusual stepwise decrease of the force gradient associated with the thin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights (≈ 0.5 N/m) and plateau lengths (≈ 1 nm). Such "quantized" elasticity is indicative of the atomic-scale stick-slip at the tip-water interface. A thermodynamic-instabilityinduced 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 the interface dynamics of a nanometric aqueous column.PACS numbers: 07.79. 07.79.Lh, 47.17.+e, 62.10.+s Water is one of the most important substances of life and has been studied extensively for hundreds of years. Nonetheless, it is still quite a unique matter that keeps surprising us and exhibits peculiarities, in particular, when confined in a nanometric configuration. For example, water and simple organic liquids exhibit solid-like orderedness in molecularly thin films [1,2,3]. Water molecules inside hydrophobic nanotubes manifest phases of ice that are not found under bulk conditions [4]. However, since bulk water possesses only short-range order [5] and water molecules move incessantly, it is usually difficult to experimentally investigate novel features of confined water structures other than thin films.In this Letter, we have employed an atomic-resolution noncontact atomic force microscope (AFM) in air [6] and achieved the spontaneous formation of a nanometric liquid column consisting of thousands of water molecules. We also have performed the sensitive measurement of the elastic property (or the force gradient) of the thin water column during its mechanical stretch. We have thereby demonstrated several novel phenomena: (i) the unusual stepwise decrease of the force gradient, associated with the atomic-scale stick-slip on the AFM-tip surface, (ii) the abrupt rupture of the thin water meniscus due to the thermodynamic instability of the liquid-vapor interface, and (iii) the manipulation of the thin aqueous column by repeated stretch-relaxation cycles, revealing the atomicscale contact angle hysteresis.Water molecules in ambient conditions produce a nanoscale water meniscus between a hydrophilic Si tip and a hydrophilic mica substrate, when capillary condensation occurs as the stiff AFM tip approaches the substrate within a nanometric distance (Fig. 1). Once the thin aqueous column is formed, it is stretched vertically upward by subsequent retraction of the tip. As the molecular water bridge of sub-zeptoliter (zepto = 10 −21 ) volume is elongated thereby, the force gradient associated with the elasticity of the system is measured by an ex- * Corresponding author: whjhe@snu.ac.kr tremely small amplitude-modulation operation of AFM [7,8]. Figure 1 presents the schematics of a home-built AFM setup used for formation of a nanometric water column by capillary condensation as well as for si...

Direct observation of self-assembled chain-like water structures in a nanoscopic water meniscus

“…Considering the high selectivity of frequency in the lock-in technique, it is extremely unlikely that such artifacts generated the same periodicity in ac signal (friction force) and dc signal (normal force). While they are consistent with the periodicity of 0.6-1.2 nm (2-4 water diameters) previously reported in the water bridge at the relative humidity of 15%, 49 they do not agree with those found in earlier studies at the solid-liquid interface. 23,44,[50][51][52]66 These oscillatory forces appear to be unique characteristics of water in air because they have not been observed with the menisci formed by soft materials such as hydrocarbon films and biofilms.…”

“…These equal distances indicate that the curved-tip area is made of crystallite terraces. 49,72 Each chain bundle was confined between the top terrace of the tip and the oxidized silicon surface in parallel. Because the solid lines meet at the zero distance all together in Fig.…”

“…10,44,[49][50][51][52] However, most of these AFM studies were only able to provide indirect evidence of the existence of layered water confined at the nanometer scale. The AFM is unable to generate force variation as a function of the controlled distance between the tip and the surface, which makes its data difficult to analyze directly with existing theories due to the missing data points.…”

Cantilever-Based Optical Interfacial Force Microscopy

“…The V A-B signal also displays a periodic change with the tip-sample distance, as shown in an enlarged inset in Figure 6(a). These periodic features are consistent with the earlier AFM-based observation of a stepwise change of the force gradient related with the thin water bridge in an ambient environment (Choe et al, 2005). The absence of modulation turns out to decrease the number of oscillations by several times, therefore suggesting that the kinetic energy due to the lateral modulation promotes layering transitions by overcoming the activation barriers between two successive layered states of the interfacial water.…”

“…The tip of the cantilever experiences the forces F x and F z by the sample surface during force measurements. A general Euler equation (Thomson, 1996) is given for the vertical displacement of the cantilever (z z ) produced by the normal force (F z ) acting at a point x = L cant as follows (Chen, 1993;Sarid, 1991),…”