Study of the rupture process of nanometer water bridge by using time-resolved noncontact AFM

Abstract

The nanometric water bridge is formed by capillary condensation between two close bodies in ambient condition, and is a commonplace phenomenon in nature. The water bridge plays an important role in the dynamic processes such as friction, adhesion and solvation, and is widely used in nanotechnology including dip-pen lithography, atomic force microscopy (AFM) and micro-electro-mechanical systems. Recently, it has been well known that the water bridge is essential in understanding the biological mechanisms of the ion-channel and protein folding. Therefore, it is important to have better and broader understanding of the mechanics, dynamics as well as kinetics of the capillary-condensed water bridge in ambient condition, from its formation to rupture.

There have been extensive studies on the formation of the capillary water nanobridges over the last several decades. In most previous theoretical studies, the Kelvin-Laplace equation has been used to determine the size and shape of the water bridge, which forms at the nanoscopic contacts of solid surfaces.

In particular, if the water bridge is formed in thermodynamic equilibrium, the radius of curvature of the bridge is equal to the Kelvin radius, and thus the bridge should break up or rupture at the distance equal to twice the Kelvin radius.

However, the applicability of this approach to the capillary bridge at the nanoscale is still in debate. Based on the experimental observation and theoretical model of the water bridge, Riedo et al. have found that the volume of the stretched water bridge is proportional to the rupture distance. Because the curvature radius of the water bridge changed during the tip retraction, the state of the water bridge is not in thermodynamic equilibrium. From the results, they found that the stretched water bridge breaks up a distance that is much larger than twice of the Kelvin radius.

In thermodynamic nonequilibrium, the rupture process of the water bridge is associated with the chemical potential gradients that may result in material transfer and trigger the irreversible bridge rupture. Thus, the Kelvin equation is not applicable in thermodynamic nonequilibrium. Men et al. applied the density functional theory, which is based on the energy barrier, in order to study the rupture process of the water bridge. Their work demonstrated the origin of the hysteresis behavior in the formation and rupture of the water bridge, and the rupture started by overcoming the energy barrier, which is so called the thermal activation process.

Despite the established theoretical results, however, experimental works on the kinetics of the nanoscale rupture process in terms of thermal activation still lack, unlike the well studied capillary formation.

In this Letter, we directly measure the activation time needed for the water bridge to be ruptured by using the quartz-tuning-fork (QTF) based AFM in ambient condition. We find that the activation time obtained at the various rupture distance exhibits random distribution and shows the characteristic exponential decay, which indicates that the rupture process follows the Poisson statistics, which is typical of the thermal activation processes. We also have measured the dependence of the rupture rate on the tip-sample distance as well as the temperature, from which we can estimate the values of the activation energy.