1. Motivation / Overview
Water meniscus plays an important role in solid-solid interfacial adhesion, friction, lubrication, and wear in wide fields. But that is not well known in nanoscopic region because it is not easy to manipulate a stable nanometric water meniscus at a fixed tip-sample distance. We use atomic force microscope (AFM) as a main instrument to form and manipulate a nanometer-sized water capillary bridge, thus we can investigate the properties of the nanoscopic water meniscus by using this home-made AFM.
Image from:
H. Choe, M.-H. Hong, Y. Seo, K. Lee, G. Kim, Y. Cho, J. Ihm, and W. Jhe, “Formation, Manipulation, and Elasticity Measurement of a Nanometric Column of Water Molecules”, Phys. Rev. Lett. 95, 187801 (2005) [Link]
2. Time-resolved dynamic force microscopy
1) Tuning-fork with cantiliever tip as a force sensor
Tuning fork is employed as a force sensor which has high sensitivity, large stiffness, and self-actuating property. By attaching a nanometer-sized cantilever tip to a tuning fork, it is possible to study a water meniscus in nanometer scale by dynamic force microscopy (DFM).
2) Experimental Setup
Many AFM systems for force spectroscopy measure the response of force sensor as a function of tip-sample distance only because they have been used to observe solid-solid interaction so far. But it is known that confined water in nano- or micrometer scale has slow dynamics in order of several tens of milliseconds. We use time-resolved DFM which measures several times at a certain tip-sample distance. It enables us to observe the response of the force sensor due to water meniscus as time passes.
3. Nucleation Time
When a nanometric water meniscus is formed between small solid surfaces, vapor molecules make a liquid by nucleation process. The nucleation time is the time lag elapsed to form a water meniscus and it is related to the energy barrier of the system. Because nucleation is thermal activated process, the time is influenced by humidity, temperature, and tip-sample distance. We can measure nucleation time directly by the time-resolved DFM and it is important to understand capillary condensation.
