Observation of Interfacial Water Chains Confined in a Nanoscale Space by Cantilever Based Optical Interfacial Force Microscopy
Abstract
We investigated interfacial water structures in an ambient environment between two silica surfaces using the cantilever based optical interfacial force microscope (COIFM). We observed—in the COIFM force-distance curve as the two surfaces approached one another—remarkable oscillatory behavior in normal and friction forces, not previously observed using conventional AFM measurements. As the gap distance decreases between the tip and the substrate, the force response of the water chains is not like that of a spring but rather a polymer. Our further analysis suggests that water molecules confined between tip and substrate in an ambient environment form a bundle of water chains through hydrogen bonding. Each chain length can be approximated by a model called “freely jointed chain” (FJC) model in which the individual segments can be thought of as performing a random walk. This model was systematically investigated for the different number of water chains in the bundle through the change of relative humidity ranging from 12 to 57 percent in increments of 5. This new understanding about water can be applied to many things. Firstly the information could be used to establish a quantitative theory about the interaction between water and hydrophilic surfaces such as silicon oxide. It can also contribute to liquid-based nano and bio-technologies as well as contribute to the improved performance of moving components in silicon-based micro-electro-mechanical system devices, where water plays a key role in interfacial interactions.