Direct Observation of Chain-Like Water Structures by Atomic Force Microscopy

Dr. Byung Kim
Department of Physics
Boise State University

The apprehension of water’s structure in local and confined spaces is extremely important for understanding the interfacial phenomena of water at nanometer scales. These phenomena include the assembly and function of biomolecules, wetting and interfacial interactions, corrosion processes, condensation and crystallization of water vapor on surfaces, and amorphous solid water in an interstellar medium. However, very little is known about the structure of confined water and its formation mechanism at the nanometer scale. We report self-assembled chain-like water structures in a nanoscopic water meniscus confined between two oxidized silicon surfaces in an ambient environment using atomic force microscopy. Sawtooth-like oscillatory forces were observed when the two surfaces approached each other in the ambient environment. The humidity-dependent oscillatory amplitude and periodicity were 3-12 nN and 3-4 water diameters, respectively. Half of each period was matched with a freely jointed chain model, possibly suggesting that the confined water behaved like a bundle of water chains. Our chain-like water structure explains many previously unexplicable properties of confined water at the nanometer scale (e.g. slow evaporation rates, positive Kelvin radii, and  long water nucleation timescales, etc. ), thus dramatically improving the understanding of a variety of water systems in nature.