Self-Assembled Water Chains and their Coil-to-Bridge Transitions in a Nanoscopic Meniscus*

Structures and behaviors of water confined between two surfaces are important in bio/nano sciences and water-based nanotechnology. I report observations of self-assembled water chains and their transitions from a coil state to a bridge state in a nanoscopic water meniscus in air. Large sawtooth-like oscillatory forces were shown when the normal and friction forces were measured as a function of distance between a sharp probe and a flat oxidized silicon surfaces using a force-feedback force microscope called “cantilever-based optical interfacial force microscope” (COIFM). In the force-distance plot, each oscillation is comprised of a rising-shaped ())  curve in the upward portion and a sigmoidal-shaped (∫) curve in the downward portion as the tip-sample distance decreases. Further analysis of each upward portion with the freely joined chain (FJC) model reveals that each portion is developed from self-assembled water chains with lengths ranging from 14 to 42 chain units in the meniscus. The analysis of downward portions reveals that each portion is generated by a “coil-to-bridge” transition of self-assembled water chains, whose lengths are between 197 and 383 chain units. The observed coil-to-bridge transitions explain many mysterious properties of confined water at the nanometer scale (e.g. long condensation distances, long nucleation timescale, high surface tension, long-range biomolecular interactions, etc.), thus dramatically improving the understanding of a variety of water systems in nature [1].

[1] Byung Il Kim, Self-Assembled Water Chains: A Scanning Probe Microscopy Approach (Springer Nature, 2023).