Origin of Friction Generated by Chain-like water Confined Between Two Surfaces

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Sunwoo Jung; Byung Kim
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Recently we reported self-assembled chain-like water structures in a nanoscopic water meniscus confined between two oxidized silicon surfaces in an ambient environment (J. Chem. Phys. 139, 054701 (2013)). A newly developed innovative cantilever-based optical interfacial force microscope (COIFM) was used to measure both normal and lateral forces between the two surfaces to avoid snap-to-contact instability that a common atomic force microscope (AFM) suffers from (Rev. Sci. Instrum. 82, 053711 (2011)). The normal force showed a sawtooth-like oscillatory pattern, associated with structural transitions by a series of water chain bundles as the two surfaces approach each other. However, little has been known about the origin of the lateral friction force generated by the chain-like water. Interestingly, the friction force data is analyzed to be dependent logarithmically on the normal force. Our independent statistical calculation of entropy (a measure of disorder) of chain-like water also showed the logarithmic dependence on the normal force. This agreement suggests that the chain-like water can dissipate energy better (worse) when the molecules is more (less) disordered along the chains. This result provides a critical insight about how friction (energy) is generated (dissipated) by chain-like water, ultimately leading to various applications in nano- and biomolecular systems.