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Thickness and Structure of Adsorbed Water Layer and Effects on Adhesion and Friction at Nanoasperity Contact

1
Tribology Research Institute, Key Laboratory of Advanced Technology of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China
2
Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
*
Author to whom correspondence should be addressed.
Colloids Interfaces 2019, 3(3), 55; https://doi.org/10.3390/colloids3030055
Received: 30 June 2019 / Revised: 30 July 2019 / Accepted: 2 August 2019 / Published: 6 August 2019
(This article belongs to the Special Issue Surface Forces, Adhesion, and Friction)
Most inorganic material surfaces exposed to ambient air can adsorb water, and hydrogen bonding interactions among adsorbed water molecules vary depending on, not only intrinsic properties of material surfaces, but also extrinsic working conditions. When dimensions of solid objects shrink to micro- and nano-scales, the ratio of surface area to volume increases greatly and the contribution of water condensation on interfacial forces, such as adhesion (Fa) and friction (Ft), becomes significant. This paper reviews the structural evolution of the adsorbed water layer on solid surfaces and its effect on Fa and Ft at nanoasperity contact for sphere-on-flat geometry. The details of the underlying mechanisms governing water adsorption behaviors vary depending on the atomic structure of the substrate, surface hydrophilicity and atmospheric conditions. The solid surfaces reviewed in this paper include metal/metallic oxides, silicon/silicon oxides, fluorides, and two-dimensional materials. The mechanism by which water condensation influences Fa is discussed based on the competition among capillary force, van der Waals force and the rupture force of solid-like water bridge. The condensed meniscus and the molecular configuration of the water bridge are influenced by surface roughness, surface hydrophilicity, temperature, sliding velocity, which in turn affect the kinetics of water condensation and interfacial Ft. Taking the effects of the thickness and structure of adsorbed water into account is important to obtain a full understanding of the interfacial forces at nanoasperity contact under ambient conditions. View Full-Text
Keywords: water adsorption; water condensation; molecular configuration; adhesion force; friction force water adsorption; water condensation; molecular configuration; adhesion force; friction force
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MDPI and ACS Style

Xiao, C.; Shi, P.; Yan, W.; Chen, L.; Qian, L.; Kim, S.H. Thickness and Structure of Adsorbed Water Layer and Effects on Adhesion and Friction at Nanoasperity Contact. Colloids Interfaces 2019, 3, 55.

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