Abstract
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.
Highlights
Understanding interfacial properties of water adsorbed on/at solid surfaces is a central theme across many scientific disciplines [1,2]
For hydrophilic surfaces, when the adsorbed water layer begins to form with increasing relative humidity (RH), the increase in Fc becomes more significant compared to the decrease in FvdW, resulting in an increase in total Fa (Figure 13c)
On hydrophilic surfaces, such as a highly-hydroxylated Si oxide surface with a contact angle less than 0◦, a strongly H-bonded solid-like arrangement of water molecules is found in the adsorbed layer formed at low RHs and a weakly H-bonded liquid-like structure grows and forms multilayers as RH exceeds 30–40% [41]
Summary
Understanding interfacial properties of water adsorbed on/at solid surfaces is a central theme across many scientific disciplines [1,2]. Water adsorption on solid surfaces is governed by the subtle balance between water-water hydrogen-bonding (H-bonding) and water-solid H-bonding as well as dispersive interactions governed. 2019, 3, 55 adsorption on solid surfaces is governed by the subtle balance between water-water hydrogen-bonding (H-bonding) and water-solid H-bonding as well as dispersive interactions governed by Lifshitz theory. The structure of the adsorbed water layer of the adsorbed water layer is determined by the extension of water-solid interactions from the is determined by the extension of water-solid interactions from the interior to the exterior of the surface. The changes in the kinetics of water condensation due to these factors and their effects on interfacial forces at the nanoasperity contact, including Fa and Ft , are discussed. Forces at the nanoasperity contact, including Fa and Ft, are discussed
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