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

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.

Synthesis of In Situ Photoinduced Halloysite-Polypyrrole@Silver Nanocomposite for the Potential Application in Humidity Sensors

“…The highly hydrophilic surface entraps more water vapors; as a result, the sensitivity of the sensing film further increased. However, in highly hydrophilic surfaces, water molecules adsorbed on the sensing film and formed a strongly H-bonding with sensing film surface that will slow the desorption process [ 33 ].…”

Reactivity of Gypsum-Based Materials Subjected to Thermal Load: Investigation of Reaction Mechanisms

“…As a consequence of the increase in temperature in the gypsum rock, liquid water may be released, which in principle could already occur at temperatures below 100 • C (e.g., in [8] or [17]). It is a recognized fact that the presence of water films on the surfaces of porous gypsum-based systems plays a major role in many material properties (e.g., [28,29]) and is increasingly becoming the focus of attention as the driving force of solution and crystallization reactions [30]. The formation of water films or the accumulation of larger quantities of water with chemical properties comparable to those of liquid water can be obtained by a kind of condensation process.…”

Antibacterial coatings on vegetable ivory obtained by cold plasma jet activation of silicone and copaiba oils

“…In turn, adhesion forces are reduced even for the 20 s sample, indicating the formation of a hydrophobic coating over the sample, hindering the formation of a surface water-contaminant layer. [42] Besides, deviation for the 20 s sample was high as compared with samples treated for a longer time, indicating that silicone film, initially, may not cover the hydrophilic surface of the seed uniformly or is not hydrophobic enough to hinder the formation of the water contamination layer. These results are in agreement with those obtained for WCA, where samples treated for a shorter time presented a lower hydrophobicity.…”

“…To evaluate the surface elastic constant and adhesion forces for the coatings, force-distance curves were obtained for samples with all treatment times, from 20 to 100 s. The surface elastic constant was calculated from the slope in force-distance retract curves, whereas F I G U R E 4 Scanning electron microscopy images for transverse and longitudinal sections of 80-s plasma-treated samples at magnifications of ×300, ×2,400, and ×4,000. Copaiba and mixture coatings' longitudinal images presented in an inset (top left corner) to evidence coating thickness adhesion forces were evaluated through capillary forces between the surface water-contaminant layer and AFM tip, [42] also measured in force-distance retract curves. Results are summarized in Figure 6.…”