Fundamental understanding of environmental effects on adhesion and friction: Alcohol and water adsorption cases

Layers of adsorbed vapor molecules have profound impacts on adhesion and friction. This article reviews fundamental aspects of alcohol and water adsorption effects on adhesion and friction. Capillary force, a component of adhesion force which arises from the liquid meniscus that forms between contacting surfaces, shows a strong vapor partial pressure dependence that is not explained by theory which neglects the adsorbed layer. Theoretical calculations accounting for the adsorbed layer give good agreement with experimentally measured adhesion forces at the nanoscale. Nanoscale friction measurements are also strongly affected by the meniscus and adsorbed layer. Conventional contact mechanics theory could not fully explain the load dependence of nanoscale friction, especially at vapor partial pressures below saturation. However, when the effect of the meniscus is included in theoretical analysis of experimental data, it is found that the friction depends on the shear strength change in the contract area and the dragging of the meniscus formed around the contact. The meniscus dragging term is dominant at low loads but becomes inconsequential at higher loads. When the adsorbed layer assumes structural ordering or causes tribochemical reactions, their adhesion and friction behaviors are further complicated and deviated from simple contact mechanics.