Liquid–Solid Nanofriction and Interfacial Wetting

Abstract

Using atomic force microscopy, the nanofriction coefficient was measured systematically for a series of liquids on planar graphite, silica and mica surfaces. This allows us to explore the quantitative interplay between nanofriction at liquid–solid interfaces and interfacial wetting. A corresponding states theory analysis shows that the nanofriction coefficient, μ = d<i>F</i>_F/d<i>F</i>_N, where <i>F</i>_F is the friction force and <i>F</i>_N is the normal force, is a function of three dimensionless parameters that reflect the intermolecular forces involved and the structure of the solid substrate. Of these, we show that one parameter in particular, β = ρ_sΔ_sσ_ls², where ρ_s is the atomic density of the solid, Δ_s is the spacing between layers of solid atoms, and σ_ls is the molecular diameter that characterizes the liquid–substrate interaction, is very important in determining the friction coefficient. This parameter β, which we term the <i>structure adhesion parameter</i>, provides a measure of the intermolecular interaction between a liquid molecule and the substrate and also of the surface area of contact of the liquid molecule with the substrate. We find a linear dependence of μ on the structure adhesion parameter for the systems studied. We also find that increasing β leads to an increase in the vertical adhesion forces <i>F</i>_A (the attractive force exerted by the solid surface on the liquid film). Our quantitative relationship between the nanofriction coefficient and the key parameter β which governs the vertical adhesive strength, opens up an opportunity for describing liquid flows on solid surfaces at the molecular level, with implications for the development of membrane and nanofluidic devices