Measurement of Adhesion and Short-Range Forces Between Molecularly Smooth Surfaces in Undersaturated Vapours and in Organic Liquids

Abstract

Results are presented of experimental studies on the adhesion of two curved molecularly smooth surfaces. in these studies adhesion forces and the elastic deformation occurring around the contact zone were measured. the experiments were carried out with surfaces (i) in an inert atmosphere of nitrogen, (ii) in under-saturated vapours, where capillary condensation around the contact zone gives rise to an additional surface tension (meniscus) force, and (iii) in organic liquids. (i) the measured adhesion of two mica surfaces each coated with a monolayer of surfactant (calcium stearate or hexadecyltrimethylammonium-bromide, CTAB) of known surface energies(γ SV ∼ 23 erg/cm 2 and ∼ 27 erg/cm 2 , respectively) show that the adhesion force is given by F ≈4πR γ SV , where R is the local radius of curvature of the surfaces. the surface profile is non-Hertzian. in particular, when pulled apart the surfaces separate spontaneously once their contact diameter has fallen to < 0.58 of the diameter at equilibrium (under zero external force). (ii) the adhesion force between two mica surfaces in undersaturated vapours of the condensable liquids benzene, cyclohexane and hexane is well described by the theoretical expression based on bulk thermodynamics: F ≈ 4πR γ SV = 4πR[γ LV cos θ + γ SL ]. Equilibrium meniscus radii (Kelvin radii) as low as 0.5 nm (relative vapour pressure < 0.1) are sufficient for these condensed liquids to behave as bulk liquid bridges, and since γ SL is small the adhesion force is effectively given by F ≈ 4πRR γ LV cos θ. (iii) in inert organic liquids, such as cyclohexane and the silicon liquid octamethylcyclotetrasiloxane, the force between two mica surfaces over the last few nanometers is spatially oscillatory – varying between attraction and repulsion – with a periodicity equal to the molecular size. These rapidly decaying oscillatory forces indicate that the molecules are layered near the surfaces, and this layering prevents the surfaces from coming into strong adhesive contact. Addition of trace amounts of water results in a purely attractive, strongly adhesive, force. the implications for lubrication and particle interactions will be discussed.