RECENT ADVANCES IN MOLECULAR-LEVEL UNDERSTANDING OF ADHESION, FRICTION AND LUBRICATION

In order to investigate the mechanism of boundary friction, adhesion and friction experiments were carried out with a variety of surfactant-monolayer-coated surfaces using the surface forces apparatus. The friction forces were measured as a function of monolayer coverage and phase state, load, temperature, shear rate and relative humidity. We report on two important findings. (i) Stick-slip motion can occur not only between a ''static'' and a ''kinetic'' state but also between a static or kinetic state and some ''superkinetic'' state that exhibits ultralow friction. (ii) The friction coefficient is not directly correlated with adhesion but rather with the adhesion hysteresis, which is a measure of the energy dissipated during an irreversible adhesion (e.g. loading-unloading) cycle. We also conclude that chain interdigitation is the most important molecular mechanism that gives rise to ''boundary'' friction and adhesion hysteresis of monolayer-coated surfaces.