Critique of the friction coefficient concept for wet (lubricated) sliding

Complications in the traditional definitions of static and kinetic friction are discussed as they pertain to lubricated sliding. First, we discuss rate criteria to observe stick-slip motion and show that stick-slip occurs only when thin liquid films are deformed faster than their intrinsic relaxation time. This conclusion comes from experiments with confined aqueous films in a surface forces apparatus modified to measure interfacial theology. The observation offers a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined fluid. Second, we consider the issue of transients and show, for the case of a polymer melt of PDMS (polydimethylsiloxane), that equilibration of the viscous forces was spectacularly slow to be achieved when the sliding velocity was changed. Overshoots of force were observed when the velocity was raised; undershoots of force were observed when the velocity was lowered. Third, we consider the question of how forces in the shear and normal directions are coupled. In systems comprised of polymer brushes in solution at near Theta solvent quality, we contrast the shear response for brush-brush sliding (both surfaces coated with end-attached polymer chains) and brush-mica sliding (one sole surface coated with end-attached polymer chains). Both systems gave nearly the same resistance to compression in the normal direction, but the latter system offered less shear resistance. The polymer brush experiments invite further theoretical interpretation.