COUPLING OF NORMAL AND TRANSVERSE MOTIONS DURING FRICTIONAL SLIDING

Using a surface forces apparatus, modified for measuring friction forc es while simultaneously inducing normal (out-of-plane) vibrations betw een two boundary-lubricated sliding surfaces, we observe load- and fre quency-dependent transitions between a number of ''dynamic friction'' states. In particular, we found regimes of vanishingly small friction at interfacial oscillation amplitudes below 1 Angstrom. The phenomenon is shown to have a molecular origin but requires the molecular relaxa tion lime to be faster than the mechanical resonance time of the macro scopic system. A parallel computer simulation study (Gao et al. J. Phy s. Chem. B 1998, 102, 5033) provides a theoretical basis for the obser ved phenomenon and indicates its broader generality. Our results also point to novel methods for realizing ultralow friction in mechanical d evices.