Coupling of normal and transverse motions during frictional sliding

Using a surface forces apparatus, modified for measuring friction forces while simultaneously inducing normal (out-of-plane) vibrations between two boundary-lubricated sliding surfaces, we observe load- and frequency-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 relaxation lime to be faster than the mechanical resonance time of the macroscopic system. A parallel computer simulation study (Gao et al. J. Phys. Chem. B 1998, 102, 5033) provides a theoretical basis for the observed phenomenon and indicates its broader generality. Our results also point to novel methods for realizing ultralow friction in mechanical devices.