A PHENOMENOLOGY OF BOUNDARY LUBRICATION - THE LUMPED JUNCTION MODEL

We propose a phenomenological model of boundary lubricated junctions c onsisting of a few layers of small molecules which describes the rheol ogical properties of these systems both in the static, frozen, and sli ding, molten, states as well as the dynamical transition between them. Two dynamical regimes can be distinguished, according to the level of internal damping of the junction, which depends on its thickness and on the normal load. In the overdamped regime, under driving at constan t velocity v through an external spring, the motion evolves continuous ly from ''atomic stick-slip'' to modulated sliding. Underdamped system s exhibit, under given external stress, a range of dynamic bistability where the sheared static state coexists with a steadily sliding one. The frictional dynamics under shear driving is analyzed in detail, it provides a complete account of the qualitative dynamical scenarios obs erved by Israelashvili et al., and yields semiquantitative agreement w ith experimental data. A few complementary experimental tests of the m odel are suggested.