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Thermal delocalization of nanosized mechanical contacts may lead to nontrivial features of atomic-scale friction. In particular, these features are specific slips, which have previously been found in atomic force experiments and are realized via a transition state, with this state being explained by the role an entropic effect in the effective interaction between bodies brought in contact. In this work, it has been shown that this effect may also lead to an atypical nonmonotonic dependence of friction force on the height of lateral potential barriers and temperature.

Atomic stick-slip patterns are readily observed in experiments. The traditional description of atomic-scale friction in terms of mechanical stick-slip instabilities (the Prandtl−Tomlinson model) appears so successful, that it obscures the actual mechanisms of energy dissipation. Here, we show that the conventional model fails completely, because it can only explain the atomic resolution of surface force maps at a level of dissipative forces that is many orders of magnitude higher than what we should expect for the slipping nano-contact. We demonstrate that we can “feel” atoms in nano-scale friction only because there is always a tiny mass that rapidly slips between atomic positions, well before the rest of the moving body follows.

One of the most surprising results of nanotribology from the physical point of view is the fact that the magnitude of the friction force in the commonly observed stick-slip regime is determined by not only the conditions of the contact between bodies under consideration, which is natural, but also and to an equal extent the stiffness of a measuring system. The nontrivial role of the stiffness in atomic friction is discussed in this work. It is shown that, under some conditions, it can play the role of a specific order parameter that determines the transitions between different friction regimes and, accordingly, different scenarios of dissipation of mechanical energy and momentum.

The progress that has been made in describing atomic friction in terms of mechanical instabilities of the stick-slip type masks the role of damping and makes researchers ignore the true mechanisms of dissipation. This study shows that there is a necessary condition for the realization of regular stick-slips occurring with atomic periodicity, which are observed in friction force microscopy (a variant of atomic force microscopy) experiments. This condition lies in the existence of a direct relation between the rate of dissipation and the characteristic frequency of a measuring system, i.e., its stiffness and mass (“critical damping”). The conclusion seems to be paradoxical; it indicates a nontrivial dissipation mechanism, which, according to the proposed explanation, essentially depends on memory effects.

An attempt is made to generalize the experience gained from solving some problems pertinent to arrangement of operations for starting a power unit.

A kinetic theory of interaction between molecules with rotational degrees of freedom and a solid surface for arbitrary ratios among the times of molecule rotation, flight through the region of surface forces, and relaxation of a molecular ensemble due to phonons has been developed. A kinetic equation for an ensemble of molecules residing in the field of surface forces has been derived from the equation for the one-particle distribution function of molecules by averaging it along the dynamical trajectories in the region of surface force action. A simple analytic expression for the probability of trapping a molecule with rotational degrees of freedom has been obtained. Experimental data on rotational cooling and rotational polarization of desorbed molecules are discussed.