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The transition from atomic-scale stick-slip to continuous sliding with ultralow friction predicted by the Prandtl-Tomlinson model has been observed with a sharp Atomic Force Microscope tip slowly scanned in contact with a cleaved NaCl crystal in UHV [1]. The extracted low contact stiffness suggests that atomic-scale stick-slip mainly involves elastic energy stored and released by a few atoms around the tip apex (nanotip). A related transition has recently been obtained upon increasing excitation of a flexural mode of the AFM cantilever [2]. In contrast to the previously observed or predicted reduction of friction in the presence of perpendicular oscillations [3], the effect is not associated with a time-average increase of the tip-sample distance. The observed dependence of the friction force on the excitation amplitude could be explained in terms of an oscillating corrugation potential experienced by the nanotip. More precisely, if the actuation frequency is much smaller than the natural frequency and damping rate of the nanotip, but much larger than the scanning frequency (velocity/lattice spacing), the computed friction values nearly collapse on a universal curve when plotted against a single parameter extracted from the measurements [2]. Very recent simulations [4] reveal that deviations from adiabatic following cause a residual viscous-like friction which is independent of damping and a concomitant energy loss in the range where friction suppression was predicted earlier [2]. Remarkably, the total loss, including the contribution from the periodic actuation, can be considerably smaller than in the absence of the latter. In the same range, thermal activation can introduce strong fluctuations in the lateral force, but has little effect on the average residual friction. Simple physically motivated approximations can explain these a priori surprising results.
[1] A. Socoliuc, R. Bennewitz, E. Gnecco, E. Meyer, Phys. Rev. Lett. 92, 134401 (2004)
[2] A. Socoliuc, E. Gnecco, S. Maier, O. Pfeiffer, A. Baratoff, R. Bennewitz, E. Meyer, Science 313, 307 (2006)
[3] M. Urbakh, J. Klafter, D. Gourdon, J. Israelachvili, Nature 430, 525 (2004)
[4] S. Maier, PhD Thesis, University of Basel (2007) |