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Friction Velocity Dependence – Breaking Friction Laws
Controlling friction is one of the main goals in modern tribology. Due to complexity of the tribological processes, up to date the problem remains mainly unsolved. While classical tribology relies on the famous friction laws, developments in nanotribology made it evident that these laws are not always obeyed. For example, in contrast to the Coulomb’s Law of Friction, kinetic friction coefficient at the atomic level depends on sliding speed.
Researchers from Massachusetts Institute of Technology, USA and Seoul National University, Korea, reported results of experimental measurements of the friction variation as a function of speed across five orders of magnitude in velocity. The friction interfaces consisted of an electrically trapped ion sliding in a sinusoidal potential. The experimental set up developed by the researchers allowed to study this idealized friction system and gain further fundamental understanding of the frictional processes.
Four regimes were observed in the experiments, as shown in figure above. In the first regime, thermal drift, the friction is small and velocity independent. This is due to the dominance of thermal hopping (motion through potential well barriers due to thermal fluctuations), e.g., slow motion in thermal equilibrium with almost vanished stick-slip. Thermal activation regime is characterized by logarithmic increase in the friction. In this regime, the stick-slip is only partially suppressed by thermal fluctuations and the friction gets larger. With further increase of the sliding speed, the thermal fluctuations are negligible and friction plateau regime is observed. Friction gets its maximum in this regime. With even further increase in velocity, the weakening regime is encountered and friction decreases with the sliding speed. This happens due to increase in the ion’s kinetic energy during the slip events between potential wells. Increased kinetic energy makes the friction reduced.
It was also shown that the existing analytical models describing velocity dependent friction are in a good agreement with the experimental data. The results obtained in the research and the level of control of the friction experiment reached by the researchers enables further fundamental understanding of friction, possibly, going into quantum regime.
The details of the research can be found in the original article: doi:10.1038/nphys3459, Velocity tuning of friction with two trapped atoms, Dorian Gangloff, Alexei Bylinskii, Ian Counts, Wonho Jhe & Vladan Vuletić.