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Friction, noise, rough surfaces and statistics

Alain Le Bot Laboratoire de tribologie et dynamique des systèmes, UMR CNRS 5513, Ecole centrale de Lyon, France.

The friction noise emitted when rubbing hands or small objects is a wide band noise generated by impacts between antagonist asperities of rough surfaces. Since microscopic shocks are random events, this is a problem of statistical mechanics. In this study, we propose to analyze the problem in terms of (vibrational) energy balance. A direct numerical simulation which consists in solving the elasto-dynamics equations with contact delivers a crucial information on the rate and strength of impacts. While experiments gives some empirical laws such as acoustical power versus sliding speed and roughness. We present an experiment which investigates the dependence of sound with the nominal contact area. It is found that in some cases, the acoustical power is proportional to the contact area while in some others, the sound is constant. This paradoxical result is explained by introducing a dissipation law of vibration at the interface of solids. In the regime where this dissipation process dominates, the sound is found to be constant.