Phononic Doppler Effect in Sliding Friction

The Doppler effect plays a fundamental role in a vast array of applications, spanning from the quantum domain to cosmic scales. Nevertheless, its manifestation in the context of phonon excitation during the friction between two objects with relative motion remains unexplored. In this study, we predict and investigate the occurrence of the phononic Doppler effect within sliding friction, employing a phononic dynamics model derived through the atomistic Green's function method. As the friction-excited phonons propagate both forward and backward relative to the moving object, we predict frequency high-shift and low-shift phenomena, respectively. We propose a phonon excitation rule, grounded in the phonon dispersion relation, to elucidate the frequency-shift behavior, essentially capturing the Doppler effect on friction-excited phonons. The predicted frequency shift, as determined by the phonon excitation rule, finds validation through molecular dynamics simulations. The observation of the phononic Doppler effect in sliding friction opens a range of potential applications, providing innovative tools for detecting energy-dissipation mechanisms at interfaces caused by friction.