The dynamics of frictional motion have been studied for hundreds of years, yet many aspects of these important processes are not understood. First described by Coulomb and Amontons as the transition from static to dynamic friction, the onset of frictional motion is central to fields as diverse as physics, tribology, mechanics of earthquakes, and fracture. We review recent studies in which fast (real-time) visualization of the true contact area along a rough spatially extended interface separating two blocks of like material has revealed the detailed dynamics of how this transition takes place. The onset of motion is preceded by a discrete sequence of rapid cracklike precursors, which are initiated at shear levels that are well below the threshold for static friction. These precursors systematically increase in spatial extent with the applied shear force and leave in their wake a significant redistribution of the true contact area. Their cumulative effect is such that, just prior to overall sliding of the blocks, a highly inhomogeneous contact profile is established along the interface. At the transition to overall motion, these precursor cracks trigger both slow propagation modes and modes that travel faster than the shear wave speed. Overall frictional motion takes place only when either the slow propagation modes or additional shear cracks excited by these slow modes traverse the entire interface. Surprisingly, in the resulting stick–slip motion, the surface contact profile retains the profile built up prior to the first slipping event. These results suggest a fracture-based mechanism for stick–slip motion that is qualitatively different from other descriptions.
Shmuel M. Rubinstein can be reached at the Racah Institute of Physics, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel; tel. 972-2-6584330; fax 972-2-6584437; and e-mail [email protected]
Rubinstein is nearing completion of his PhD degree studies at the Racah Institute of Physics of the Hebrew University of Jerusalem. He received his BS and MS degrees from the Hebrew University of Jerusalem. The recipient of the Charles Clore fellowship in 2007, Rubinstein's interests include the dynamics of friction, fracture, and nonequilibrium electro-osmotic instabilities.
Gil Cohen can be reached at the Racah Institute of Physics, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel; tel. 972-2-6585720; fax 972-2-6584437; and e-mail [email protected]
Cohen has been a laboratory researcher at the Hebrew University of Jerusalem since 2003. He received his PhD degree from the Hebrew University of Jerusalem in 2000. His research interests include friction, fracture, nonlinear phenomena, and biophysics.
Jay Fineberg can be reached at the Racah Institute of Physics, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel; tel. 972-2-6585207; fax 972-2-6584437; and e-mail [email protected]
Fineberg has been a faculty member at the Hebrew University of Jerusalem since 1992 and holds the Max Born Chair in Natural Philosophy. He received his PhD degree from the Weizmann Institute of Science in 1988. His interests include fracture, the dynamics of friction, nonlinear wave interactions, and nonlinear phenomena. The recipient of a number of prestigious awards, Fineberg also has authored numerous scientific papers.