Lecturer: Prof. Jay Fineberg, The Racah Institute of Physics, HUJI 
Abstract:
 Rapid rupture fronts akin to earthquakes, mediate the transition to frictional motion. Once formed, their singular form dynamic  and arrest  are well-described by fracture mechanics. Ruptures, however, first need to be created within initially rough frictional interfaces.  Hence, ``static friction coefficients” are not well-defined; frictional ruptures nucleate over a wide range of applied forces. A critical open question is, therefore, how the nucleation of rupture fronts actually takes place. Here, we experimentally show that rupture fronts are prefaced by slow nucleation fronts; self-similar entities not described by fracture mechanics. They emerge from initially rough frictional interfaces at a well-defined stress threshold, evolve at characteristic velocity and time scales governed by stress levels, and propagate within a frictional interface to form the initial rupture from which fracture mechanics take over.  These results describe how friction starts and shed light on earthquake nucleation as well as processes governing material failure.