During my sabbatical year, I am a visiting associate professor in Prof. Jay Fineberg’s group at the Racah Institute of Physics of the Hebrew University in Jerusalem. This is one of the leadings groups in the world in the experimental study of friction. Together with colleagues, Prof. Fineberg has recently shown experimentally that friction is similar to the Mode II crack propagation. The idea was suggested theoretically some 20 years ago, however, how to prove it experimentally? In took very clever designed experiments with two transparent PMMA blocks, a high-speed camera (600,000 frames per second) and small strain gages to show that one-over-square-root singularity (known for crack propagation) is also found during friction. You can read about it at “Was da Vinci Wrong? New paradigm shows that friction and fracture are interrelated, say Hebrew University physics researchers” or in the paper I Svetlizky, J Fineberg, Classical shear cracks drive the onset of dry frictional motion, Nature 509 (2014), 205-208.
Interestingly, Prof. Fineberg’s research also provided experimental evidence of the so-called Adams instabilities (or Adams-Martin instabilities), discovered theoretically by Prof. George G. Adams from Northeastern University in 1995 (see, for example, “George Adams’ work leads to popular theory bearing his name”). George Adams was my PhD advisor, and my doctoral thesis was in part about the frictional Adams instabilities. The experimental evidence of the Adams instabilities is discussed in H Shlomai, J Fineberg, The structure of slip-pulses and supershear ruptures driving slip in bimaterial friction, Nature Communications (2016) 7.
According to the frictional rupture theory, the effective fracture energy Γ reflects the local strength of the interface, as determined by the normal stress σ(x), which is proportional to the real area of contact at every point. On the other hand, the shear stress τ(x) is proportional to the density of strain energy stored; therefore, τ(x)/σ(x) reflects the balance between the potential energy available before rupture, in the vicinity of each point and the energy needed to rupture the interface.
However, there are still several questions which need to be solved. Among them is what controlled the nucleation of friction, or the transition from the static to the kinetic/dynamic friction. The Linear Elastic Fracture Mechanics (LEFM) theory does not provide a simple answer to that, since it tells how a crack grows but not how the initial defect originates. Another interesting questions are phase transitions between different friction regimes associated with friction and found in the experiments.