What is it about?

This paper describes how tiny amounts of slip on the rough frictional interface between rock samples control the increase in friction during periods of (nearly) no motion. This goes against the decades-old notion that increase in real contact area, and not slip, on these microscopically rough surfaces determines changes in frictional strength during periods of no slip.

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Why is it important?

The strengthening of frictional interfaces during periods of reduced motion control the dynamics of stick-slip instabilities across most materials, including geological faults and their ability to host repeated earthquakes. The most widely used constitutive equation for friction used to model systems ranging from lubricants, micromachines to earthquake nucleation considers frictional strengthening to be insensitive to slip. We not only show that this constitutive equation has little experimental support, but also show that friction evolves dominantly with slip across more than 5 orders of magnitude variations in slip rate.

Perspectives

Friction has been studied since the time of Leonardo da Vinci but yet the details of the physical processes that control friction are pretty nebulous. Since the 1990s the scientific community had pretty much converged on the paramount importance of contact area creep in determining the strengthening of frictional interfaces. Our finding that frictional strengthening is instead dominantly controlled by interfacial slip opens up a whole new bag of questions on this millenia old puzzle. I hope this encourages people, most importantly students, to look back at the physics of friction and realize that it remains an important unsolved problem with immense practical implications.

Pathikrit Bhattacharya
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This page is a summary of: The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates, Proceedings of the National Academy of Sciences, July 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2119462119.
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