Shmuel M. Rubinstein, Gil Cohen
and Jay Fineberg
The dynamics of friction have been studied for hundreds
of years, yet many aspects of these everyday processes are not understood. One
such aspect is the onset of frictional motion (slip). First described more than
200 years ago as the transition from static to dynamic friction, the onset of
slip is central to fields as diverse as physics1, 2, 3, tribology4,
5, mechanics of earthquakes6, 7, 8, 9, 10, 11 and fracture12,
13, 14. Here we show that the onset of frictional slip is governed by
three different types of coherent crack-like fronts: these are observed by
real-time visualization of the net contact area that forms the interface
separating two blocks of like material. Two of these fronts, which propagate at
subsonic and intersonic velocities, have been the
subject of intensive recent interest12, 13, 14, 15, 16, 17. We show
that a third type of front, which propagates an order of magnitude more slowly,
is the dominant mechanism for the rupture of the interface. No overall motion
(sliding) of the blocks occurs until either of the slower two fronts traverses
the entire interface.