An earthquake may be regarded as the abrupt slip of a fault and therefore it is important for seismologists to understand the friction law that governs the fault motion. However, we cannot do experiments using faults, which are generally larger than kilometer scale. Therefore, we have to somehow extrapolate the results of laboratory experiments to the fault scale. To this end, a theoretical consideration is vital because the extrapolation cannot be verified without considering the scale dependence of underlying physical processes. Here we review a recent understanding of such mechanisms behind some standard friction laws known in the field of earthquake physics and discuss its scale dependence.
When soft and sticky materials are detached from or slid against a rigid substrate, specific types of meso-scale dynamics accompanying large deformation and pattern formation are observed. In this paper, we introduce our studies on the in-situ visualization of cavitation during debonding of soft adhesives and also of the Schallamach waves in sliding friction of adhesive gel-sheets.
We developed a new apparatus using the probe-tip-quartz-crystal resonator technique, and measured the dynamical frictional force acting on HOPG and C60 substrates by a sliding Si3N4 tip as a function of sliding distance. It was found that the dynamical frictional force undergoes a drastic change when the oscillation amplitude is approximately the lattice constant of each substrate: For a small case, it is directly proportional to the amplitude, while for a large case, it does not depend on the amplitude. The observed behavior is qualitatively understood by a simple one-dimensional Tomlinson model. Then, we can conclude that the principal mechanism of energy dissipation due to the frictional force does not depend on sliding distance.
For the better understanding of friction mechanism, it is important to study real contact points at the frictional interface. For this purpose, we originally built an experimental setup with micro electric mechanical system (MEMS) probes and a transmission electron microscope (TEM). Thanks to this setup, nanoscaled real contact points were formed and separated under in-situ TEM observation. The formation and the separation processes of real contact points were well visualized and those of silicon and gold real contact points were compared at the nanoscale.
We report the atomic-scale peeling of a single-layer graphene on a graphite substrate, in which stick-slip sliding of the single-layer graphene occurs at the atomic scale while maintaining AB-stacking registry with the graphite substrate. The amplitude of the peeling force depends on the lattice orientation of the surface, which is affected by the sliding force at the interface between the graphene and graphite surfaces. This study of peeling at the atomic scale will clarify the relationship among peeling, friction, adhesion, and superlubricity.