Friction

Granular friction: We experimentally investigated the response of a sheared granular medium in a Couette geometry. The apparatus exhibits the expected stick-slip motion and we probe it in the very intermittent regime resulting from low driving. Statistical analysis of the dynamic fluctuations reveals notable regularities. We observe a possible stability property for the torque distribution, reminiscent of the stability of Gaussian independent variables. In this case, however, the variables are correlated and the distribution is skewed. Moreover, the whole dynamical intermittent regime can be described with a simple stochastic model, finding good quantitative agreement with the experimental data. Interestingly, a similar model has been previously introduced in the study of magnetic domain wall motion, a source of Barkhausen noise. Our study suggests interesting connections between different complex phenomena and reveals some unexpected features that remain to be explained.

Vibration induced slip: Mechanical vibrations are known to affect frictional sliding and the associated stick-slip patterns causing sometimes a drastic reduction of the friction force. This issue is relevant for applications in nanotribology and to understand earthquake triggering by small dynamic perturbations. We studied the dynamics of repulsive particles confined between a horizontally driven top plate and a vertically oscillating bottom plate. Our numerical results show a suppression of the high dissipative stick-slip regime in a well-defined range of frequencies that depends on the vibrating amplitude, the normal applied load, the system inertia and the damping constant. We proposed a theoretical explanation of the numerical results and derive a phase diagram indicating the region of parameter space where friction is suppressed. Our results allow to define better strategies for the mechanical control of friction.

Frictional creep: Understanding the microscopic mechanisms that govern friction represents a fundamental scientific problem with important practical applications. According to the macroscopic description dating back to Amontons and Coulomb, two bodies in contact under a normal force start to slide when subject to a lateral force exceeding the static friction force , while sliding motion can be sustained under a dynamic friction force. The transition from static to dynamic friction is not completely well defined, because even when the lateral force is below the nominal static friction, a body can slowly creep forward due to thermal activation. In addition, direct visualization of the contact area at the onset of slip and numerical simulations  indicate the formation and propagation of detachment fronts fracturing the multicontact interface. These findings suggest that the onset of slip is due to microscopic processes, ultimately due to the interactions between individual atoms lying on the surfaces in contact, propagating up to the macroscale to yield collective sliding. We have analyzed the onset of slip of a xenon (Xe) monolayer sliding on a copper (Cu) substrate. We consider thermal-activated creep under a small external lateral force, and observe that slip proceeds by the nucleation and growth of domains in the commensurate interface between the film and the substrate. We measured the activation energy for the nucleation process considering its dependence on the external force, the substrate corrugation, and particle interactions in the film. To understand the results, we use the classical theory of nucleation and compute analytically the activation energy which turns out to be in excellent agreement with numerical results. Our results are relevant to understand experiments on the sliding of adsorbed monolayers.