3. Elementary friction processes

Friction is not a fundamental force: it is the result of several phenomena. Understanding the fundamental mechanisms of friction is difficult, because they depend on the properties of materials, both in volume and surface, and on the environment of a given system, but also on the spatial and time scales probed by shear. A few elements will give an idea of the complexity of the subject:

• dry friction is generally different from lubricated friction;

• Friction due to the propagation of vibrations (phonons) in materials and electronic friction in the case of metal surfaces can be significant;

• Surface roughness and chemical inhomogeneity often play a major role in the friction of macroscopic objects;

• adhesion between surfaces and the elastic properties...