Macroscopic versus microscopic description of friction: from Tomlinson model to shearons

In this Letter we investigate the response of an embedded system that is su bject to an external drive, which is chosen to be either a constant velocit y or a harmonic shear. Two approaches are introduced that cover macroscopic and microscopic aspects of the problem and mimic recent measurements on fr iction using the surface forces apparatus. The first approach is based on t he Tomlinson model and its generalization, and allows for the investigation of the coupled lateral-normal response to a lateral drive. We propose a me thod for analyzing both linens and nonlinear response of confined systems d riven harmonically. The method provides a way to deduce the microscopic par ameters responsible for dissipation. The second approach focuses on the mic roscopic level. The shear is shown to excite "shearons", which are collecti ve modes of the embedded system with well-defined spatial and temporal patt erns that dominate the frictional properties of the driven system. We demon strate that the slip relaxation in stick-slip motion and memory effects are well described in terms of the creation and/or annihilation of shearons.