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

In this Letter we investigate the response of an embedded system that is subject to an external drive, which is chosen to be either a constant velocity or a harmonic shear. Two approaches are introduced that cover macroscopic and microscopic aspects of the problem and mimic recent measurements on friction using the surface forces apparatus. The first approach is based on the Tomlinson model and its generalization, and allows for the investigation of the coupled lateral–normal response to a lateral drive. We propose a method for analyzing both linear and nonlinear response of confined systems driven harmonically. The method provides a way to deduce the microscopic parameters responsible for dissipation. The second approach focuses on the microscopic level. The shear is shown to excite “shearons”, which are collective modes of the embedded system with well-defined spatial and temporal patterns that dominate the frictional properties of the driven system. We demonstrate that the slip relaxation in stick–slip motion and memory effects are well described in terms of the creation and/or annihilation of shearons.