Stick-slip motion in spite of a slippery contact: Do we get what we see in atomic friction?

Shortly after the invention of the atomic force microscope (AFM) this instrument has been applied [1 ]a s a friction force microscope (FFM). This seemingly trivial extension of the AFM to the lateral direction has enjoyed great interest [2], since it is believed to provide direct atomic-scale access to the phenomenon of friction. Experiments with atomic resolution typically show periodic, sawtoothlike behavior of the lateral force known as stick-slip. The FFM tip is thought to be held periodically in lattice positions of the surface, which is easily modeled using a quasistatic approach, first proposed by Prandtl [3] and often referred to as the Tomlinson model [4 –6]. An object (the tip) is considered to move in a periodic potential field formed by the substrate lattice, while being dragged along the surface by an external spring (the cantilever), which is at the same time used to measure the lateral force experienced. Generalizations of the model, from dynamical modeling to nonequilibrium statistical mechanics [7], have advanced our understanding of atomic-scale friction, e.g., its velocity dependence, transitions from stick-slip to other regimes, and the role of thermal effects [4,8–10].