Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces

Friction between solids is responsible for many phenomena such as earthquakes, wear or crack propagation(1-4). Unlike macroscopic objects, which only touch locally owing to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel-Kontorova model, which considers a monolayer of interacting particles on a periodic substrate potential(5-8). In addition to the well-known stick-slip motion, such models also predict the formation of kinks and antikinks(9-12), which greatly reduce the friction between the monolayer and the substrate. Here, we report the direct observation of kinks and antikinks in a two-dimensional colloidal crystal that is driven across different types of ordered substrate. We show that the frictional properties only depend on the number and density of such excitations, which propagate through the monolayer along the direction of the applied force. In addition, we also observe kinks on quasicrystalline surfaces, which demonstrates that they are not limited to periodic substrates but occur under more general conditions.