Reverse Stick-Slip on a Periodic Wedge-Shaped Micrograting

We have investigated the time evolution of sliding friction between colloidal spheres of PMMA (with diameters of 3 μm and 9.6 μm) supported by an AFM cantilever and a SiO2 grating consisting of alternated slopes of ± 55° and flat areas, and periodicity comparable or below the sphere diameters. With the larger sphere we recognize a reproducible ‘reverse stick-slip’, which is explained by mere momentum balance consideration. The friction is proportional to the normal load, as expected for elastic contact with a wedge. With the chosen method, a friction coefficient μ ≈ 0.30 and an interfacial shear strength τ ≈ 0.7 GPa can be estimated with no need of force calibration. This response is very different from the regular stick-slip with the periodicity of the grating, which would be observed if the well-known Prandtl–Tomlinson model for the friction experienced on a periodic potential could be scaled up to the present case.