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Reverse Stick‑Slip on a Periodic Wedge‑Shaped Micrograting
05.02.2019 12:00

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.

Publication

Alper Özogul, Stephan Gräf, Frank A. Müller, Enrico Gnecco: "Reverse Stick-Slip on a Periodic Wedge-Shaped Micrograting". Tribology Letters (2019) 67:29.


Fig. 1

Fig. 1: (a) Sketch (with true proportions) of the TGG1 grating and of the two PMMA spheres with diameters below (green) and above (yellow) the grating period λ used in the experiment. (b) Force diagram used to derive Eqs. (1–3) in the text.

Fig. 2

Fig. 2: Sphere-grating convoluted topography, lateral force and contact resonance frequency shift maps (top to bottom) as recorded with (a) R = 4.8µm and (b) R = 0.75µm PMMA colloidal probes CP-CONT on the SiO2 grating described in the text, frame sizes: 10µm.

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