Fig. 4: Large changes in surface topography and chemistry do not affect the frictional aging rate.
From: Predicting frictional aging from bulk relaxation measurements
a Smooth and b rough polypropylene spheres (Supplementary Fig. 4) with root-mean-square (rms) roughness heights of 0.23 and 1.81 μm, respectively, have very different real contact areas under the same normal force of 0.35 N. c These polypropylene (PP) spheres, however, show the same aging characteristics as for the pristine spheres with rms-roughness of around 0.63. The roughness values are obtained after correcting for the macroscopic spherical curvature. The solid line is, as in Fig. 3c, \(\frac{{F}_{s}}{{F}_{d}}=1+0.0417\,{{{{{\rm{ln}}}}}}\,(\frac{{t}_{w}}{1{{{{{\rm{s}}}}}}})\). The frictional aging for the polypropylene spheres on a silanized glass substrate (green squares) is also consistent with the slope \({{n}}=0.0417\) determined from the bulk stress relaxation experiments, although in this case, there is a clear vertical shift associated to decrease of the friction due to the change in surface chemistry. The dashed line is \(\frac{{F}_{s}}{{F}_{d}}=0.95+0.0417\,{{{{{\rm{ln}}}}}}\,(\frac{{t}_{w}}{1{{{{{\rm{s}}}}}}})\). d While silanization of the glass substrate does not change the frictional aging rate, it reduces the absolute dynamic friction by a factor of 3 to 4.
