The root-mean-square roughness Rq was measured using the Hommel-Etamic T8000 Nanoscan scanning profilometer: Rq of the steel disc was 0.17 μm and Rq of the rubber balls was 1.30 μm. In
the latter case, the roughness was measured on the mould used for producing the balls, as the rubber is too compliant for stylus profilometry.