  Dry solid friction is often accompanied by force modulations originating from
stick-slip instabilities. Here a distinct, quasi-static mechanism is evidenced
leading to quasi-periodic force oscillations during sliding contact between an
elastomer block, whose surface is patterned with parallel grooves, and finely
abraded glass slides. The dominant oscillation frequency is set by the ratio
between the sliding velocity and the period of the grooves. A mechanical model
is proposed that provides a quantitative prediction for the amplitude of the
force modulations as a function of the normal load, the period of the grooves
and the roughness characteristics of the substrate. The model's main ingredient
is the non-linearity of the friction law. Since such non-linearity is
ubiquitous for soft solids, this "fingerprint effect" should be relevant to a
large class of frictional configurations and might in particular have important
consequences in human (or humanoid) active digital touch.
