Figure 9.

Schematic multi-scale two-dimensional representation of the sliding of the spherical probe on the skin surface. The cutaneous surface is not smooth, but has a specific relief. The skin relief can be observed at two different scales: the macroscopic scale, where the skin is composed of cutaneous plates and primary lines (or wrinkles) which separate the cutaneous plates; and the microscopic scale, which corresponds to the skin micro-relief and is composed of the secondary lines on the cutaneous plates. Like skin, in the microscopic scale, the spherical probe is not smooth. The probe roughness has been assimilated to spherical roughness for the model. The parameters d and d₀ are the skin micro-relief parameters. d is the critical vertical displacement to initiate sliding and d₀ is the lateral critical distance to initiate sliding [33]. The spherical geometry of the probe roughness has been chosen arbitrarily.