Figure 1.

Typical trial. (a) Illustration of the dynamics during a normal exploration. While the finger contacts the plate, the skin expands outward, creating residual stress. This stress is released by rapid change of friction. (b) Evolution of the normal force and ultrasonic amplitude. The vibration amplitude changes when the normal force reaches 0.7 N. (c) Image of the real contact between the plate and the skin during the transition. (d) Temporal evolution of the contact area and the average skin movement. (e) Displacement fields show the expansion at a selected instant after the trigger.