Figure 9.

(a) Typical data for the time evolution of the normal and tangential forces involving a finger pad in contact with optically smooth glass, (b) associated image frames and (c) derived optical flow images. During the tangential preloading period (grey box in (a)), the tangential force increases until gross slip occurs (black arrow in (a)). Frames were selected during the preloading phase and compared with obtain the optical flow images. The apparent contact area is not correlated with the tangential force. The stick area within the contact (black areas in (c)) decreases with increasing tangential force and tends to zero with the initiation of gross slip. Adapted from André et al. [95].