Frictional dynamics of finger pads are governed by four length-scales and two time-scales

The evolution of the contact area of a finger pad against a surface is critical during tactile interaction, whether for gripping or discriminating surfaces. The contact area made by a finger pad is commonly considered at two distinct length scales corresponding to the gross area, A_gross, and to the smaller ridge area, A_ridge, that excludes the interstitial spaces between the ridges. Here, these quantities were obtained from high-resolution imaging of contacts during loading and stress relaxation. While A_gross rapidly reaches an ultimate value, the contact made by the ridges is initially formed from unconnected junctions with a total contact area, A_junct, which continues to increase for several seconds during the holding period. Thus, the contact area grows in a two-step process where the number of junctions made by the ridges first increases, followed by a growth of their size and connectivity. Immediately after contact the stratum corneum is in a glassy state and the individual junctions form a multiple asperity contact. At longer contact times, the asperities soften owing to the occlusion of moisture excreted from the sweat pores in the ridges. Thus, the real area of contact, A_real, which drives the creation of friction, grows with time at a relatively slow rate. It is concluded that multi-asperity dynamic contact models should be preferred compared with static models in order to describe the physics of finger pad contact mechanics and friction.