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, Agross, and to the smaller ridge area, Aridge, 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 AgroSS rapidly reaches an ultimate value, the contact made by the ridges is initially formed from unconnected junctions with a total contact area, Ajunct, 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, Areal, 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.