Finger pad friction and its role in grip and touch

Many aspects of both grip function and tactile perception depend on complexfrictional interactions occurring in the contact zone of the finger pad,which is the subject of the current review. While it is well established thatfriction plays a crucial role in grip function, its exact contribution for discriminatorytouch involving the sliding of a finger pad is more elusive. Fortexture discrimination, it is clear that vibrotaction plays an important rolein the discriminatory mechanisms. Among other factors, friction impactsthe nature of the vibrations generated by the relative movement of the fingertipskin against a probed object. Friction also has a major influence on theperceived tactile pleasantness of a surface. The contact mechanics of afinger pad is governed by the fingerprint ridges and the sweat that isexuded from pores located on these ridges. Counterintuitively, the coefficientof friction can increase by an order of magnitude in a period of tensof seconds when in contact with an impermeably smooth surface, suchas glass. In contrast, the value will decrease for a porous surface, such aspaper. The increase in friction is attributed to an occlusion mechanismand can be described by first-order kinetics. Surprisingly, the sensitivity ofthe coefficient of friction to the normal load and sliding velocity is comparativelyof second order, yet these dependencies provide the main basis oftheoretical models which, to-date, largely ignore the time evolution ofthe frictional dynamics. One well-known effect on taction is the possibilityof inducing stick–slip if the friction decreases with increasing sliding velocity.Moreover, the initial slip of a finger pad occurs by the propagationof an annulus of failure from the perimeter of the contact zone and thisphenomenon could be important in tactile perception and grip function