Finger pad friction and its role in grip and touch.

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick-slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function.

A continuous measure of fingertip friction during precision grip.

When humans manipulate an object, the minimal grip force (GF) required to avoid slipping depends on the frictional properties between the fingers and the object. As a consequence, fingertip skin friction plays a critical role during object manipulation. Here, the effects of the normal force and moisture content on the skin's static coefficient of friction (CF) for human fingertips were studied. Ten subjects were asked to pinch an object with a given normal force. Slippage of the object on the fingertips was generated for different ranges of normal force using a linear translation stage. The exerted forces and moisture of the fingertips were then measured, and the static coefficient of friction was calculated as the ratio between the tangential force and normal force at slippage. These results demonstrate that the effects of the normal force and moisture content on the CF exhibit a complex interaction. For a given moisture condition, the CF varies as a power function of the normal force; in contrast, for a given normal force, the CF is described by a "bell-shaped" function of moisture. A global expression of the CF as a function of the normal force and moisture content is derived, and a method is proposed for a continuous measure of the CF. This new method shall be of particular interest in investigating dexterous manipulation.