Finger pad friction and its role in grip and touch

Abstract

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.

Figure 1.

A microstructural cross-section of a human finger pad (middle finger) obtained using Optical Coherence Tomography (Spectral Radar OCT – OCP930SR, ex Thorlabs). Typical fingerprint height and spacing are shown with four (spiral) sweat ducts (filled circles) and a drop of sweat (open circle) emerging from a pore on a ridge surface. The sweat ducts are visible within the stratum corneum layer which is relatively dark. The thickness of the stratum corneum is estimated to be approximately 300 µm. This is based on the difference of 400 µm in the optical depth between the light skin surface and the upper boundary of the intermediate light layer, which is the remainder of the epidermis [48,49], and using an assumed value of the refractive index for skin of 1.4. The inset photograph shows the same skin region from above, bounded top and bottom by the edges of the OCT contact probe. (Online version in colour.)

Figure 2.

The relationship between (a) the contact area and (b) the dynamic frictional force of individual finger pads as a function of the applied normal force, for three different fingers held flat against an acrylic sheet. Index finger, filled black squares; middle finger, filled grey circles and thumb, unfilled black squares. The lines in (a) and (b) are the best fits to equations (2.3) and (2.6), respectively. Index finger, black line; middle finger, grey line; and thumb, dashed line. The data are taken from fig. 2 in the study of Warman & Ennos [73].

Figure 3.

(a) Moisture levels and (b) static grip forces for the finger pad of a single subject as a function of sequential grip and release procedures on a manipulandum defined in terms of a trial number. The points are the mean values obtained from 20 blocks of trials, and the associated bars represent±1 s.d. The subject (S2) was judged to have dry skin and the data demonstrate that the sequential contact in this case results in an increase in the moisture level with a corresponding reduction in the grip force required to stabilize the position of the manipulandum. Adapted from André et al. [80].

Figure 4.

The static grip force as a function of moisture level for the finger pads of eight subjects each involving 20 blocks of 25 trials of the type described in figure 3, which is for one of the subjects. The line is the best fit to the data and exhibits a minimum at approximately 7.75 arbitrary units corresponding to the optimal moisture level for grip. Adapted from André et al. [80].

Figure 5.

The dynamic coefficient of friction (W = 0.2 N) as a function of the dynamic occlusion time corresponding to sliding speeds of (a) V = 6 mm s⁻¹ and (b) V = 24 mm s⁻¹ for glass, (c) V = 6 mm s⁻¹ and (d) V = 24 mm s⁻¹ for PP. Best-fit curves to equation (3.1) are shown. Adapted from Pasumarty et al. [62].

Figure 6.

Tangential force data for a dry finger pad sliding against optically smooth glass as a function of the dynamic occlusion time, followed by the addition of water in the contact (W = 0.2 N and V = 6 mm s⁻¹ ). Adapted from Pasumarty et al. [62].

Figure 7.

Typical dynamic coefficient of friction data for a finger pad sliding on news print as a function of time; the line is the best fit to equation (3.1). The data are taken from fig. 2 in the study of Skedung et al. [90].

Figure 8.

The dynamic coefficient of friction for different types of paper as a function of surface roughness. Uncoated: unfilled black triangles; coated mechanical: filled grey squares; and woodfree-coated: filled black circles. The data are taken from fig. 6 in the study of Skedung et al. [90].

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].

Figure 10.

The stick ratio as a function of the tangential force for two subjects with very different hydration levels (filled circles, GC wettest; unfilled diamonds, AG driest) and for two target normal forces: (a) 0.5 N and (b) 5 N. Each point corresponds to the mean of five trials for a given subject and normal force. The dashed and full lines are best fits to equations (4.3) and (4.4) and equations (4.7) and (4.8) for (a) and (b), respectively, using the values of the parameters given in table 2. The data are taken from fig. 6 in the study of André et al. [95]. (Online version in colour.)

Figure 11.

The dynamic coefficient of friction (W = 0.2 N) for a finger pad as a function of sliding velocity for (a) an optically smooth glass surface, and (b) a smooth PP surface. The steady-state dry values of the coefficient of friction, μ_∞: unfilled circles and the wet values of the coefficient of friction, μ_w: filled circles. The lines are the best fits to equation (5.4) using the parameter values given in table 3. The data are taken from figs 18 and 19 in the study of Pasumarty et al. [62]. (Online version in colour.)