Effect of skin hydration on the dynamics of fingertip gripping contact

Abstract

The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.

Finger pad friction and its role in grip and touch

“…Figure 10a shows the best fits to the low normal force data reported by André et al [95], which demonstrates that equation (4.3) provides a first-order description of the trends for these datasets. The best-fit values of m for the data corresponding to W ¼ 0.6 N are given in table 2.…”

“…Initially at small applied tangential loads, André et al [95] observed in some cases that there was a small reduction in the gross area of contact before a slip annulus was initiated. Presumably this involves a peeling mechanism.…”

“…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].…”

“…At some critical tangential stress, Westling & Johansson [40] first reported that localized slip occurred at the periphery of the contact zone before the onset for sustained sliding. André et al [95] employed an optical technique to measure the contact region of a finger pad in the static state and with increasing tangential force for normal loads of approximately 0.5 and 5 N, which is particularly relevant to tactile exploration and grip, respectively. Some typical images are shown in figure 9.…”

Why pens have rubbery grips

“…The evolution of the apparent area of contact arising from the deformation of a finger pad has been examined using optical imaging during incipient slip (11); at different force levels (12,13); at different moisture contents (14) and tangential loads leading to slip (15); under rotation and lateral sliding movements over flat, raised, or indented glass surfaces (10); during stickto-slip transitions in distal, proximal, radial, and ulnar directions (16); over complete stick-to-slip epochs (17); and under the effects of oscillating loads (18,19).…”