A continuous measure of fingertip friction during precision grip

Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is therefore important to understand how hand moisture varies between people, during different activities and the effect of this on friction.In this study a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture.Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration.

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“…As noted earlier, this effect has been observed by other authors [8,9]. Other authors, however, did not note a bell curve [10][11][12].…”

Section: Finger Friction Response With Moisturesupporting

confidence: 72%

“…There is an initial increase in friction as moisture levels rise, before a threshold is reached and the levels reduce. This has been described as a bell-curve response and has been reported by a number of authors [8,9].…”

Section: Introductionmentioning

confidence: 65%

Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

et al. 2010

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“…For example, André et al [67] observed that this was the case for normal loads of less than 3.5 N. It is possible to account for such observations by considering equations (2.1), (2.2) and (2.4), which provide a relationship between the frictional and normal forces:…”

Section: Rsifroyalsocietypublishingorg J R Soc Interface 10: 20120467mentioning

confidence: 99%

“…More systematic studies were carried out by André et al [67], who reported values of the coefficient of friction measured during grip studies as a function of the moisture content at the skin surface of a finger pad using a device that had been developed previously [79]. The subjects exhibited moisture levels that ranged from dry to very wet and it was found that there was a maximum value of the coefficient of friction at intermediate moisture levels.…”

Section: Occlusionmentioning

confidence: 99%

Finger pad friction and its role in grip and touch

Adams

Johnson

Lefèvre

et al. 2013

J. R. Soc. Interface.

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228

201

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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.

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“…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).…”

mentioning

confidence: 99%

Why pens have rubbery grips

Dzidek

Bochereau

Johnson

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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The process by which human fingers gives rise to stable contacts with smooth, hard objects is surprisingly slow. Using highresolution imaging, we found that, when pressed against glass, the actual contact made by finger pad ridges evolved over time following a first-order kinetics relationship. This evolution was the result of a two-stage coalescence process of microscopic junctions made between the keratin of the stratum corneum of the skin and the glass surface. This process was driven by the secretion of moisture from the sweat glands, since increased hydration in stratum corneum causes it to become softer. Saturation was typically reached within 20 s of loading the contact, regardless of the initial moisture state of the finger and of the normal force applied. Hence, the gross contact area, frequently used as a benchmark quantity in grip and perceptual studies, is a poor reflection of the actual contact mechanics that take place between human fingers and smooth, impermeable surfaces. In contrast, the formation of a steady-state contact area is almost instantaneous if the counter surface is soft relative to keratin in a dry state. It is for this reason that elastomers are commonly used to coat grip surfaces.finger friction | true contact area kinetics | biotribology | fingerprints

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A continuous measure of fingertip friction during precision grip

Cited by 66 publications

References 23 publications

Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

Finger pad friction and its role in grip and touch

Why pens have rubbery grips

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