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Why pens have rubbery grips

  1. Michael J. Adamsa,1
  1. aSchool of Engineering, University of Birmingham, Birmingham B15 2TT, United Kingdom;
  2. bSorbonne Universités, Université Pierre et Marie Curie Univ Paris 06, Institut des Systèmes Intelligents et de Robotique, F-75005 Paris, France;
  3. cUnilever Research & Development Port Sunlight, Bebington, Wirral CH63 3JW, United Kingdom
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved August 22, 2017 (received for review April 14, 2017)

Significance

Why does gripping a pen, tool, or handle feel more secure when it is coated with a rubbery material? The keratin of the skin outer layer is stiff and rough at a small scale. When encountering a smooth, stiff, and impermeable surface, such as polished metal or glass, the actual contact area is initially small as is the friction. Because the keratin softens when it is hydrated by the moisture secreted from the sweat pores, it requires many seconds for the contact area to increase to the value reached almost instantaneously with a soft material, such as a rubber. This mechanism might be used by our tactile sense to identify materials and has implications for the design of tactile displays.

Abstract

The process by which human fingers gives rise to stable contacts with smooth, hard objects is surprisingly slow. Using high-resolution 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.

Footnotes

  • ?1To whom correspondence should be addressed. Email: m.j.adams{at}bham.ac.uk.
  • Author contributions: B.D., S.B., S.A.J., V.H., and M.J.A. designed research; B.D., S.B., S.A.J., V.H., and M.J.A. performed research; B.D., S.B., S.A.J., and M.J.A. analyzed data; B.D. and S.B. conducted the experiments and analyzed the results; B.D., S.B., S.A.J., V.H., and M.J.A. discussed the results; and B.D., S.B., S.A.J., V.H., and M.J.A. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1706233114/-/DCSupplemental.

Freely available online through the PNAS open access option.

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