Why pens have rubbery grips

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.

Keywords: biotribology; finger friction; fingerprints; true contact area kinetics.

Fig. 1.

Temporal evolution of the contact area on a glass surface for participant A corresponding to trials a₁ and a₂ (Table 1). Each row is associated with framed enlarged image portions depicting the creation, growth, and coalescence of regions of the junction area. The last two rows show the contact evolution on a PDMS surface (Materials and Methods) under similar conditions for participant B, dataset h.

Fig. 2.

Typical evolution of the load force, $A_{\text{gross}}$, and $A_{\text{junct}}$ as a function of contact time for a glass (red) and for an elastomer (blue) counter surface. The evolution of $A_{\text{gross}}$ is independent from the material of the counter surface.

Fig. 3.

Evolution of the relative junction area $A_{\text{E}}=A_{\text{junct}}/A_{\text{gross}}$ and of the junction density $N/A_{\text{gross}}$ as a function of hold time in contact with glass for participants A and B and different loading rates and applied loads. Solid lines show first-order kinetics best fits.

Fig. 4.

(A) Contrasted kinetics of contact formation for a peak compression of 2 N showing images at the beginning and end of the subsequent hold period. (B) Time course of the evolution of friction for a finger sliding on an elastomeric surface or on a glass surface (fitted to a first-order kinetic relationship) at a velocity of 0.02 m s⁻¹ and under a load of 0.2 N. (C) Pairs of enlarged grayscale images from randomly selected trials with an elastomeric surface.

Fig. S1.

(A) Schematic diagram of the frustrated total internal reflection technique used to measure the contact area between a finger pad and a glass prism. Light rays are entirely reflected unless there is intimate contact resulting in a dark image against a light background. (B) A 3D grayscale rendering of a fingerprint image.