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 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

The Perception of Ultrasonic Square Reductions of Friction With Variable Sharpness and Duration

“…Thus, for a fast finger movement, the ultrasonic stimulation started sooner after the onset of the tactile exploration. There is evidence that stable contact between the finger and the surface takes time to build [47] and that specific phenomena such as partial slip of the finger-surface contact area take place at the start of the sliding [48]. These results suggest that the onset dynamics of the sliding finger are influenced by its speed and that their characteristics for fast speed interfere with the frictional reduction induced by stepwise ultrasonic stimulation.…”

Tactile Sensors for Friction Estimation and Incipient Slip Detection—Toward Dexterous Robotic Manipulation: A Review

“…Furthermore, the friction of the fingerpad skin is affected by normal force and hydration [14,15,37,43], contact area, surface roughness, motion direction and sliding speed [44]. Additionally, for some materials the friction increases dramatically after the grasp is initiated, due to the reconfiguration of keratin molecules at the interface, meaning that an estimate of low friction made at initial contact may lead to a substantial overestimation of the grip force required a few seconds later [45]. Therefore, while sensing friction at initial contact is beneficial for securely lifting an object, it may not be appropriate for ongoing maintenance of a secure grip.…”

Physical Variables Underlying Tactile Stickiness During Fingerpad Detachment

“…We designed an experimental apparatus that can measure contact forces and finger contact area over time to test how these quantities are related to human stickiness perception (see Figure 1). We also measured the moisture of the participant's fingerpad, as moisture tends to change physical interactions between the skin and a surface (Tomlinson et al, 2011;Derler et al, 2015;Dzidek et al, 2017;Gueorguiev et al, 2017). Moreover, we took the comfort of the participants into account by not attaching any fixtures to the finger and by making the finger-glass interaction direction downward.…”