When you grip something — a pen, a tool or even the handlebars of your bike — it immediately feels more secure if the handle is made of something rubbery. But why are our hands not as good at gripping stiffer materials? As Brygida Dzidek and co-workers have now shown, the answer lies in the composition of our finger pads (Proc. Natl Acad. Sci. USA 114, 10864–10869; 2017).
Dzidek et al. pressed a prism onto the fingertips of a number of individuals, and then tracked the evolution of their fingerprints on the surface. Across variable experimental conditions, what emerged was that the contact area between fingers and surface is initially extremely small, but then grows over time. For rubbery surfaces, the maximum contact surface was reached extremely quickly, whereas for stiff materials like glass, saturation of the contact surface took as long as 20–30 seconds.
Looking at the fingerprints left on glass, at first they resemble a scattered collection of marks left by the ridges that we have on our finger pads — often by the bigger ones that are found right in the middle of the fingertip (pictured). The marks left later are influenced by the plasticity of our superficial skin layer, the stratum corneum, which is largely composed of keratin — a stiff material in dry conditions. As our fingertips sweat, moisture softens the keratin, which in turn leads to an increase of the contact area between fingertip and surface. This happens gradually over the time it takes for sweat to diffuse in the skin layer. Conversely, when we touch a surface that is softer than keratin, there is no need to wait for the emollient effect of sweat — hence the immediate firm grip we have on rubber.
As a next step, the researchers studied the evolution of the surface friction over time. Despite the fact that plasticized keratin loses interfacial shear strength, leading to a potential decrease in friction, the substantial increase of the surface contact area more than compensated for this loss. As a result, the friction between our fingers and a glassy surface was found to double as our skin gets sweaty. The fact that this might come as a surprise is a testament to the ability of our body to adapt our behaviour in light of vastly changing microscopic conditions.