Urban myth disproved: Fingerprints do not improve grip friction

Fingerprints do not increase friction

Fingerprints mark us out as individuals and leave telltale signs of our presence on every object that we touch, but what are fingerprints really for? According to Roland Ennos, from the University of Manchester, other primates and tree-climbing koalas have fingerprints and some South American monkeys have ridged pads on their tree-gripping tails, so everyone presumed that fingerprints are there to help us hang onto objects that we grasp. This theory that fingerprints increase friction between the skin and whatever we grab onto has been around for over 100 years, but no one had directly tested the idea. Having already figured out why we have fingernails, Ennos was keen to find out whether fingerprints improve our grip, so he recruited Manchester undergraduate Peter Warman to test out fingerprint friction and publishes his results on June 12 2009 in the Journal of Experimental Biology at http://jeb.biologists.org.

Because the friction between two solid materials is usually related to the force of one of the materials pressing against the other, Ennos and Warman had to find a way of pushing a piece of acrylic glass (Perspex®) against Warman's finger before pulling the Perspex® along the student's finger to measure the amount of friction between the two. Ennos designed a system that could produce forces ranging from a gentle touch to a tight grip, and then Warman strapped his index finger into the machine to begin measuring his fingerprint's friction.

But after days of dragging the Perspex® along Warman's fingers and thumbs, it was clear that something wasn't quite right. Instead of the friction between each finger and the Perspex® increasing in proportion to the amount that the Perspex® pushed against Warman's fingers, it increased by a smaller fraction than Ennos had expected. Ennos realised that instead of behaving like a normal solid, the skin was behaving like rubber, where the friction is proportional to the contact area between the two surfaces.

To check that skin behaves more like rubber than a normal solid, the duo varied the area of each fingerpad that came into contact with the surface by dragging narrow and wide strips of Perspex® along Warman's fingerpads. They found that the friction did increase as more of the fingerprint came in contact with the surface, so the skin was behaving just like rubber.