The role of the skin on our fingertips, palm and soles of the feet is to grip other objects, and they all have characteristic “friction” ridges. Nevertheless, very little research has been carried out about how well fingers perform, how friction is achieved and why we have soft fingerpads with fingerprints at all. Recent research carried out in the laboratory of the University of Manchester suggests that finger skin has frictional properties rather like rubber. The friction force rises with contact area rather than normal force. Therefore it appears strange that we have fingerprints at all since these reduce contact area.
Why do we have fingerprints? Possible answers are:
1) Fingerprints may increase friction on rough materials
2) Fingerprints may increase friction on wet surfaces by channeling away water like a car tyre;
3) Fingerprints may help prevent blistering by allowing shear of the skin.
Dr. Roland Ennos started his project to test these hypotheses by carrying out friction tests on surfaces of contrasting roughness and wetness, using a mechanical testing machine, and investigate the strains of the finger skin using optical methods available in the School of Materials. The first results have disproved the theory that fingerprints improve grip by increasing friction between people's hands and the surface they are holding.
WHY OUR FINGERPRINTS DO NOT PROVIDE GRIP!
Dr. Roland Ennos' research indicates that our skin behaves like rubber:
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.
In order to check study skin behavior, in the experiments the researchers 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.
Finally, the friction issue was clinched when Warman measured his fingerprints' surface area. The area of skin in contact with the Perspex was always 33% less than if the fingerpads were smooth resulting in the maximum contact area. Fingerprints definitely don't improve a grip's friction because they reduce our skin's contact with objects that we hold, and even seem to loosen our grip in some circumstances.
THEN... WHY DO WE HAVE FINGERPRINTS?
So if fingerprints don't tighten our grasp on smooth surfaces, what are they for?
Ennos explains that our fingerprints might have evolved to grip onto rough surfaces, like tree bark; the ridges may allow our skin to stretch and deform more easily, protecting it from damage; or they may allow water trapped between our finger pads and the surface to drain away and improve surface contact in wet conditions. Other researchers have suggested that the ridges could increase our fingerpads' touch sensitivity.
For example, Dr Jon Barnes, a biomechanics expert at the University of Glasgow, has carried out research into the adhesive properties of tree frogs' feet. He said other work, using artificial surfaces, had produced similar results to those in Dr Ennos' work.
And Loesch et al. (1990) found that the the size and intensity of fingerprint patterns correlates with complex precision grip tests involving complicated manipulation of objects.
Whatever our fingerprints are for, it seems that the idea that they provide friction for grip is just another urban myth. In the near future Ennos and his team will also test if and how fingerprints prevent blisters.
Sources: Manchester.ac.uk, Physorg.com, and Bbc.co.uk
COPYRIGHT © 2002-2013: Martijn van Mensvoort (Dutch version: Hand Analyse)