What are fingerprints good for, besides aiding police investigations? That’s the question that biomechanics researcher Roland Ennos recently set out to answer. This notion that human fingerprints (and presumably footprints) evolved because they act like tire or boot tread–increasing the friction against a smooth surface so we don’t slip or drop stuff–is a 100-year-old urban myth that, apparently, had never been put to the test [NPR].
To test the impact of fingerprints, Ennos rigged a machine that measured the amount of friction generated by a fingertip (belonging to study coauthor Peter Warman) when it was pressed against a piece of acrylic glass. Warman gradually increased the pressure, going from a light touch on the glass to a tight grip, but the corresponding friction didn’t increase as much as the researchers expected. Soon they realised that the skin was not behaving like a normal solid, where friction is proportional to the strength of the contact. Instead, it was behaving like rubber, where the friction is proportional to the contact area between the two surfaces [BBC News].
Explains Ennos: “Because there are all the gaps between the fingerprints, what they do is reduce the contact area with the surface.” … Measurements taken from ink marks then revealed that fingerprints actually reduce the area in contact with a surface by about one third, compared with smooth skin. Therefore fingerprints actually reduce friction [New Scientist]. The study appears in the Journal of Experimental Biology.
But if fingerprints don’t help us keep a grip, what do they do? Ennos suggests that fingerprints may allow our skin to stretch and deform more easily, protecting it from damage. Alternatively, 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 [BBC News], possibly by amplifying the vibrations caused when a fingertip sweeps over a finely textured surface.
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Image: flickr / kevindooley