Friday, June 12, 2009
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.
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.
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.
So if fingerprints don't tighten our grasp on smooth surfaces, what
are they for? Ennos explains that our fingerprints may function in
other ways. They 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. Whatever our fingerprints are for, it seems that the
idea that they provide friction for grip is just another urban
myth.
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Article available at the Journal of Experimental Biology
Thanks to The Company of Biologists for this article.