COMMENT & ANALYSIS
umans spend a lot of time picking
things up and putting them down
again. We hardly ever think about
it. But whatever we’re manipulating – pens,
cake, keys, tennis balls or this magazine – it’s
only possible because our fingers can grip.
Wandering round a stationery shop the other
day, I spotted one of those sponge pads that
you sometimes see in post offices. Bank tellers
use them to moisten their fingers and improve
their grip when they’re counting banknotes.
But then I remembered the years I’ve spent on
climbing walls and on gymnastic equipment. In
those environments, it’s exactly the opposite.
You put chalk on your hands to improve your
grip, because the chalk dries your hands out
and drier fingers grip better. So which method
actually works?
Water is pretty sticky stuff. Water molecules
are constantly forming brief alliances with the
molecules around them, and those temporary
bonds can be pretty strong. But we don’t think
of water as ‘sticky’ all by itself, because a water
molecule is very small and switches allegiance
often. That’s why water isn’t very viscous – it’s
easy for those molecules to flow past each
other. Water will stick to lots of surfaces, but it won’t hold
on to other water molecules for long periods of time. Sugar
syrup has the same sorts of bonds, but the sugar molecules
are far larger, and each one is attached in many places to the
ones around it. That’s why sugar is thick and viscous – lots of
bonds have to be broken for a molecule to move anywhere,
and that takes time. But water isn’t like that. It’s thin and
runny, and so it isn’t acting like glue.
It turns out that climbers use chalk precisely because water
is so runny. Climbing is an intensive activity, so your hands
can get quite sweaty. If there’s enough water on your fingers
to form a complete layer, your fingers don’t actually touch the
rock directly because of the water in the way. In this case,
your fingers have almost no grip. Water sticks to the rock and
it sticks to your fingers, but the water layer in between just
slides over itself. If you rely on that, you’re going to fall off the
rock face pretty quickly! So the chalk is there to absorb the
water and prevent it ever forming a single thin layer.
The interesting bit is what happens when there’s a little bit
of water, but not enough to form a complete layer. This is
where the bank teller’s damp sponge comes in useful. If you
start with completely dry fingertips, you don’t have much grip.
But as you add tiny amounts of water, your grip improves and
keeps improving right up to the point where there’s enough
for the water to form a proper layer, and then it all gets worse
again. What seems to be happening is that a small amount
of water softens the top layer of your skin, just a tiny bit. That
makes your skin more flexible, and so when you press your
fingertip on to a banknote, your skin squishes more into the
rough surface. That means that a greater area of your skin is
in direct contact with the note. Slightly damp fingertips have
much more grip than dry ones.
I am one of those people who always struggles to prise
apart the two sides of plastic bags in supermarkets. But now
I know the solution: slightly damp fingertips when you try to
slide the two sides past each other. It may not look elegant,
but science says that it works! ßILLUSTRATION: EIKO OJALAH
HELEN CZERSKI ON... STICKY FINGERS
“BANK TELLERS MOISTEN THEIR FINGERS FOR GRIP, BUT CLIMBERS USE
CHALK. WHICH METHOD WORKS?”
DR HELEN CZERSKI IS A PHYSICIST AND BBC SCIENCE PRESENTER