Comment & Analysis
Why a spot of rain turns you into a frizzball
Next time you’re having a bad hair
day, blame it on the weakness of
hydrogen bondshile cycling, I don’t really mind
being rained on. But I do mind
arriving at my destination looking
like a drowned rat. This week, I’ve been
caught by showers during my commute
on three separate days. I’ve got towels at
work to dry off, and I can always change
my clothes, but my hair gives the game
away every time. My cycling helmet is full
of ventilation holes and is basically useless
as an umbrella. And so I arrive with my
hair plastered down flat and weirdly frizzy.
As I felt the first heavy drops of rain on the
third occasion this week, I found myself
wondering why hair changes shape so easily.
It’s just strands of protein. Why does getting
it wet make such a difference?
Hair is simultaneously very strong and
very easily sculpted. You could suspend a
100g mass from a single human hair and
it wouldn’t break. But hair can easily be
shaped by an enthusiastic hairdresser armed
with combs, hairdryers and a few mysterious
potions. The secret of its strength comes
from the long, coiled strands of a protein
called keratin that makes up the majority of
each hair. The secret of the shape-shifting is
in how those coils are attached together.
Before the rain started, each of the hairs
tucked under my cycling helmet had a fixed
shape. The coiled proteins are connected
to the coils next to them with two types
of bond. The strongest (called disulphide
bonds) are pretty stable. But there are plenty
of others (called hydrogen bonds) and
these are fickle. As the first drops of rain
found their way through my helmet, the
water trickled into the gaps between the
hairs, and then into the centre of the hairs
themselves. This is where the trouble starts.
When water reaches the hydrogen bonds,
it disrupts them. So wet hair is weak – it’s
only held together by some of the bonds
that it had when dry. One of the things that
hairspray does is to provide a barrier so that
water can’t get inside the hairs themselves.
The weakening of these bonds also makes
the coils stretch out a tiny bit – damp hair is
longer than dry hair. In fact, this was used in
the 1800s as a measure of humidity.
So as I was cycling along, my hair was
wet, weak, and crushed by the helmet. I’ve
got straight hair, and that’s because the hairsthemselves are round. Naturally curly hair is
oval, and this means that when it gets wet, it
tends to coil up a bit more. When I arrived at
work, the wet hairs finally began to dry out,
and individual water molecules floated off into
the air in my warm office. The problem with
that is that as soon as they go, the hydrogen
bonds reform, and they bond the protein coils
together again as they are, in the flat, squashed,
frizzy shape that the helmet pushed the hairinto. This is why hair doesn’t hold its shape
on humid days – as soon as water gets into
the hairs, the hydrogen bonds will continually
break and reform, allowing the shape to
change gradually.
Hair-styling is basically chemistry, and
it wouldn’t be possible without these two
types of bonds. It allows such amazing
variety – just the act of washing and drying
your hair gives you the opportunity to give
it a whole new shape. Unfortunately, if you
don’t have the appropriate tools to re-shape
your hair as you’d like, you spend the day
looking like a drowned rat long after the
water has evaporated. I think I’m going to
invest in a new cycling helmet, and possibly
pay a bit more attention to the rainfall radar
before setting out! ß“If you don’t have the
appropriate tools, you
spend the day looking
like a drowned rat
long after the water
has evaporated”
W
DR HELEN CZERSKI is a physicist and BBC presenter whose
ILLUSTRATOR: ANDREW LYONS most recent series was Colour: The Spectrum Of Science