“You can see a
world in a grain of
sugar, the world of
tiny molecules and
miniscule forces”
Investigate capillary
action with a cup of
S tea and a sugar cubeHave fun manipulating a world of tiny forces - have sugar with your tea
Comment & Analysis
ugar lumps are so much fun - I
had no idea. I knew that horses
like them, and that you find them
in coffee shops in France (where they are
wrapped up and stacked like bricks). But
my interest in sugar lumps stopped there,
until a friend suggested that we indulge in
proper afternoon tea in Oxford. We went
into the sort of teashop that has very thin
china cups, an army of doilies and a constant
background melody of refined clinking
noises. After my friend’s coffee arrived, he
picked up a white sugar lump with the posh
tongs, but he didn’t drop it into his cup. He
held it with the lower side barely touching
the liquid surface, and the white sugar was
suddenly invaded by black coffee. It only
took a couple of seconds for the whole lump
to go dark brown. And then he let go of it
and it fell into the hot coffee below.
This is just beautiful, because it shows
why scale matters. We assume that liquids
can be poured into containers and will then
just stay at the bottom of the container, but
that’s only the case for anything bigger than
a few millimetres across – gravity usually
dominates. But if you’re smaller than that,
other forces matter more, and liquids don’t
necessarily stay in their containers.
A sugar lump is made of lots of crystals
packed together, with tiny spaces in between
them. Imagine the jostling molecules in the
coffee touching the sugar. Coffee is mostly
water, and water molecules are attracted to
sugar, so the coffee will slide up the surface
of the sugar crystal a little way. But water
is also strongly attracted to other water
molecules, and will change its shape to
touch as little air as possible. So the water
molecules sliding up the sugar surface bring
along some other water molecules to reduce
the surface area on the non-sugar side.
The channels through the sugar lump
are so narrow that very few extra water
molecules have to be pulled up against
gravity to minimise the surface area. So
the coffee can keep creeping upwards,
just because the sugar is attracting water
so strongly. This is capillary action – the
combination of sugar-water adhesion and
surface tension. These adhesive forces are
tiny, but when the channels are tiny that’s all
you need to overcome gravity. The balance
of forces is different down at the bottom.Liquids don’t just fall down.
When I got home, I bought some sugar
lumps and food dye and had a bit of a play.
Milk rises about three times more slowly
through a sugar lump than water, and I
think that’s because it’s more viscous. Oil
only rises about 6mm and then stops, so
that’s less strongly attracted to the sugar than
the water is.
The lovely thing about this is that we can
watch something as large as a sugar lump
and see the effect of tiny forces on individualmolecules. All the molecules in a liquid are
free to move around, and each one is just
responding to the forces on it. It’s a bit weird
to think of water creeping around by itself,
but it’s happening everywhere. It’s why
towels are absorbent, and why sports tops
wick sweat. Even though we live up here in
the macro world, we can engineer materials
that have structure on a tiny scale, and
that can take advantage of the rules being
different when you’re small.
I have never had sugar in my tea, but
the sugar bowl was empty by the time we
left the tearoom. You can see a world in a
grain of sugar, the world of tiny molecules
and miniscule forces. And even better than
seeing, you can play with them!DR HELEN CZERSKI is a physicist, oceanographer
and BBC science presenter who appears regularly on
ILLUSTRATOR: CIARA PHELAN Dara O Briain’s Science Club