© the hitman
Avogadro’s lab
Foams
Toothpaste for
elephants?
‘Elephant’s toothpaste’ is a
spectacular demonstration
that generates a huge
amount of foam in no time
at all – take a look on
YouTube:
http://bit.ly/149oMja
Stephen Ashworth tells us about a conversation
between Avogadro the Mole and his friend Bob…
Cheers!
Some people get all the
fun! Find out about the
team whose job it is to
study the physics and
chemistry of the bubbles in
champagne:
http://rsc.li/M6n2dY
‘I say, Avogadro,’ said my friend Bob, ‘the cappuccino
foam here is magnificent.’
‘I couldn’t agree more,’ I replied, as we found seats in
the bustling coffee shop.
‘It’s such a shame that you only get foam on a
cappuccino and in a washing-up bowl.’
‘On the contrary, dear Bob,’ I corrected, ‘but what we
should do first is decide what we mean by foam.’
‘Go on then, what is foam?’ challenged Bob.
© shutterstock
‘A foam is a series of pockets of gas trapped in either
a solid or a liquid (bubbles). There are lots of different
foams all over the place.’
‘Well really, Avogadro,’ said Bob indignantly ‘give me an
example.’
6 | The Mole | March 2013
Cushions, sandwiches and volcanoes
‘Let’s look at the chair you are sitting on – it probably
has foam rubber in the cushion.’
‘Of course!’ exclaimed Bob. ‘I should have thought of
that, but surely there aren’t many others?’
‘Think about the sponge cake you’re eating – that’s a
foam. The whipped cream in the middle is a foam too.
Those sandwiches on the counter are made of bread,
another foam.’
‘Well I never,’ said Bob, ‘I’d never thought of it like that.
Does that mean that your meringue is a foam too?’
‘That’s right.’ I replied. ‘There are also foams in nature.
A sponge is foam – air pockets trapped in a solid much
the same as pumice – the rock from volcanoes. You find
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Egg white on the left and washing
up liquid foam on the right
foams in plant stems and some insects use foam to
protect their eggs.’
same, does it?’
‘Yes, that’s right,’ I
confirmed. ‘Then you take
your sample and ideally
Surface tension
put it in a very clean,
‘What we need to make a foam is a substance that helps
narrow, straight-sided
to stabilise a surface. If you think about a single soap
transparent container. A
bubble, it’s just a thin film of water with two surfaces –
measuring cylinder would
inside and outside.’
be perfect, but as long as
the vessels have the same
‘So what will stabilise such a large surface?’
shape and size the results
‘Any molecules that collect at the surface of a liquid
will be comparable.’
and reduce the surface tension will help make a foam.
‘Can I use glasses?’
Surface tension is the force that makes drops of liquid
spherical – the molecules in the liquid don’t like to
‘Yes, if they are all the same.’ I said. ‘Then you blow
be at the surface, so the drop forms the shape with
into your sample until there is enough foam to fill
the minimum surface area possible. The substances
the container. If you can’t even fill the container you
that collect at the surface are known as surface active
know your foam is not stable. As gravity makes the
agents, or surfactants.’
liquid drain out of the bubbles to the bottom of the
foam, some of them will burst. The speed the foam
‘That’s fine, but what sort of molecules make good
drops in the container gives an indication of the
surfactants?’
stability of the foam.’
‘Washing-up liquid is a good example. It’s made up of
‘What can I use as a sample?’
molecules that have one end that likes to dissolve in
water and one end that doesn’t.’
‘You can use anything that will make a foam. I tried
it first with a teaspoon of washing-up liquid in about
‘Wait… wait… that means that one end is hydrophilic, it
100 cm3 of water, and then with some egg white
likes water, and the other end is hydrophobic, does it?’
dissolved in about the same amount of water. The
said Bob, excitedly.
proteins in the egg white act to stabilise the foam.’
‘That’s correct,’ I replied, ‘so the molecules collect at
Is it a fair test?
the surface with the hydrophilic end in water and the
‘Egg white will have water in it too, so isn’t it hard to
hydrophobic end sticking out. The result is a lower
devise a fair test for this?’ asked Bob, frowning with
surface tension, which stabilises the liquid foam.’
concentration.
‘What makes a foam then?’ asked Bob.
Bubbles
In 1999 the world record
for the most people
simultaneously blowing
bubbles was set in
London. 23 680 people
gathered to blow bubbles
for one minute.
Try it yourself – testing stability
‘Are some foams more stable than others?’
‘Oh yes.’
‘Can you test the stability of a foam?’
‘Well, there is a nice little experiment that you can
do when you get home,’ I said, ‘first you have to find
something that will make lots of little bubbles. I did this
by tying or gluing a piece of foam rubber to the end of
a straw.’
‘That helps to make lots of small bubbles that are all the
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‘That’s a very good point – I assumed that the egg
white was around 90% water and used half water
and half egg white, but even that has its problems.’
‘I think I will go home and try it out myself!’ said Bob
eagerly, as he stood up to leave.
‘A least we know that the milk foam is pretty stable,’
he chortled
Find out more
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‘How?’ I asked, puzzled.
‘You’ve still got some on the end of your nose!’
March 2013 | The Mole | 7