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Institute of Continuing Education

 

Marshmallows and liquorice allsorts. Tuck shop merchandise perhaps – but these can also be used to illustrate fundamental concepts in materials science (of which more below).

Attending the recent Materials Education Symposium in Cambridge, participants extolled the virtues of silly putty as a teaching material, and demonstrations included liquid nitrogen, bursting balloons, and over-stretched springs (reshaped with a hairdryer).

You might be forgiven for imagining that we were not treating our subject with sufficient seriousness, but underlying all of this is an important issue. Any communication or teaching makes assumptions about prior knowledge – what can you take for granted that your students already know?

In the case of students studying materials science, it is normal for their academic backgrounds to be quite varied, and it is important that we do not make too many specific assumptions. Despite that, most people already know far more than they realise about the behaviour of materials. We have abundant experience of materials from the everyday world around us, and we can use that experience to illustrate and inform our understanding of new concepts.

So how do children’s sweets come into this? We’ll start with the marshmallow – and be prepared for sticky fingers.

Squeeze it gently between your thumb and forefinger (so that it can spring back again), and notice how it bulges out sideways. Most solid materials approximately conserve their volume under stress, so if you squash them, they get wider. Normally we don’t notice this because the contractions and expansions are so small, but with a marshmallow the effect is big enough to see (and of course it is also obvious for rubber bands, but you shouldn’t eat those). Engineers need to know exactly how much wider or narrower a component will become under a load, so we need to measure how big this effect is and we call the resulting number Poisson’s ratio. For our marshmallow, we can work out its Poisson’s ratio by dividing the fraction by which it gets wider, by the fraction by which it gets shorter, and the same calculation can be done for any other material as well. Because the conservation of volume is not exact, this number varies for different materials, and it is a fundamental ‘elastic constant’. Students are often surprised to discover that one of the few materials that really does conserve its volume under compression is rubber – very few other materials are truly incompressible in this way. Cork is also unusual in having a negative Poisson’s ratio (when you compress it, it becomes narrower instead of wider, which is why it works as, well … a cork). This happens because natural cork has a cell structure that can collapse like a concertina, and the same property is now being developed and exploited in ‘auxetic materials’. To learn more about these, you could start with this article from the internet magazine Plus, which is part of the Millenium Mathematics Project.

Next let’s consider the thin-layer sandwich variety of liquorice allsort – three layers of sugar paste with two of liquorice in between. Imagine squeezing the sandwich held flat between your thumb and forefinger. The sugar paste is softer, so these layers compress relatively more than the liquorice sheets. Now squeeze it sideways (it might be best to eat the first one and start again with a fresh one!). This time the sandwich is harder to compress, because we cannot compress the soft sugar paste without also compressing the harder liquorice by the same amount. You have learnt that a composite (combination of two materials) responds to the same force differently, depending on how it is oriented. Actually you already know this, unless you never played with your food as a child. We can go on to describe this mathematically, working out how much thinner the composite becomes for the same force applied in different directions. The mathematics applies not only to liquorice allsorts, but also to plywood, carbon fibre composites and to any other engineering material with more than one distinct component.

I could go on from here - ham sandwiches, for example, are useful in visualising defects in the crystal structure of metals (you can learn about those from this learning package on the DoITPoMS website) - but I think it is time for a snack. I wonder what I packed amongst my teaching resources?

If you are interested in learning more about Materials Science, you might like to have a look at the range of Physical Science courses we offer at ICE.

Dr Erica Bithell, ICE Academic Director in Physical Sciences

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