Why do some mugs change colour with temperature?

12 March 2019

MUG-DRINK

White mug on wooden table

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Question

What material can change colour when heated and reverse back when cooled in things such as cups and mugs, or children's plastic spoons?

Answer

Have you ever had a mug that, when you put a hot cup of tea or coffee in it, it changes colour? Well Ranjay got in touch to ask actually how this works. Chris Smith asked chemist Ljiljana Fruk from Cambridge University to tackle the topic.

Ljiljana - Actually, I do have one of these cups. I love them.

Chris - You've got one?

Ljiljana - Because when you put hot tea in it you see this dinosaur suddenly appearing on the cup which is really beautiful. These kind of materials have been around for ages and they can be of two types. So the one which is used in cups or some toys, it's made of organic dye which are called leuco dyes. So basically, they have structure which is changing with the heat. So they can be, for example, transparent because they don't absorb light and then you heat them up and their molecular structure changes a little bit and suddenly they can absorb light of a different wavelength. And you have many different kinds of dyes, so you can basically draw these patterns on your cup and the moment you put something hot into your cup it will change the colour. Usually they are not very sensitive so it will just detect hot or cold and that's why you would use them for this material. But if you want to be a little bit more precise you might be using liquid crystals which are more expensive. So they can also be made of some kind of organic molecules but they are assembled in a particular way, and as you are heating them you are changing their assembly, you are changing their structure a little bit so they interact with the light in a different way.

Chris - Tim?

Tim - So are these materials used for something else? Because they sound amazing and then a mug is a fairly underwhelming use of such an amazing material!

Ljiljana - Well they are. I mean liquid crystals are used in monitors, in lots of sensors as well, so we use them continuously. So you have LCD displays, they are liquid-crystal displays, so you use them in technology and high-tech industry quite a lot. You would not probably use liquid crystals to make a cup because it will be quite expensive. But what is also really interesting is something that in biology liquid crystals are often used. So for example, materials which are similar will be used by lizards for example, or some types of birds. They also have particular structure which can change and then it will change the colour and this can be with the humidity or the temperature. Even an octopus which can camouflage itself very well uses some kind of structural materials that change with impulses. It's all kinds of materials that are really useful.

Chris - Thank you Ljiljana. So there you go Ranjay, basically a molecule that when you heat it, temporary changes shape and that change in shape means it interacts with one colour of light but not another and reflects a different colour temporarily which is why the colour appears to change. When it cools down, it flips back to how it started and that's why you get your original colour back.

Are you gonna have it in your restaurant, that colour changing cutlery?

Ljiljana - Well actually, I am talking right now to a chemist at the University of Cambridge to give us some of the structural colour to decorate the food. You can have an artistic display on your plate.

Chris - I can't wait!

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