DIY Science: Walnuts, Rice and Avalanches

Here's a demo for you to try at home.
12 March 2019

Interview with 

Amalia Thomas, University of Cambridge


A road running through snowy mountains


Grab a jar, fill it with some walnuts, uncooked rice and give it all good shake. Amalia Thomas is a mathematician from Cambridge University who’ll be taking this demo and her research to Parliament as part of a competition called STEM for Britain. She explained to Chris Smith how this work helps to save people from avalanches... 

Amalia - Thank you Chris. Adam has prepared this jar thank you very much. And as you might be doing it home I’m shaking it really well.

Chris - That’s worked already.

Amalia - Yeah. Three of four shakes and can you tell me what you see?

Ljiljana - The nuts came on the top.

Amalia - Yeah, the nuts came to the top. So this is a phenomenon that happens in granular systems; these are systems that are made of small solid things, particles - in this case rice and nuts. And it's a very interesting thing that happens in granular systems only that the large materials will always come to the top so this is called size segregation.

Chris - Would that work, Amalia, if the nuts were really heavy? So if had ball bearings in there would it still work?

Amalia - That's a very good point. Size segregation, is only one type, there are other factors which contribute and one of them is density, so if it's larger but a lot denser it would still not rise. Another factor is say surface properties, so if it's very rough it will go up faster.

Chris - And what's the physics of that? What's actually driving that because that was extraordinary, all of the nuts are now perched on top of the rice. All of them and it's within two or three shakes it was phenomenal so why does that happen?

Amalia - Well, that is what my PhD is about. This is something that is still not completely well understood. You might think that the smaller particles are more likely to fit into gaps so they fall but here we've only got a few big ones and they are pushed upwards. The key to that, and for this I'm going to make an analogy with fluid systems, so say with a mudslide, this would not happen in fluid continuous systems because in granular systems forces are not distributed evenly. What that means is some granules carry a lot of weight and some of them don't carry any weight at all and the large particles are more likely to carry weights and, therefore, they’re more likely to be subjected to forces and, as a result they’re levered upwards.

Chris - So it's an average thing is it? Resolves that on average you end with a force being exerted against the big stuff because they're more likely to be pushed somewhere, whereas a small stuff would just slip and slide away and distribute towards the bottom?

Amalia - Yes, to carry weight than small ones.

Chris - Where do we see the same science being manifested in the real world?

Amalia - We see it all the time; we don't know it. This is actually called the breakfast cereal effect and for this I stole my housemate’s granola.

Chris - You really have got a tub of granola.

Amalia - I've got a transparent tub made with a mixture made by my housemate Emily. In here she's got granola, she's got peanuts, walnuts, and coconut.

Chris - And it's all nicely mixed up at the moment? There's no obvious pattern to anything?

Amalia - So we put it on its side and I just give it a little shake, and just with that little shake...

Chris - She could eat three different breakfast with that but now because you've got three different layers in there.

Amalia - I'll put this above my head. Can someone else tell me what they see?

Ljiljana - The bigger things come up.

Amalia - And all the coconut has gone to the bottom, and it's a same phenomenon. Sometimes it's a good thing so this is used in recycling plants when you know you want the different types of garbage separated. And it's a bad thing say in pharmaceutical companies when they want all their active ingredients well mixed.

One big interesting application is in avalanches which is where I actually study this phenomenon. Imagine your free skiers so you're skiing in the mountains and not going through any specific path you get stuck in an avalanche. There is a procedure for that, you have to wear what look like airbags so that if you're caught in an avalanche you pull it and you suddenly become a big particle and then you will rise in the avalanche.

Chris - You literally float to the top of the snow?

Amalia - You will, just like a walnut.

Chris - Does it work?

Amalia - Yeah they do. And they save lives that way because like the walnut you're brought to the top and then your not crushed like everything else that the avalanche has taken with it.

Chris - You might be beaten up a bit but at least you'll be at the top and findable?

Amalia - And findable, which is key, yeah.


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