Ever Wonder Why Big Cereal Chunks Are Always On Top?

14:06 minutes

a waterfall of corn flakes falling into a blue bowl on a blue table with a blue carton of milk nearby
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You may not have heard of it, but you’ve probably seen the “brazil nut effect” in action—it’s the name for the phenomenon that brings larger nuts or cereal chunks to the top of a container, leaving tinier portions at the bottom of the mix. But the process by which granular materials mix is weirdly hard to study, because it’s difficult to see what’s going on away from the visible surfaces of a container. 

In recent work published in the journal Scientific Reports, researchers turn the power of three-dimensional time-lapse x-ray computer tomography onto the problem. By using a series of CT scans on a mixed box of nuts as it sorted itself by size, the researchers were able to capture a movie of the process—finally showing how the large Brazil nuts turn as they are forced up to the top of the mix by smaller peanuts percolating downwards. 

Parmesh Gajjar, a research associate in the Henry Moseley X-ray Imaging Facility at the University of Manchester, talks with SciFri’s Charles Bergquist about the imaging study, and the importance of size segregation in mixing of materials—with applications from the formation of avalanches to designing drug delivery systems. 

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Segment Guests

Parmesh Gajjar

Parmesh Gajjar is a research associate in the Henry Moseley X-Ray Imaging Facility at the University of Manchester in Manchester, UK.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.


OK, where are those little nuts? Always on the bottom somewhere.


IRA FLATOW: Charles Bergquist! Hi, Charles. You want a nut?

CHARLES BERGQUIST: Oh, thanks. Have you ever noticed how in a box of nuts like this the nuts aren’t mixed thoroughly?

IRA FLATOW: Really? Always seems like the nuts are in layers. Here, like the big stuff is on the top, and the smaller pieces are way, way down on the bottom here.

CHARLES BERGQUIST: Right. It’s actually something called the Brazil nut effect, after the really big nuts that tend to clump up at the top of the mix. And there’s new research explaining why that happens.

IRA FLATOW: People actually study that?

CHARLES BERGQUIST: They do, indeed. And I asked Dr. Parmesh Gajjar of the X-ray Imaging Facility at the University of Manchester just why he was spending brain power and instrument time looking at boxes of mixed nuts.

PARMESH GAJJAR: It’s known as the Brazil nut effect, but, actually, it’s a much wider phenomenon known as the phenomenon of size segregation. So that is large-size objects gather at the top of a pile. So if they’re shaken, if they’re shared, agitated, any of those types of motion cause the large particles to come to the top and separate from smaller particles, which are closer to the bottom.

And this has huge implications across many industries and many environmental processes. For example, in industry, if you’re filling a silo full of material, as you fill the silo, large separates from small. And then you empty the silo, you’re not going to get an even mixture. And in the environment around us, say, avalanches, a snow avalanche or a rock avalanche, the large particles come to the top of the avalanche and then get to the front of the avalanche and increase the power of the avalanche. So it has an effect on the way the avalanche travels and, hence, its devastation.

CHARLES BERGQUIST: So haven’t we known about this for years? I feel like this is something that we’ve all seen.

PARMESH GAJJAR: So actually, during my PhD, because my PhD was on this process of size segregation, my mom used to laugh all the time and say the process that you’re studying, this process by which large particles rise to the top and small go to the bottom, is something that people in the kitchen have known for years. And it’s true.

We’ve known about this for years. But how does it really happen? That’s the key question here.

CHARLES BERGQUIST: You’re using something called time-lapse 3D X-ray computer tomography to find the answer to this question. Why can’t you just look at the box of nuts?

PARMESH GAJJAR: The problem with all granular materials is a problem of, how do we understand what’s happening in 3D away from the surface or away from things that we can see? So if I just take an example– so here’s a cereal box, and we want to know what’s happening inside of the cereal box. Now, if you look closely, Charles can you see the window that’s there?


PARMESH GAJJAR: So we could shake this, and we could monitor what’s happening at the window. But that doesn’t tell us what’s happening on the inside of the box because we can’t see on the inside of the box. And that’s the difficulty with granular materials. We can only kind of see one layer, and we can’t actually understand what’s happening in 3D on the inside of materials. So to kind of take a step forward, people started to do things with spheres.

Spheres are great. We can understand a lot. We can see the process of large particles coming to the top with spheres. But most of the things, most of the materials that we have in the real world are not spheres.

So I work in an X-ray imaging facility. So this is one of the largest X-ray imaging facilities in the world. And we scan material science applications. So we put anything basically that’s non-human or nonclinical inside of our machines, and we get a 3D image of them. And so what we could do in this way is we can build up a 3D picture of what’s happening to the nuts over time. We take a series of CT scans, and we build up the time picture.

CHARLES BERGQUIST: Walk me through when you’re using this series of CT scans to see inside the material inside the box, what did you see happening?

PARMESH GAJJAR: What we could see by examining each of the individual nuts was a really unique motion. The Brazil nuts that rose to the top initially started horizontal in the pile. And then slowly, as the box was shaken, these nuts started to go from a horizontal more towards a vertical position. Then when they became vertical, more vertically aligned, they then started to rise upwards. And then when they reached the top at the surface, they then started to fall back to being horizontal again. We had this changing orientation of the Brazil nuts in different parts of its motion.

CHARLES BERGQUIST: So should I think of this as the Brazil nuts wiggling their way up or as everything else sort of shoving its way down?

PARMESH GAJJAR: That’s actually a really, really good question. So as the box is shaken, the small particles kind of fall their way through the gaps. It’s called a process of kinetic sieving, or percolation. And as they find their way down, they gather towards the bottom.

But the math has to be preserved. If small particles are coming down, something has to go up. And so that kind of levers the large particles upwards. It’s given the phrase squeeze expulsion, but, essentially, you have the small particles kind of just percolating down through gaps, and that, in turn, causes a mass balance, the mass– some large particles move upwards, and that, in this case, is the Brazil nuts.

CHARLES BERGQUIST: Does this finding apply to other particles in general as well? Can I use this to predict the chunks of granola in my box of granola in addition to these uniquely shaped nuts?

PARMESH GAJJAR: The nuts are quite unique because of their shape. But I think this opens massive doors. Up to now, nobody has really been able to examine the process by which irregular shaped particles segregate. So a box of muesli, nothing in our box of meusli is really that spherical, right? So I think this– the technique that we’ve unlocked here really opens a lot of doors. In fact, I’d really love to get muesli– put muesli in my experimental box and see how that segregates.

CHARLES BERGQUIST: So if I give the box of nuts a shake, does that just randomize everything again? Or does it speed up the sorting process?

PARMESH GAJJAR: Actually, shaking will kind of speed up the sorting process. This is one of the problems that they have in industry. When you do any kind of shaking, any kind of blending, it actually causes the anti-mixing, which you’re trying to avoid in the first place.

CHARLES BERGQUIST: Does what you’re learning by looking at your box of nuts still apply if you scale it way down to, say, particles in a pharmaceutical mixture?

PARMESH GAJJAR: Pharmaceuticals have a range of different sizes. When you’re down at the very small range, breathable medicines that we use for, say, asthma, respiratory medicines that we use for asthma need particles that are very small down to smaller than 5 microns to reach into the deep part of the lungs. At this kind of size range, there are a lot of other effects.

The inter-particulate affects between powders cause them to be very cohesive, stick together. And so at that size range, this kind of segregation becomes almost secondary. But for more tabulating materials, for larger, say, granules, yeah, the process of size segregation certainly has a big effect. And it’s one of the problems that they have when producing tablets.

CHARLES BERGQUIST: I imagine there must be other important factors like the smoothness of the particle or the density and things like that. How do those interact in this problem?

PARMESH GAJJAR: Yeah, actually, that’s a really, really good question. So size is an almost kind of an unintuitive way of sorting particles out. We don’t imagine that the large ones necessarily come to the top until we recall our Brazil nut experience. Density is one that we are more familiar with. Denser objects are likely to sink to the bottom. And as you said, the smoothness, the surface friction, shape, all of these factors have an effect as well.

So we just take an example– large, dense objects and small, light objects. Now, do the large ones rise? Or do the large ones sink? It’s actually a really interesting question, and that actually depends on the ratio between the density and the size, in this case. And there are actually cutoffs where we can predict where the large ones rise up or actually the large ones sink because they’re denser.

CHARLES BERGQUIST: How did you come to be interested in this? What made you start studying This

PARMESH GAJJAR: When I was looking around for a PhD project, I think it was the sheer simplicity of it. Such a simple problem, like large particles coming to the top, small coming to the bottom. I think that caught my imagination, that how we could apply math theories to understand and model it.

So actually, my PhD thesis was on modeling this phenomenon, seeing if we could put mathematical theories to understand and predict how large particles rise, how small particles come, after what kind of time range does that happen, what kind of height they rise. These are the questions that we were trying to predict with mathematical theories.

And actually, the mathematical theories of this are really good. The difficulty is on the experimental side. And that’s what we’ve managed to uncover in the last few years, using X-ray computer tomography.

CHARLES BERGQUIST: So how well does the real world match up with the theories that you came up with before, the models?

PARMESH GAJJAR: I think that the models that we were able to apply actually fit really well. There’s still areas where the model can be improved. And I think that’s where we need really good experimental data to really help us in that regard.

CHARLES BERGQUIST: Interesting. You’re listening to “Science Friday” from WNYC Studios. In case you’re just joining us, I’m Charles Bergquist, talking with Parmesh Gajjar about a problem known as the Brazil nut problem. So in the practical world, if I wanted to use this knowledge to create an even distribution of something, is the answer just, don’t have weird things in your mix that are shaped like Brazil nuts, or make everything the same shape? Or is there something more fine-grained that I should be thinking about?

PARMESH GAJJAR: Making everything the same shape is certainly, I think, helpful, but not always practical. Meusli would be pretty uninteresting if everything was the same shape. I think the knowledge of what’s happening in the real world is vital to the way we design processes that help us mix something together. Just a challenge to try at home– take a box of meusli, say, or take a jar and fill a jar full of, let’s say, peanuts and Brazils or, a simpler example, if you have lentils, split lentils and whole lentils. Fill it up, and then try and get it well mixed.

You could shake it. You could tumble the jar. You could very rotate it back and forth. You’ll actually find it, I think, quite difficult to get an even mix of the two. And this is what, in industry, they’re trying to do all the time.

I think there’s a lot more work to be done. This is, really, the first insight into how things are moving in 3D. But this opens doors. If we can really understand how the process is happening, we can then put in steps to mitigate it, to stop it happening in the first place. That could be, potentially, designing our mixing cell in such a way that it causes an even mix. And we can even then test it, look at it in 3D to make sure that what we’re trying to do is actually happening in the real world.

CHARLES BERGQUIST: Thank you so much for taking the time to talk to me about this.

PARMESH GAJJAR: No, thank you. It’s been a real pleasure to talk to you and to just discuss this work with you.

CHARLES BERGQUIST: Dr. Parmesh Gajjar is a research associate in the Henry Mosley X-ray Imaging Facility at the University of Manchester in Manchester, UK. I’m Charles Bergquist. Fun fact, it turns out Dr. Gajjar is allergic to nuts, so he had to get someone else to fill up his experimental box of nuts.

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About Charles Bergquist

As Science Friday’s director and senior producer, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

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