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3 October 2007

Mystery Grains


DR ANTIONETTE TORDESILLAS: Granular materials constitute a wide range of everyday common materials, the sorts of materials that you encounter from the time you wake up in the morning through to the soils, the materials that roads are made of are also granular in form, through to powders through to natural grains such as nuts, rice, wheat grains and mineral resources, such as iron ore and coal.

Apart from the fact that they're so prevalent and technologically important we know so very little about them. That I think is the mystery of these materials is that we're surrounded by them every day but we know so little about them.

They exhibit a wide range of highly unpredictable and counter intuitive properties. They're very weird. My students would say they're extremely weird, bizarre, unpredictable. They have multiple personalities for a start.

Take vacuum packed coffee for example. This is very much solid like behaviour because it's stiff, stiff as a brick and at the same time it's strong enough to hold your weight. And yet, if we open the pack, I can pour it just like I would pour water and you can see it flows like a liquid and now it's very weak. It can't even support my little finger.

Another interesting thing about these materials is that they tend to sort themselves out according to size and shape. And so you take this ball for example and here is a granular material composed of very fine particles.
If I pour this on top, so you can see it was on the bottom, and I close this container and shake it a little bit, you'll see that the largest, in this case the one single ball, rises to the top.

That's counter intuitive because you would expect the heaviest, largest particle to stay on the bottom. So this is sorting by size.

Over here I have mixed in equal parts sugar and hundreds and thousands, which is a cake decoration. Watch what happens when I rotate this cylinder.

So you can see the hundreds and thousands which are the more mobile particles because of their spherical shape. They tend to roll down the hill as I roll the container much further than the sugar granules which tend to get stuck with each other because of their angular shape.

So let's take this same container and rotate it just a little bit faster, and so now, watch what happens. You see the sugar granules will try and separate out from the hundreds and thousands in bands, so you have sugar, hundreds and thousands, sugar.

This is one of the great mysteries of this material: you get order from disorder. In life, typically, we get the reverse.

Why does this matter?

Well, because they're highly unpredictable we can't control them and they affect a large range of our industries, so chemical, pharmaceutical industries, also minerals processing. Processing efficiency levels for these industries typically are very low, now more than 60%. Compare that with 96% on average efficiency levels for liquids.

In many cases you have this material having to flow from a container. Typically it gets blocked in the narrow part and obviously once that's blocked then it stops the whole processing.

It's not just inefficiencies. It can also be quite dangerous. In fact, in the United States over 1000 silos fail every year and the reason why this is so is because again we're back to that unpredictability in the behaviour of these materials. You would expect that in a silo much of the weight of the grains would be pushing on the bottom of the silo and not on the walls. And in fact what happens in this case is that the forces go from the centre to the sides so a lot of the forces are directed towards the walls of silo and hence it's collapsed in this case.

So one way of doing this is by modelling individual grains and so here we're going to look at a simulation of a pile that's being driven into the ground which is what happens when you build foundations. All these dots are all the particles, coloured according to the amount of force that they carry.

But the problem with this technique is it's just so expensive and so time consuming so this simulation here is very small scale. It only involves 5000 particles which is the number that you would find in a teaspoon full of sand.

So, if you wanted to do a simulation on a handful of sand, which consists of a few million grains you'll need a super computer and even then it will take you several months.

We've developed a faster and cheaper technique because we don't simulate every single grain. Rather what we do is we get an overall behaviour by averaging out the details of the individual grains.

Here we see the force being pushed down on our model of the ground and here we're not seeing the particles. Rather we're just seeing the overall effects so you can see the flow pattern around the object.

Our model can also handle large scale processes. Here's a classic example where the material has failed. The ground has collapsed and can no longer support the building.

Ultimately if we can design models that can predict when the ground will collapse then we can design better buildings to safeguard against such events.

With understanding comes control and the ability to predict those highly unpredictable behaviours.

Because of the sheer prevalence of these materials and the fact that they're an integral part of so many industries the smallest advance in our understanding can significantly benefit a wide range of industry.