The Nitty Gritty on the Physics of Sand

This just in: The faster an object slams into a pile of sand, the quicker it will come to a halt. Physicist Douglas Durian of the University of Pennsylvania talks about new research into the properties of sand.

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DEBBIE ELLIOTT, host:

And now onto more basic science in our weekly segment Science Out of the Box.

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This week's topic: a breakthrough in physics. No, it's not the uncovering of another subatomic particle or a revelation about dark matter. It's a discovery about something you're likely to dig your toes into soon - sand.

Douglas Durian, a professor at physics at the University of Pennsylvania, is one of the two scientists who made the breakthrough that we're going to talk about, and we're going to let him explain it. Dr. Durian?

Professor DOUGLAS DURIAN (Physics, University of Pennsylvania): So what we did is we dropped some projectiles into a simple bucket of sand. What could be more simple dropping a ball into sand? And what we discovered, to our surprise, was that the faster the ball hits the sand, obviously, the farther it goes in, but the quicker it comes to rest - completely the opposite of what you might expect.

ELLIOTT: You would expect the faster it was going, the longer it would take to stop.

Prof. DURIAN: That's right.

ELLIOTT: Why the faster you throw it, the faster it stops?

Prof. DURIAN: It's because the force exerted from the sand onto the ball grows as a function of both the speed and the depth at which the ball is located. So if the ball were set gingerly on top of the sand, initially, it feels almost no stopping force, so it can actually grab - you can pull it down deep, and it takes a long time to do so. But always you have a very large force acting on the ball, if you throw it in hard.

ELLIOTT: So is it almost like the sand is pushing back on the ball?

Prof. DURIAN: The sand is, certainly, pushing back on the ball, and it's doing so with ever more vigor the deeper the ball it gets into the material.

ELLIOTT: Now I spend a lot of time at the beach, and my kids love to play in the sand, but why is it that physicist would be interested in the way sand behaves?

Prof. DURIAN: Well, the interesting thing is, in fact, even though it's very common and ordinary, the mechanics of the material puzzle us. Sand is, obviously, composed of many grains. Well, a grain is just a little ball of glass, essentially. And we can know all there is to know about the behavior of an individual grain of sand, or we can know lots about how two grains of sand interact. But when we put a million grains of sand together, they exhibit behaviors that you could not begin to predict by knowing all about one or just two grains of sand at a time.

ELLIOTT: Give me some examples of those behaviors.

Prof. DURIAN: Usually when we think about the mechanics of a material, we asked is it behaving like a solid. If you push on it, it springs back? Is it behaving like a liquid? Or it will flow into form into and can fill any shaped vessel? Or is it behaving like a gas where it can expand easily and fill space? In fact, sand can exhibit all three of these hallmark behaviors. It can behave like a solid. It can behave like a gas. It can behave like a liquid.

ELLIOTT: So when the ball hits the sand, is the sand acting like a solid, a liquid, or a gas?

Prof. DURIAN: It's actually acting like a solid and a liquid. So the way the sand grains are flowing as the ball is pushing in, sand is being pushed slowly out of the way, it's doing that in a nice smooth uniform way much like fluid would flow around a sinking ball. So right near the ball itself as the ball is moving, the sand is behaving like a liquid.

Now, far away from the ball, and also as the ball comes to rest, the sand is behaving like a solid. If the sand remains in a, sort of, a liquid-like state, the ball would just sink like a rock to the bottom, like a rock in a pool, instead it comes to rest. And the solid-like character of the sand will support the weight to the ball forever.

ELLIOTT: Okay, so let's talk about the practical applications of this. Is this going to help me keep my golf ball out of the bunker at all?

Prof. DURIAN: It sure won't.

ELLIOTT: Oh, well. But what are the practical applications?

Prof. DURIAN: Our motivation doing this research wasn't practical, so we don't have specific applications in mind we're seeking to improve. But our results, we think, are a simple case that you can use to build intuition, and that could be something as simple as thinking about what it takes to design a vehicle to roll over sand-like on a dessert, or say you're designing a lunar rover or something like that, to space craft touching down on the surface of Mars. As an application, too, something like designing, oh, say, you need to pick the right kind of sand for your sand trap.

ELLIOTT: Douglas Durian is a physics professor at the University of Pennsylvania. His findings with Professor Hiroaki Katsuragi appeared in the online edition of Nature Physics.

Thank you so much for speaking with us.

Prof. DURIAN: Thank you.

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