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This is Scientific American 60-Second Science. I'm Christopher Intagliata. Got a minute?

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Ever run along wet sand, and it hardens up almost like concrete under your feet?

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But pick up that same sand and it drizzles through your fingers?

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"So that's the essence of why granular materials are interesting..."

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Yale physicist Abe Clark.

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"…sometimes they can behave like solids."

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And other times like fluids.

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Understanding the transitions between liquid and solid, "that's really nontrivial."

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Grains of sand or otherwise are Clark's specialty.

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He and his colleagues recently investigated how a bucket of beads responds when another object falls into them.

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It's analogous to dropping a stone on sand, and then observing how the stone's force transfers to the grains.

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"The top grain is contacted by the intruder, and then it tells a friend, and it tells a friend and so forth, and it moves along a little chain.

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So what this looks like is basically little lightning bolts of force shooting off the intruder."

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But the faster that impact gets, think meteor or missile strike, the more extensive that chain-like network between the grains becomes, meaning the grains behave more like a solid.

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Which in some cases made the intruding projectiles bounce right off.

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The findings are in the journal Physical Review Letters.

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The work was funded in part by the Defense Threat Reduction Agency, part of the DoD, so of course some of the applications are military.

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Granular materials might be the perfect thing to thwart a missile attack, for example.

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And on a smaller scale, "well if I was gonna go build sandbags for the military I'd tell them to use little rubber pellets to fill their sandbags with, instead of sand."

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Because he says rubber beads would create a stronger, longer-lasting repellent force against bullets.

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But remember, Abe and his colleagues are physicists.

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"I've seen a couple crazy blogs or something saying, you know, 'scientists are building bunker busters' sorts of, something like that, so no, we're definitely not doing anything like that."

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After all, that would be a task for engineers.

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Thanks for the minute, for Scientific American 60-Second Science. I'm Christopher Intagliata.

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