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1 .This is Scientific American 60-Second Science. I'm Christopher Intagliata. Got a minute?
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2 .Ever run along wet sand, and it hardens up almost like concrete under your feet?
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3 .But pick up that same sand and it drizzles through your fingers?
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4 ."So that's the essence of why granular materials are interesting..."
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5 .Yale physicist Abe Clark.
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6 ."…sometimes they can behave like solids."
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7 .And other times like fluids.
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8 .Understanding the transitions between liquid and solid, "that's really nontrivial."
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9 .Grains of sand or otherwise are Clark's specialty.
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10 .He and his colleagues recently investigated how a bucket of beads responds when another object falls into them.
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11 .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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12 ."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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13 .So what this looks like is basically little lightning bolts of force shooting off the intruder."
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14 .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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15 .Which in some cases made the intruding projectiles bounce right off.
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16 .The findings are in the journal Physical Review Letters.
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17 .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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18 .Granular materials might be the perfect thing to thwart a missile attack, for example.
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19 .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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20 .Because he says rubber beads would create a stronger, longer-lasting repellent force against bullets.
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21 .But remember, Abe and his colleagues are physicists.
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22 ."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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23 .After all, that would be a task for engineers.
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24 .Thanks for the minute, for Scientific American 60-Second Science. I'm Christopher Intagliata.
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