美国科学60秒 SSS 2015-04-16

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Ever run along white sand and it hardens up almost like concrete under your feet? But pick up that same sand and it drizzles¹ through your fingers.So that's the essence of why granular materials are interesting. Yale physicist² Abe Clark. Sometimes they can behave like solids. Another times like fluids.Understanding the transitions between liquid and solid. That's really nontrivial.

Grains of sand and otherwise are Clarks's specialty³. He and his collegues recently investigated how a bucket of beeds responds when another objects falls into it. It's analogous⁴ to dropping a stone on sand and then observing how the stone's force transfers to the grains.

The top grain is contacted by an intruder, and then it tells a friend and it tells a friend and so forth⁵ and it moves along a little chain. So what this looks like is basically a little lightning bolts of force shoot off the intruder. But the faster that impact gets, think me*** or missiles track, the more extensive that chain like network between the grains becomes, meaning the grains behave more like a solid, which in some cases make the intruding⁶ projectiles⁷ bounce right off. 

The finding is in the journal Physical Review Letters.

The work was funded in part by the Defence Threat Reduction Agency, part of the DOD. So of course some of the obligations are military. Grainulor materials might be the perfect thing to thwart⁸ a missile attact, for example. And on a smaller scale... Well if I was gonna go build sandbags for the military, I would tell them to use rubber pellets to fill their sanbands with instead of sand. Because he says rubber beeds would create a stronger longer-lasting repellent force against bullets. 

But remember, Abe and his collegues are physics. I've seen a couple of crazy blogs or something saying, you know, scientists are building b***b***, sorts of, you know, something like that. So no. We are definitely not doing something like that. Afterall, that would be a task for engineers.

Thanks for the minute, for Scientific American 60 Second Science. I'm Christofer Tagiya.

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