Ever run along wet sand, and it hardens up almost like concrete under your feet? But pick up that same sand and it drizzles through your fingers? "So that's the essence of why granular materials are interesting...” Yale physicist Abe Clark. “…sometimes they can behave like solids." And other times like fluids. Understanding the transitions between liquid and solid, "that's really nontrivial."



Grains—of sand or otherwise—are Clark’s specialty. He and his colleagues recently investigated how a bucket of beads responds when another object falls into them. It's analogous to dropping a stone on sand, and then observing how the stone's force transfers to the grains. "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. So what this looks like is basically little lightning bolts of force shooting off the intruder."



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. Which in some cases made the intruding projectiles bounce right off. The findings are in the journal Physical Review Letters. [Abram H. Clark et al, Nonlinear Force Propagation During Granular Impact]



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. Granular materials might be the perfect thing to thwart a missile attack, for example. 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." Because he says rubber beads would create a stronger, longer-lasting repellent force against bullets. But remember, Abe and his colleagues are physicists. "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." After all, that would be a task for engineers.



—Christopher Intagliata



[The above text is a transcript of this podcast.]