We haven't gotten to the essential question: what held the balls together? Apparently tiny bridges of oil that formed between them. Almost all of the oil was doing something else - like coating the spheres, or hiding in crevices. Then, rather suddenly, "Clumping takes over, and instead of particles moving as individuals, they move as a clump," Schiffer says.Īt this point, the sand-castle builder could start crafting parapets, crenellations, and other architectural details.īecause they were using plastic spheres instead of irregular sand grains, the Notre Dame researchers could calculate the tiny amount of fluid - less than one-ten-millionth of a cubic millimeter - needed to hold each ball together. As they gradually increased the oil content, the glop began holding a steeper angle. ![]() Then they put this mess in a container, and measured the angle of the cone that formed when they pulled the plug on the bottom. To measure the phenomenon, Schiffer and his group rounded up some polystyrene spheres, each 0.8 millimeter in diameter, and mixed them with a smidgen - less than 1 percent by weight - of oil. On a Ferrari, it's called "surface tension." In a sandbox, damp grains stick together by what Schiffer calls "interstitial liquid bridges." In research reported on June 18, 1997, in the science journal Nature, Notre Dame University physicists Peter Schiffer, Albert-Laszlo Barabasi and colleagues have pinned the clumping of damp grains to the same phenomenon that causes water to bead up on a waxy Ferrari. ![]() (Plugged plumbing is not just a problem in the cardiac biz.) Still, the influence of dampness on tiny particles is a big deal for any factory that handles powder, in industries ranging from pharmaceuticals to agriculture. That's where you'll find damp sand - dry sand just won't hack it.īut why does damp sand magically hold the shape of a yogurt container after you dump it out?Īmazingly enough, only recently have scientists approached this earth-shaking question as a problem of basic physics. ![]() It's one of the key lessons for beach-bound toddlers: if you're going to make sand castles, you've got to plop your tushy down near the water.
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