Prize for the best scientific party trick,

[Ichingcarpenter]

f there were a Nobel Prize for the best scientific party trick, this would be a contender. Take two tin cans, one with the top removed and the other with both ends cut out to form a tube. Turn the closed-bottomed can upside down and push both cans into dense sand. The closed-bottomed can will sink more easily than the open tube. That's weird because if you push the two cans into water, exactly the opposite will happen.

Raphaël Clément and another graduate student at the Université Paris Diderot in France discovered the effect while playing around in the lab with empty tea canisters. "It works quite well with an upside-down glass, too," Clément says. "You can try it at home." The effect highlights the subtle and often surprising physics of granular materials. Physicists have fundamental theories of solids, liquids, gases, and even ionized gases called plasmas, but they have no such theory to predict the behavior of a common dirt pile.

To see why the effect is surprising, first imagine pushing the inverted closed-bottomed can and the tube into water. Naturally, the open-ended tube slips right in. The closed-ended can, however, traps air that creates a huge upward buoyant force as the air is forced down into the water. Trying to submerge the closed can is like trying to dunk a balloon.


That's because the air trapped in the closed-ended cylinder affects the sand in a dramatic way, the researchers argue. Ordinarily, still sand acts somewhat like a solid. But if the air around the sand grains moves quickly enough, the sand will flow like a fluid, as physicists and engineers have long known. And when the closed-ended cylinder sinks into the sand, the air trapped in it rushes out from beneath the cylinder's rim, fluidizing the sand at that point and making it easier for the stuff to get out of the can's way. The sand has to be relatively densely packed, and the can must drop quickly. If the researchers slowly added weight to the can, the air seeped away gradually and the can descended no farther than the tube.




.....SNIP.....SNIP


video at the link


http://news.sciencemag.org/sciencenow/2011/02/convince-your-friends-youre-a-ge.html

[Jim__]

1. I definitely agree with the last paragraph in the article.

The researchers were probably lucky to stumble onto the effect, as it likely works only for certain grain sizes and cylinder sizes, says Stephan Koehler, a physicist at Worcester Polytechnic Institute in Massachusetts. "I think they really hit the sweet spot in a huge parameter space," he says. Troy Shinbrot, a physicist at Rutgers University in New Brunswick, New Jersey, says he wishes Douady and colleagues had explored exactly how the effect depends on such factors. Still, he says: "It's a cool effect. I might use it in one of my classes to show the difference between granular materials and fluids or solids."

The demonstration in the video doesn't appear to actually perform the test described in the text. There is no test with an open-ended tube. The tube they can they use has a small opening at the top; they put a heavy weight on top of the can. My guess is that this has the effect of trapping air between the weight and the remaining top of the can - the air in the can escapes through the hole in the top; but is then trapped. So in this case, the air doesn't escape out the bottom and doesn't fluidize the sand; but does the not-so open-ended tube behave the same way a truly open-ended tube would behave?

[QED]

2. I always thought the disappearing spoon was kind of fun.