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New jets are made of powder and air, yet behave like an ultra-cold fluid

National Science Foundation : 15 December, 2005  (Company News)
Using nothing more than a container of loosely packed sand and a falling marble, a research team led by University of Chicago physicist Heinrich Jaeger has discovered a new state of fluid matter.
This new matter takes the form of a large, sharply focused jet of sand shooting upward from the impact point, explains Jaeger, who is director of the National Science Foundation-funded Materials Research Science and Engineering Center at the University of Chicago. Jaeger's team describes the surprising phenomenon in the Dec. issue of the journal Nature Physics.

It looks much like the vertical jets that form when a marble falls into a pool of water, or some other liquid, except that in those jets, the rising column is held together by the liquid's surface tension. In loose-packed sand, notes Jaeger, there is no surface tension.

Instead, he says, the granular jet seems to behave more like ultra-cold matter at temperatures near absolute zero (minus 497.6 degrees Fahrenheit). Although the sand grains themselves are at room temperature, says Jaeger, 'the jet acts like an ultra-cold, ultra-dense gas in terms of how we define temperature via the random motion of particles. Inside the jet there is very, very little random motion.'

In addition, by taking high-speed X-ray images at both atmospheric pressure and in a vacuum, Jaeger and his colleagues have shown that most of the energy driving the jets is coming from air compressed between the sand grains.

More precisely, they found that the jet actually forms in two stages. Air pressure plays little role during the first, when the jet takes the form of a thin stream of particles that breaks up into droplets,. But it plays a critical role during the second stage, when the jet forms a thick column of particles with ripples on its surface.

Why doesn't air pressure just blow the sand grains apart? 'One of the biggest questions that we have still not solved is why this jet is so sharply delineated,' says Jaeger. 'Why are there these beautiful boundaries?'

Jaeger and his colleagues conducted their study with support from both NSF and the Department of Energy. But to observe the basic effect at home, he says, you just have to take a cup of powdered sugar, pour it into another container to ensure that it is loosely packed, and then drop in a marble. 'Once you drop that marble in there you see that jet emerging,' he says, 'but you have to look fast.'
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