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[交流] 美国物理学家发明神奇物质

【科研中国SciEi.com整理】
University of Chicago physicists have created a novel state of matter using nothing more than a container of loosely packed sand and a falling marble. They have found that the impacting marble produces a jet of sand grains that briefly behaves like a special type of dense fluid.

"We're discovering a new type of fluid state that seems to exist in this combination of gas--air in this case--and a dense arrangement of particles," said Heinrich Jaeger, Professor in Physics and Director of the Materials Research Science and Engineering Center at the University of Chicago. "It's just a most amazing phenomenon."

Jaeger's team describes the surprising phenomenon in the December issue of the journal Nature Physics. The team consists of graduate students John Royer and Eric Corwin; 2003 University of Chicago physics graduate Andrew Flior; visiting physics graduate student Maria-Luisa Cordero from the Universidad de Chile; Peter Eng, a Senior Research Associate at the University's James Franck Institute; and Mark Rivers, Associate Director of the University's Consortium for Advanced Radiation Sources.

Scientists typically have produced new states of matter at ultra-cold temperatures, those nearing absolute zero (minus 497.6 degrees Fahrenheit). In this case, granular materials take on unusual characteristics at room temperature. "The jet acts like an ultra-cold, ultra-dense gas, not in terms of ambient temperature, but in terms of how we define temperature via the random motion of particles. Inside the jet there is very, very little random motion," Jaeger said.

The jetting phenomenon was first reported in 2001 by Sigurdur Thoroddsen and Amy Shen, who were then at the University of Illinois at Urbana-Champaign. Studying the way the characteristics of granular materials changes from solid to fluid has long been a research theme at Chicago's Center for Materials Research. Thoroddsen and Shen's work led Jaeger to suggest that Floir reproduce the experiment as the subject of his undergraduate honors thesis.

Meanwhile, a group led by Detlef Lohse at the University of Twente in the Netherlands used high-speed video and computer simulations to infer how the jet was caused by gravity as material rushed in to fill the void left behind by the impacting object.

全文见：http://www.physorg.com/news8765.html

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