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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.
But to actually demonstrate the underlying cause of
the jet's formation, the Chicago team needed very fast, non-invasive
tracking of the interior of the sand. To this end, the Chicago
scientists used high-speed X-ray radiography. Taken at 5,000 frames
per second, the X-ray images were the fastest ever taken at Argonne
National Laboratory's Advanced Photon Source, which produces the most
brilliant X-ray beams for research in the Western Hemisphere.
The experiments, conducted at both atmospheric
pressure and in a vacuum, showed that air compressed between the sand
grains provides most of the energy that drives the jet. The University
of Twente's Lohse said he regards the work of Jaeger's team as "very
important."
"The result is totally unexpected," Lohse. "One
would think that the effect of air would weaken the jet, but what is
the case is just the opposite."
Systematically reducing the pressure, Jaeger's team
observed that the jet, in fact, consisted of two stages. Air pressure
exerted little influence on the jet's initial stage, a thin stream of
particles that breaks up into droplets. But air pressure played a key
role in forming the jet's second stage, characterized by a thick
column of particles with ripples on its surface.
"One of the biggest questions that we have still
not solved is why this jet is so sharply delineated. Why are there
these beautiful boundaries? Why isn't this whole thing just falling
apart," Jaeger asked.
Jaeger's team needed advanced scientific equipment
and support from the National Science Foundation and the U.S.
Department of Energy to conduct this study. To observe the basic
effect, however, "You can do this experiment at home," he said. Take a
cup of powdered sugar and pour it into another container to ensure
that it is loosely packed. Then, drop a marble into the cup. "Once you
drop that marble in there, you see that jet emerging, but you have to
look fast." |
