[tt] Physics News Update 845

Fri Nov 2 17:37:35 UTC 2007

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From: physnews at aip.org
Date: Fri, 2 Nov 2007 11:47:52 -0400
To: eugen at LEITL.ORG
Subject: Physics News Update 845
Reply-to: physnews at aip.org

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 845 November 2, 2007  by Phillip F. Schewe
www.aip.org/pnu

MARTIAN DUNES TAKE THEIR TIME: they need 1000 years to travel a few
meters.  This is the conclusion of a new study that tries to
simulate the observed structures of dunes on the red planet and to
determine whether present conditions could have been responsible.
On Earth, a sand dune is shaped by wind and water.  On Mars, there
doesn*t seem to be any surface water movement (at least not any that
would shape dunes), and as for wind, there isn*t much of that
either.  With an atmosphere only 1/100 the density of Earth*s the
wind speed on Mars would have to be considerable to move sand
around.  Eric Parteli of the Universitaet Stuttgart in Germany and
his colleague Hans Herrmann of the Universidade Federal do Ceara  in
Brazil calculate that on Mars (where the gravity is only 1/3 the
Earth strength) a dune at a height of 1 meter would require a wind
velocity of 35 m/s (roughly 75 mph) to be moved appreciably.  This
speed occurs only a few times a decade, hence the glacial pace of
dunes on Mars.  Their most surprising finding, Parteli said, comes
from their study of bimodal sand dunes, those that bear evidence of
being shaped by winds from two perpendicular directions.  They
deduce a wind oscillation period on Mars of 50,000 years (the time
it takes for winds to shift around by 90 degrees), roughly the same
as the period for the precession of Mars*s axis.  (Physical Review
E, October 2007; parteli at icp.uni-stuttgart.de; more information
at http://www.icp.uni-stuttgart.de/~hans/dunes.html)

GRANULAR LIQUIDS WITH ZERO SURFACE TENSION.  New experiments with
spherical glass beads show that liquid behavior can arise simply
from rapid collisions among a sufficiently dense stream of
particles. The experiment was undertaken by Xiang Cheng, Heinrich
Jaeger and Sidney Nagel and their colleagues at the University of
Chicago, experts on discovering novel effects with  granular
materials (see http://www.aip.org/pnu/2005/split/725-3.html and
http://www.aip.org/pnu/2005/split/759-2.html).  If one or two beads
are dropped from above on a horizontal surface, they will bounce
back in the direction from which they came.  If, however, many beads
are dropped all at once---constituting a dense granular stream
hitting a target---then something else happens: the grains deflect
out laterally in the form of a very thin, symmetrical sheet or cone
as if they were a liquid.  Indeed, the experiments using granular
matter quantitatively reproduce results obtained with streams of
water.  However, with beads, the *liquid* is one in the limit of
vanishing surface tension.  (To ensure there was no cohesiveness
between the beads, which range in size between 50 microns and 2
millimeter, they were baked in a vacuum oven beforehand, evaporating
any lurking moisture.)  During the short interval the beads inside
the stream collide with each other in front of the target,
liquid-like
conditions are established whose observable consequence are the thin
sheets. This novel, zero-surface-tension liquid state, the
experimenters believe, might be of interest to physicists at the
Relativistic Heavy Ion Collider (RHIC), where heavy nuclei colliding
at high energies (see http://www.aip.org/pnu/2005/split/728-1.html)
form a plasma  of quarks and gluons that also resembles a liquid.
Intriguingly, the
collision pattern produced by the completely classical, macroscopic
granular liquid can match that produced by the quark-gluon plasma.
(Cheng et al., Physical Review Letters, 2 November 2007)

***********
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