PHYSICS NEWS UPDATE -- Number 687 June 4, 2004 by Phillip F. Schewe, Ben Stein

From: Sam Wormley (swormley1_at_mchsi.com)
Date: 06/04/04 

Date: Fri, 04 Jun 2004 17:42:48 GMT

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 687 June 4, 2004 by Phillip F. Schewe, Ben Stein

REVERSING TIME TO CATCH SNIPERS. At last week's 75th anniversary
meeting of the Acoustical Society of America in New York City,
researchers presented a system that uses "time-reversed" acoustics
to pinpoint the exact locations of gunfire and explosions in an
urban environment. Coming from the U.S. Army's Cold Regions
Research and Engineering Laboratory and the University of
Connecticut, the researchers (Donald.G.Albert@erdc.usace.army.mil
and Lanbo.Liu@erdc.usace.army.mil) tested the system in a small
"training" village consisting mainly of two-story concrete-block
buildings. In their tests, they fired a gun at an arbitrary location
within the village. The gunshot echoed from building walls and
other surfaces. A network of simple audio sensors recorded the
reverberations at unique vantage points. The researchers then
turned to a computer, which contained a 2-D computer model of the
village. Inside this "virtual village," the computer generated a
backwards version of each recorded sound wave. Displaying a map of
the village, the computer broadcasted the time-reversed waves from
the locations corresponding to the sensors that recorded the
original waves. In the computer map of the village, the
time-reversed waves eventually returned and converged at the spot
corresponding to the source of the gunshot. The researchers are
hoping to develop the system for real-world use, for example by
reducing the amount of computer processing time associated with the
procedure so that it can potentially pinpoint snipers and explosions
in real-time. (Paper 5aPAb5; pictures, movies and lay-language text
at http://www.acoustics.org/press/147th/liu-albert.html)
                
OBSERVING SUPERFLUIDITY IN HYDROGEN MOLECULES is difficult since the
predicted temperature at which liquid H2 would become superfluid
(losing all viscosity), about 2 K, is well below the triple point of
hydrogen (14 K), the temperature below which H2 exists only as a
solid. To make H2 into a superfluid, H2 molecules would have to be
supercooled, cooled rapidly below their freezing point. A new
experiment at the Instituto de Estructura de la Materia-CSIC in
Madrid has not yet observed superfluid H2, but physicists there
have, for the first time, proved that tiny H2 droplets---tiny
clusters, with up to 8 molecules, in a gas jet---are liquid in
form. The scientists (from Madrid, a Max Planck Institute in
Goettingen, and Washington State University) determined the liquid
status of the individual cluster sizes through Raman scattering, the
process in which the energy of a laser beam is depleted ever so
slightly when it passes through a molecular medium (in this case the
H2 droplets) by the excitation of the molecules. This proved for the
first time that a Raman spectrum can be obtained for H2 clusters.
Why so much fuss over whether hydrogen can be made superfluid? If
successful it would be the first direct evidence for the existence
of another superfluid besides helium, at present the only known
liquid superfluid. H2 is the simplest and most abundant molecule in
the universe, and scientists rely on it to point to properties in
other atoms and molecules. Furthermore, hydrogen is the primary
fuel in
stars, while on Earth hydrogen might also play an important role as
fuel since it has the highest chemical energy density by mass.
(Tejeda et al., Physical Review Letters, 4 June 2004).

MICROFLUIDIC TANGO: SORTING WITHOUT DIFFUSION. Separations of
complex biological mixtures such as the contents of a cell require
biomolecules to be sorted by their size or density. To accomplish
this, molecular biologists usually employ methods that rely on
diffusion, the often gradual migrations of particles from one zone
to another. However, diffusion-based sorting requires patience,
since the particles must randomly wander over a large number of
possible paths. Now, a multidisciplinary Princeton team (Robert
Austin, Austin@princeton.edu) has produced a potentially faster,
non-diffusion-based sorting method. The researchers tap into the
power of microfluidics, the control of liquids using microscopic
structures. Their microfluidic method allows them to sort objects
in a nonrandom (deterministic) fashion. In their technique, a
smooth fluid carries the biomolecules of interest in a downward
stream. Encountering arrays of obstacles staggered in a certain
way, smaller molecules zig-zag back and forth through the obstacles
but must proceed on the average straight down. However, if a
biomolecule is big enough, it moves steadily at an angle to the
zig-zag motion, taking tango-like dance steps as it veers to the
left or right, thereby separating itself from the smaller molecules.
In their initial demonstrations, the researchers have sorted
fragments of artificial bacteria chromosomes to within 12% of their
molecular weight in 10 minutes, already an order of magnitude faster
than conventional methods. In tests with sub-micron polymer bead
particles, the device can rapidly and continuously sort them into an
array of output channels with a resolution of 1% of the particles'
radius or less. Thus the device may find applications in the area
of sorting inorganic nanoparticles as well. (Huang et al., Science,
14 May 2004.)

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
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