PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 635  May 1, 2003   by Phillip F. Schewe, Ben Stein, and James Riordon

A "WATER HAMMER" POWERS UP SONOLUMINESCENCE.  In household
plumbing, a water hammer can occur when the flow of water suddenly s
lows, generating a temporary vacuum and a shock wave that together
violently shake the plumbing.  At this week's meeting of the
 Acoustical Society of America in Nashville, Seth Putterman of UCLA
(310-825-2269) described a new "water hammer" method for ge
nerating sonoluminescence (SL), the transformation of sound into light. 
This new approach yields SL flashes with much higher p
owers than before.  In the ordinary SL process, a sound wave enters a
liquid tank, and produces bubbles that collapse and relea
se ultrashort flashes of light. In the SL version of the water hammer,
researchers shake a 20-inch-long, 1.5 inch diameter cyli
ndrica
l tube with a force of 2 g's.  Filled with water and a small amount of
xenon gas, the tube shakes so that water in each half of
 the tube travels in an opposite direction and temporarily creates a
centimeter-long region of vacuum in the center.  As the va
cuum closes, it launches a large shock wave that generates SL in the water,
producing an output of approximately 300 million ph
otons (about a hundred times greater than earlier SL experiments) that add
up to a peak power of about half a watt.  The scaled
-up photon output, Putterman says, makes it possible to perform more and
better measurements of the hard-to-understand SL pheno
menon.  (Su et al., Physics of Fluids, tentatively June 2003.) In a
separate experiment that uses the traditional approach of a
iming
 sound at a liquid tank, Putterman and colleagues have successfully
achieved SL with 1 MHz sound waves, as opposed to the 20-40
 kHz waves that are conventionally used. While MHz sound waves are
currently used in various acoustics applications, megahertz
SL from a single bubble has not been achieved before: The small wavelength
of a MHz acoustical wave makes it very challenging t
o control the local sound field in water to the point that a single bubble
can collapse synchronously with sound.  Compared to
the kilohertz version, megahertz SL produces a markedly different spectrum
of light, and therefore the researchers are planning
 further investigations in this new high-frequency realm.

NICARAGUA IS WET UNDERNEATH.  A new seismic study of a rock slab deep
underneath Nicaragua shows that the slab has the highest
concentration of water of any comparable slab associated with volcanoes. 
Just as radar can be used to tell you about landforms
 and vegetation at the surface, so seismic waves can tell you about the lay
of the land 150 km down.  Geoffrey Abers and Terry
Plank, scientists from Boston University, and their collaborator from UCSB,
Bradley Hacker, observed that seismic waves at dept
hs of 100-150 km beneath a string of Nicaraguan volcanoes traveled as if
the rock slab down there were acting like a waveguide.
  From the wave speeds, the researchers deduced that the water content of
the slab was about 5%, some 2 to 3 times greater than
 for o
ther subducted slabs.  Since water subducted along with oceanic crust
sometimes returns to the surface along with lava, one can
 check the elevated water content finding.  Indeed, the fluid concentration
of Nicaraguan lavas is quite high.  Abers (abers{at}bu
.edu) says that the Nicaraguan slab, and another very "wet" slab
he has studied near Guam, are quite steep (the angle of subduc
tion in the Nicaraguan case is about 70 degrees), which he believes makes
the slab a better conduit for fluids.  (Geophysical R
esearch Letters, 1 April 2003.)

CARBON NANOWIRE (CNW), a one-dimensional string of carbon atoms threaded
through a carbon nanotube, has been observed for the f
irst time.  Carbon chains have been observed before, but never inside a
nanotube.  Yosinori Ando and his colleagues at Nagoya U
niversity (Japan) produced the CNWs amid a welter of nanotube whiskers by
shooting an electrical arc between two carbon electro
des, and employ not the usual helium atmosphere but one of hydrogen.  (This
same team has produced the smallest nanotubes---onl
y 0.4 nm in diameter---and multiwalled nanotubes with the thinnest inner
diameter---only 1 nm.)  Carbon nanowires should  have
interesting mechanical properties; e.g., as ultrastrong fibers they might
serve in Space Shuttle nosecones or as friction-free
rotati
onal bearings (see figure at http://www.aip.org/mgr/png/2003/186.htm ). 
Their chemistry is also new.  The allotropes of carbon
 are usually classified according to the type of chemical bonding, whether
of the "s" type (the electron residing in a spherica
l orbital cloud) or the "p" type (dumbbell shaped orbital).  The
three known carbon bondings are sp^3 (diamond), sp^2 (graphite
, fullerene, and nanotubes), and sp (carbon chain).  The CNW allotrope,
however, partakes of both the sp and sp^2 bondings.  In
 the electronic realm, CNWs might provide the smallest possible metal-metal
junction, or provide highly coherent point sources
of mono-energetic electron beams.  Finally, CNWs provide a quick way to
study 1-dimension carbon chains, which might account fo
r some
 of the mysterious emissions from interstellar space.  (Zhao et al.,
Physical Review Letters, upcoming article; contact Yoshino
ri Ando, 81-52-832-1151, x5280, yando{at}ccmfs.meijo-u..ac.jp; website,
http://www.meijo-u.ac.jp/ST/coe/ENGLISH/index2.html )

***********
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