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 slows, 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 generating sonoluminescence
(SL), the transformation of sound into light. This new approach yields
SL flashes with much higher powers than before. In the ordinary SL process,
a sound wave enters a liquid tank, and produces bubbles that collapse
and release ultrashort flashes of light. In the SL version of the water
hammer, researchers shake a 20-inch-long, 1.5 inch diameter cylindrical
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 vacuum closes, it launches a large shock
wave that generates SL in the water, producing an output of approximately
300 million photons (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 phenomenon. (Su et al.,
Physics of Fluids, tentatively
June 2003.) In a separate experiment that uses the traditional approach
of aiming 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 to 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 depths 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 other 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
says that the Nicaraguan slab, and another very "wet" slab
he has studied near Guam, are quite steep (the angle of subduction in
the Nicaraguan case is about 70 degrees), which he believes makes the
slab a better conduit for fluids. (Geophysical
Research Letters, 1 April 2003.)
Carbon Nanowire (CNW)
CARBON NANOWIRE (CNW), a one-dimensional string of carbon atoms threaded
through a carbon nanotube, has been observed for the first time. Carbon
chains have been observed before, but never inside a nanotube. Yosinori
Ando and his colleagues at Nagoya University (Japan) produced the CNWs
amid a welter of nanotube whiskers by shooting an electrical arc between
two carbon electrodes, and employ not the usual helium atmosphere but
one of hydrogen. (This same team has produced the smallest nanotubes---only
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 rotational bearings (see
PNG figure). 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 spherical orbital
cloud) or the "p" type (dumbbell shaped orbital). The three
known carbon bondings are sp3 (diamond), sp2 (graphite,
fullerene, and nanotubes), and sp (carbon chain). The CNW allotrope,
however, partakes of both the sp and sp2 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 for some of the mysterious emissions
from interstellar space. (Zhao
et al., Physical Review Letters, 5 May 2003; contact
Yoshinori Ando, 81-52-832-1151,
x5280; visit the website
for more information)
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