Natural Science Forum / Physics / General Physics / May 2007

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 826 May 30, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu

MICROFLUIDIC ACCELERATOR.  Microfluidics is the science of carrying out
fluid chemical processing on a chip whose channels are typically
millimeters or microns across. In such a constricted space, viscosity
becomes large, and the fluid flow can slow way down, thus limiting the
kind of mixing or testing that can be done. Physicists at the
University of Twente in the Netherlands, however, use tiny exploding
bubbles to speed things up. The bubbles are produced by shooting laser
light into the fluid. (See movie at
http://stilton.tnw.utwente.nl/people/ohl/controlled_cavitation.html)
The light brings a tiny volume of fluid above its boiling temperature,
causing a local bubble explosion, which accelerates the surrounding
fluid along the channel, now at speeds of up to 20 m/sec, twenty times
higher (and still another factor of 10 within reach) than would be the
case without the bubble. (The same researchers have produced
sonoluminescence in the same way.) An extra advantage of using flexibly
positioned laser light is that for transparent microfluidic chips fluid
pumping can be accomplished without external connections to the chip.

Besides being the first to apply such a cavitation technique for
speeding up fluids on a chip, the Twente scientists are the first to
achieve flow visualization at rates of a million frames per second at a
size scale of 100 microns. The leader of the Twente group, Claus-Dieter
Ohl (c.d.ohl@utwente.nl, 31-53-489-5604) says that he and his
colleagues are currently using the bubble acceleration technique for
improving mixing in various enzyme reactions and in producing tiny
pores in membranes. (Zwaan et al., Physical Review Letters, upcoming
article)

WARM THE WORLD, SHRINK THE DAY. Global warming is expected to raise
ocean levels and thereby effectively shift some ocean water from
currently deep areas into shallower continental shelves, including a
net transfer of water mass from the southern to the northern
hemisphere. This in turn will bring just so much water closer to the
Earth*s rotational axis, and this-like a figure skater speeding up as
she folds her limbs inward-will shorten the diurnal period. Not by
much, though. According to Felix Landerer, Johann Jungclaus, and Jochem
Marotzke, scientists at the Max Planck Institute for Meteorology in
Hamburg, the day should shorten by 0.12 milliseconds over the next two
centuries. (Recent issue of Geophysical Review Letters.)

LISTENING TO MUSCLE NOISE. Muscles make noise. For example, you can
hear the sound of the masseter muscle-a jaw muscle used in chewing
food-by propping your head (ear down) in the palm of your hand. The low
rumbling comes from the shortening of the actomyosin filaments in the
muscle fibers. Muscle noise can be measured using various sensors, such
as microphones and even skin-mounted accelerometers.  Scientists at the
Scripps Institute of Oceanography listen to muscle noise in order to
detect muscle stiffness, which in turn can provide information about
neuromuscular disease, such as muscle dystrophy. Muscle stiffness was
traditionally measured using external radiation sources (such as a
vibrating piston). But the Scripps researchers use a process called
passive elastography, a low-cost, in-vivo, non-invasive technique in
which an array of surface sensors follow the passing of natural shear
waves traveling along the muscle fibers. The new results will be
delivered next week by Karim Sabra (ksabra@mpl.ucsd.edu) at the meeting
of the Acoustical Society of America (ASA) taking place June 4-8 in
Salt Lake City.  By the way, the Scripps scientists were originally
interested in underwater noise effects and only later adapted their
work to noise in muscle. (ASA paper 2pUW9; meeting website at
http://www.acoustics.org/press)

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