Text 290, 81 rader
Skriven 2004-12-27 17:39:54 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 713
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 713 December 27, 2004
by Phillip F. Schewe, Ben Stein

WHY DO HEART ATTACKS OCCUR MOST FREQUENTLY BETWEEN 9 AND 11 AM? Studying five
healthy volunteers for 10-day periods in pioneering efforts to ultimately
answer this question, a collaboration of Boston University physicists and
Harvard physiologists has found evidence that the body's circadian clock (a
part of the brain that regulates daily biological activities) influences
patterns in the heart's "interbeat intervals," the lengths of time between
successive heartbeats. At around 10AM for all the healthy individuals, the
values of successive interbeat intervals displayed increased signs of
randomness, statistically resembling that seen in previous studies of
individuals with heart disease.  In their studies, the researchers took special
care to isolate the effects of a person's internal circadian clock (which has a
24.2-hour rhythm, marked by a regular rise and fall of body temperature) from
the effects of behavior (such as physical activity and a person's wake/sleep
time) or external stimuli (such as the rising or setting of the sun). Towards
these ends, the researchers made sure  to "desynchronize" the individuals'
internal body clocks from these other factors by keeping the volunteers in a
dimly lit room and by varying their sleep and wake times from day to day while
keeping activity levels relatively constant. The researchers next plan to
explore how an individual's behavior may interact with the circadian clock to
influence the correlations in interbeat intervals.  The researchers have not
yet studied patients with heart disease and are far from being able to make
clinical recommendations. However, their further research may obtain insights
into the underlying causes of increased cardiac risk and could lead to improved
therapy, such as more appropriately timed medication to coincide with phases of
the body clock.  (Hu et al., Proceedings of the National Academy of Sciences,
December 28, 2004; contact Plamen Ch. Ivanov, Boston University, 617-353-3891,
plamen@argento.bu.edu; Steven Shea, Harvard Medical School, 617-732-5013,
sshea@hms.harvard.edu)

A PEA-SIZED MAGNETOMETER can do the job of much bigger units, and measure
magnetic fields with a sensitivity of 50 pico-tesla. Researchers at NIST
exploit the fact that rubidium atoms possess quantum levels whose energies will
depend on the ambient magnetic field.  By encapsulating a tiny portion of atoms
in a cell and making precision measurements of laser light traveling through
the atoms, a field reading can be made.  All of this is packaged in only about
12 cubic millimeters.  Furthermore, the device can be manufactured in large
batches through lithographic means.  For geophysical applications, such as for
detecting underwater or underground iron objects such as pipelines, tanks, and
shipwrecks, the device's tiny power consumption, compact size, and low price
should move it ahead of several existing magnetometer designs with a few more
years of development work.  (Schwindt et al., Applied Physics Letters, 27
December 2004; contact Peter Schwindt, schwindt@boulder.nist.gov, 303-497-7969;
lab website at
 www.boulder.nist.gov/timefreq/ofm/smallclock/CSAM.htm )

DNA STRETCHING CROSS-STREAM.  A new experiment shows that in specially
engineered fluid flows typical of coating processes, single DNA molecules can
sometimes enter into a kind of flow instability in which the DNA orients itself
perpendicular to the plane of the flow.  The experiment, conducted at Rice
University by Matteo Pasquali and Rajat Duggal, was part of a broader study of
how polymer molecules behave in moving fluids, a subject pertinent to many
biological and technological research areas, such as inkjet printing, paper
coating, the movement of air in lung alveoli, and DNA arrays. Studying polymers
in complex fluid flows is difficult because single polymers are hard to resolve
(being typically only 10-100 nm in size) and because polymers can influence
each other and the flow itself even at very low concentration (down to few
parts per million).  That's why DNA (above 10 microns in contour length) was
chosen and why the DNA was kept "ultradilute," so that it would not influence
the flow and that only DNA molecule is visible at a time. In the Rice
experiment, a dilute suspension of DNA in water thickened by sugar is taken up
by a rotating drum which moves past a glass knife edge. In this way a thin
slice of solution can be moved as if on a conveyor belt past a lens.  The lens
focuses a blue-green light on the DNA and picks up green-yellow light emitted
by the previously fluorescently-stained DNA molecules.  The resulting
30-frame-per-second film clearly can image individual DNAs at a time with a
spatial resolution of 250 nm (the thickness of the molecule cannot be resolved
but its length can be).  The researchers had expected that in the complex flow
(a flow in which the velocity of the fluid varies across the width of the
channel) the DNA would deploy itself with the flow rather than at right angles.
Indeed, this happened at the lowest drum rotation speeds; the direction of
stretching changed once the drum speed became high enough to induce ripples on
the surface of the liquid moving past the glass knife. (Journal of Rheology,
July/August 2004)

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 * Origin: Big Bang (1:106/2000.7)