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{at}argento.bu.edu; Steven Shea, Harvard Medical School,
617-732-5013, sshea{at}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{at}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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