PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 725 April 1, 2005
by Phillip F. Schewe, Ben Stein
                        
ZEPTOGRAM MASS DETECTION---WEIGHING MOLECULES.  Michael Roukes and his
Caltech colleagues produce some of the finest nanoscopic electromechanical
systems (NEMS) devices in the world.  His latest achievement is performing
mass measurements with nearly zeptogram (zg) sensitivity, that is, with an
uncertainty of only a few times
10^-21 grams.  At this level you can start to weigh molecules one at a
time.  In experiments, the presence of xenon accretions of only about 30
atoms (7 zg, or about 4 kilodaltons, or the same as for a small protein)
have been detected in real time.  Minuscule masses are measured through
their effect on an oscillating doubly clamped silicon carbide beam, which
serves as the frequency-determining element in a tuned circuit.  So, in
practice, the beam would be set to vibrating at a rate of more than 100 MHz
and then would be exposed to a faint puff of biomolecules. Each molecule
would  strike the beam, where its presence (and its mass) would show up as
a changed resonant frequency.  After a short sampling time, the molecule
would be removed and another brought in.  Through this kind of
miniaturization and automation, the NEMS approach to mass spectroscopy
could change the way bioengineering approaches its task, especially in the
search for cancer and its causes.  The Roukes (roukes{at}caltech.edu,
626-395-2916
) group reported its findings at last week's meeting of the American Physical Society
(APS) in Los Angeles.

LASER SCATTERING OF MITOCHONDRIA, the "power plants" of cells,
can immediately identify early-stage liver cancer cells and potentially
monitor stem cells as they undergo various stages of development.  At the
APS March Meeting, Paul Gourley of Sandia
(plgourl{at}sandia.gov) reported the latest uses of the "biocavity
laser," an aluminum-gallium-arsenide based design that continuously
pumps in single human cells into a chamber for analysis.  The laser's beams
are altered in their passage through the cells. The 800-nanometer light in
the experiments is not absorbed by most of the cell, except by its hundreds
of mitochondria, which are responsible for scattering 90-95 percent of the
light.  By analyzing the scattering patterns, the researchers determined
the distribution of mitochondria in the cell, and could instantly determine
whether the cell was healthy (in which case the mitochondria cluster
cooperatively around the cell nucleus) or cancerous (in which case they are
apathetically sprawled across the cell).  The process is highly accurate,
works much more quickly than traditional techniques, and does not require
the usual pre-treatment of cells with chemical reagents or fluorescent
molecules.  Co-author Bob Naviaux of UC-San Diego added the biocavity las
er technique also has the potential to rapidly identify the in-between
states of stem cells as they transform into their final identities.  (Also
see Sandia News release at http://www.sandia.gov/news-center)

NO SPLASH ON THE MOON.  Sidney Nagel's lab at the University of Chicago has
explored the behavior of liquid drops---how and when they fall from a
faucet---granular materials, crumpling, and other
everyday-but-difficult-to-explain phenomena.  At the APS meeting, Nagel's
graduate student, Lei Xu, revealed a surprising discovery concerning one of
the commonest physical effects: the splash a liquid drop makes when it
strikes a flat surface.  Under ordinary atmospheric conditions a liquid
drop will flatten out on impact, splay sideways, and also raise a
tiara-like crown of splash droplets.  Remove some of the ambient
atmosphere, and surprisingly the splash becomes less.  At about one-fifth
atmosphere the splash disappears altogether, leaving the outward going
splat but no upwards splash (see movie at kauzmann.uchicago.edu ). 
Apparently it is the presence of the air molecules that give the impacting
liquid something to push off of; remove the surrounding atmosphere, and the
splash stops

---