Paradox of the Cyrano among bats solved after 58 years
The extremely long nose of a bat from the remote rainforests of South
East Asia has perplexed scientists ever since the species was first
described under the scientific name "paradoxolophus" ("paradoxical
crest") 58 years ago.
Now, scientists from Shandong University in China, Virginia Tech in the United States, and the Vietnamese Academy
of Sciences have presented results demonstrating that this long nose
can be predicted exactly from its impact on the ultrasonic beams the
bats emit. Using computer methods similar to the ones that transform
the characters in animated movies, the scientist conducted a
"Pinocchio experiment" creating a whole set of modified nose lengths
around the value encountered in nature. Some of these modifications
"cosmetically shortened" the bat's nose whereas others made it even
longer. By predicting the width of the ultrasonic beam for each of
these nose jobs with a computational method, the scientist found that
the natural nose length has a special value: Nose shortening caused
significant loss in ultrasonic focus, whereas artificially elongated
noses provided only negligible additional benefits. Hence, this
unusual biological shape can be predicted accurately from its physical
function alone which makes it a clear-cut example for how physical
forces can determine the outcome of evolution.
***
LD12022
Galactic Positron Annihilation Not a Dark Matter Signal
The intense flux of gamma rays created by the annihilation of
electrons with positrons - their anti-matter counterparts -
from the inner part of our Galaxy has been called a great mystery
because of its spatial distribution, and has been evoked by
cosmologists as evidence of a signal of dark matter. In a new
paper in the Physical Review Letters, however, scientists at
the University of California, San Diego and the Claremont Colleges
show that the observed distribution of gamma rays is entirely
consistent with a less exotic explanation - positrons emitted
by the radioactive decay of nickel, titanium and aluminum,
created in the end-of-life supernova explosions of stars more
massive than our Sun. Contrary to the key assumption made in
the dark matter hypotheses that positrons annihilate close to
where they were born, within about a light-year, Drs. Lingenfelter,
Higdon, and Rothschild show that the positrons, like the well-
measured cosmic-ray electrons of the same energy, travel large
distances of over a thousand light-years before they annihilate.
This large distance is dictated by the interaction of the positrons
with magnetic fluctuations, which the authors show are very weak
throughout most of the Galaxy. With such propagation the authors
predict a spatial distribution of annihilation gamma rays that
matches the distribution peaked towards the inner Galaxy, as
recently observed by the INTEGRAL gamma-ray satellite. They
further show that this propagation explains other basic spectral
properties of the annihilation radiation seen by INTEGRAL. Thus,
the authors show no new and unexplained signal for dark matter
is necessary to explain the observed gamma rays.
***
ECR1044E
Microfluidic Mixers
There are many cases where improved methods of fluid mixing are needed, especially in
the small, confining geometries present in microfluidic reactors. In such small cavities it
is notoriously difficult to induce the turbulence that creates efficient mixing. One
approach is to fabricate electromechanical mixing cells or micron-size stir bars, but we
have developed a simpler, more robust approach that uses commonly available magnetic
particles. These particles can be any size Ð from nanoparticles to traditional
micropowders Ð and the concentration required in the fluid is quite low. When a special
type of magnetic field (which we call a "vortex" field) is applied to the particle
suspension the particles chain into countless stir bars that whirl around as rapidly as 1000
times each second, mixing every corner of the fluid volume. These self-assembled stir
bars are strange actors: if the vortex field is altered to make them whirl around faster the
mixing doesn't get stronger! Instead, stronger mixing is achieved by increasing the
strength of the vortex field. This is just backwards from what one would expect, and we
have traced this strange behavior to the volatile, adaptive nature of the particle chains.
Finally, as a practical matter, the vortex field can be created by small magnetic coils
placed around the mixing cell, and when the mixing is over, the particles can be retrieved
with a magnet for reuse.
