Number 653, September 12, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
Nanotube Velcro
Joining two or more nanochips, such as nano-electromechanical systems
(NEMS), can be done by welding or gluing or with tiny nuts and bolts.
But what if you could gently just fasten them the way fabrics are fastened,
with velcro? Conventional velcro fastening works by pairing one patch
of mm-scale hooked protuberances with a patch of looped protuberances.
In the microscopic version, both patches would bristle with carbon nanotubes,
grown upright except for a hook on the top end. David
Tomanek and his colleagues at Michigan State (517-355-9702) are
studying how to make nano-velcro work (see
movies). His calculations so far show that the nanotubes will remain
in place on each separate substrate (they can be grown on selective
pieces of surface geometry using lithographic-like patterning techniques)
and will also remain locked together when mated with its counterpart
on another substrate. A typical application for nano-velcro would be
to fasten a diamond coating onto specific parts of a metal surface.
(Berber et al., Physical Review
Letters, upcoming article; co-authors, Savas
Berber, and Young-Kyun Kwon)
Good Vibrations Help a Frog Locate Tasty Prey
Living in southern Africa, the aquatic frog Xenopus catches insects
by detecting critters' vibrations on the water surface. Not able to
see well in a liquid environment, the frog gets a wealth of information
from the water waves that insects produce as they slosh around. The
waves tell Xenopus the direction in which the insect is located. They
even give the frog a general idea of the type of insect that is making
the waves. To detect the water waves on its skin, the frog has about
180 receptors known as "lateral-line" organs, which are found
on the skin along both sides of the body, around the eyes, and also
on the head and neck. Now, researchers in Germany (Leo
van Hemmen, TU Munich, +49-89-289.12362) have developed a simple
model that explains how the lateral-line organs enable Xenopus to locate
and classify its prey. Strikingly, the model suggests that the frog
can reconstruct the shape of the water wave (its "waveform")
from limited information, namely the movement of water recorded by the
180 simple sensory organs. In the frog, water gets deflected by 4-8
flag-like structures (called "cupulae") in the lateral line
organs. Each deflection stimulates nearby hair cells to generate electrical
spikes that are synchronized in time with the deflection. The timed
electrical spikes from the 180 sensory organs, the researchers show,
contain enough information for the frog to "estimate" the
shape of the water wave pretty accurately. This is true even if some
of the lateral-line organs are not functioning properly. Furthermore,
they show how the frog can localize and distinguish between two different
water waves coming simultaneously from two insects in different directions.
This model may also be applicable to the mechano-sensory systems of
other animals, such as crocodiles (Soares, Nature,
16 May 2002), which have similar receptor organs (Franosch et al.,
Physical Review Letters, upcoming).
Horizontal Brazil Nut Effect
A new twist on the Brazil-nut effect appears to be a good way to harvest
large particles from a granular mixture, according to recent experiments
and simulations performed at the University of Texas at Austin. The
Brazil-nut effect is an odd but well-known phenomenon in agitated granular
mixtures. Depending on the conditions, shaking containers filled with
grains of various sizes will cause the larger grains to rise to the
top of the mixture (Update
132), or sink to the bottom. The Texas researchers (contact: Sung
Joon Moon, 609-258-2977), however, showed that they could also control
the horizontal distribution of large grains by using kinks that spontaneously
arise in granular layers for sufficiently large container accelerations.
A kink separates two regions oscillating with opposite phase: the granular
layer on one side of a kink is moving up while the layer on the other
side is moving down. Larger particles flow from the two oscillating
regions and collect in the kink. The researchers can control the location
of a kink by adjusting the driving signal, and harvest the large grains
by sweeping the kink to one side of the container. The research shows
that trapping results from avalanches that form at the kink as falling
fluid-like regions move past rising, effectively solid, regions. The avalanches lead to internal convection rolls that carry the large particles toward a kink. The horizontal Brazil-nut effect may eventually lead to new commercial methods for segregating granular material by size. (S. J. Moon et al., Phys.
Rev Lett., upcoming)
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