PHYSICS NEWS UPDATE
                                        
The American Institute of Physics Bulletin of Physics News
Number 772  April 5, 2006  by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                
LIQUID FLOWING UPHILL; MIGHT BE USED TO COOL CHIPS.  In a phenomenon
known as the "Leidenfrost effect," water droplets can perform a
dance in which they glide in random directions on a cushion of vapor
that forms between the droplets and a hot surface. Now, a
US-Australia collaboration (Heiner Linke, University of Oregon,
linke{at}uoregon.edu) shows that these droplets can be steered in a
selected direction by placing them on a sawtooth-shaped surface.
Heating the surface to temperatures above the boiling point of water
creates a cushion of vapor on which the droplet floats.  The
researchers think that the jagged sawtooth surface, acting as a sort
of ratchet, redirects the flow of vapor, creating a force that moves
the droplet in a preferred direction. The droplets travel rapidly
over distances of up to a meter and can even be made to move up
inclines. This striking method for pumping a liquid occurs for many
different liquids (including nitrogen, acetone, methanol, ethanol
and water) over a wide temperature range (from - 196  to + 151 C).
A practical application of this phenomenon might be to cool off hot
computer processors.  In a concept the researchers plan to test,
waste heat in a computer would activate a pump moving a stream of
liquid past the processor to cool it off. Such a pump for coolants
would need no additional power, have no moving parts, and would
spring into action only when needed, when the processor gets warm.
(Linke et al., Physical Review Letters, upcoming; extensive visuals
and explanations at http://www.uoregon.edu/~linke/dropletmovies/ )

ENTANGLED PHOTON HOLES.  In some semiconductor devices, such as
light-emitting diodes, an applied voltage can dislodge electrons
from some atoms, leaving behind a hole which behaves in some
situations as if it were a positively charged particle in its own
right.  A "current" of holes can move through the material and the
holes can recombine later with electrons to produce light.  In very
loose analogy, James Franson (Johns Hopkins) suggests that photonic
holes might be created; a photon hole, to give one example, would be
a place in an otherwise intense laser-beam wavefront where a photon
had been removed (by passing the laser beam through vapor, for
instance).  Not only can there be photon holes, Franson
(443-778-6226, james.franson{at}jhuapl.edu) suggests, but the holes can
be entangled, meaning that their quantum properties would be
correlated, even if far apart from each other.  Such entangled
photon-holes would be able to propagate through optical fibers just
as well as entangled photons, but might be even more robust against
the decoherence (the undoing of the quantum correlations) that
plagues present efforts to establish quantum information schemes.
Franson expects to do put his idea to experimental test in the next
few months.  (Physical Review Letters, 10 March 2006)

SUNLIGHT ON A CHIP.  A new LED design employs a handy combination of
light and phosphors to produce light whose color spectrum is not so
different from that of sunlight.  Light emitting diodes (LEDs)
convert electricity into light very efficiently, and are
increasingly the preferred design for niche applications like
traffic and automobile brake lights.  To really make an impression
in the lighting world, however, a device must be able to produce
room light.  And to do this one needs a softer, whiter, more color
balanced illumination.  The advent of blue-light LEDs, used in
conjunction with red and green LEDs, helped a lot.  But producing
LED light efficiently at blue, red, and yellow wavelengths is still
relatively expensive, and an alternative approach is to use
phosphors to artificially achieve the desired balance, by turning
blue into yellow light.  Scientists at the National Institute for
Materials Science and at the Sharp Corporation (in Japan) have now
achieved a highly efficient, tunable white light with an improved
yellow-producing phosphor (see figure at
http://www.aip.org/png/2006/257.htm ).  Their light yield is 55
lumens per watt, about twice as bright as commercially available
products operating in the same degree of whiteness.  (Xie et al.,
Applied Physics Letters, 6 March 2006; contact Rong-Jun Xie,
xie.rong-jun{at}nims.go.jp)

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