Sunday, April 09, 2006

Liquid Flowing Uphill to Cool Chips, Entangled Photon Holes, Sunlight from LED

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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, 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;)

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, forinstance). Not only can there be photon holes, Franson (443-778-6226, 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. 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,


The American Institute of Physics Bulletin of Physics News
Number 772 April 5, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

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