Download 2006 Term 3 No 1

5. Physics News from the Web
Items selected from the bulletins of the IOP and the American Institute of Physics
a)
Liquid Flowing Uphill; Might be used to Cool Chips (check out the videos)
b)
Sunlight on a Chip.
c)
A Submersible Holographic Microscope.
a)
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, [email protected]) 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/ )
b) 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, [email protected])
c) A Submersible Holographic Microscope. A new device allows scientists to form 3D images of tiny
marine organisms at depths as great as 100 m. The device allows the recording of behavioral characteristics
of zooplankton and other marine organisms in their natural environment without having to bring specimens to
the surface for examination.
Scientists at Dalhousie University in Halifax, Canada, used the hologram arrangement originally invented by
Denis Gabor: light from a laser is focused on a pinhole that acts as a point source of light if the size of the
hole is comparable to the wavelength of light. The spherical waves that emanate from the pinhole illuminate
a sample of sea water. Waves scattered by objects in the sea water then combine at the chip of a CCD camera
with un-scattered waves (the reference wave) from the pin hole to form a digitized interference pattern or
hologram. The digital holograms are then sent to a computer where they are digitally reconstructed with
specially developed software to provide images of the objects. The Dalhousie researchers packaged their
holography apparatus in such a way that the laser and digital camera parts are in separate watertight
containers, while the object plane is left open (see figure at http://www.aip.org/png/2006/255.htm ).
One difficulty was to get container windows of optical quality that are thin enough for high resolution
imaging but thick enough to resist sea pressure. The new submersible microscope can also record the
trajectories of organisms in the sample volume so that movies of the swimming characteristics of micron size
marine organisms can easily be produced. Holograms with 1024 x 1024 pixels can be recorded at 7 to 10
frames/s. This requires a large bandwidth for data transmission to a surface vessel and was accomplished
with water tight Ethernet cables. Imaging volumes can be several cubic centimeters depending on the desired
resolution. The Gabor geometry allowed the Dalhousie researchers to design a very simple instrument
capable of wavelength limited resolution of marine organisms in their natural environment. Past generations
of submersible holographic microscopes had lower resolution, weighed several tons, had to be deployed from
large ships, and used high-resolution film as the hologram recording medium. This meant that only a small
number of holograms could be recorded. In contrast, the Dalhousie instrument only weighs 20 kg, can be
deployed from small boats or even pleasure vessels, and can record thousands of holograms in a few minutes
so that the motion of aquatic organisms can be captured in detail. (Jericho et al., Review of Scientific
Instruments, upcoming article; contact M.H. Jericho, Dalhousie University, [email protected], and also
the Universidad Nacional de Columbia)