Number 740, August 5, 2005
by Phil Schewe and Ben Stein
Tracking Fluid Flow Inside a Porous Material
Tracking fluid flow inside a porous material can now be performed
with remote MRI viewing. MRI is an important means for sub-surface
viewing of soft objects like biological tissue or moist in solid
things like rice grains. In a new approach, scientists at Lawrence
Berkeley National Laboratory and UC Berkeley in collaboration with
Schlumberger-Doll Research have developed a style of MRI that can be
used to see how a gas flows through a porous rock, an experimental
tool with possible applications in oil exploration, in situ
monitoring of natural and manmade structures, and industrial
processes where the flow of a fluid through an opaque material is
important.
To accomplish this, Josef Granwehr
(joga@waugh.cchem.berkeley.edu) and Yi-Qiao Song (ysong@SLB.com) and
their colleagues use not one radio coil but two, separated in
space. In MRI it is customary to cause atomic nuclei in a sample
(given an orientation by an external magnet) to be disturbed by
magnetic waves induced by the coil. The same coil is used a moment
later to detect the radio waves given back out by the target nuclei,
thus providing information about their whereabouts.
In the Berkeley
setup, one coil surrounds the porous sample and can, in combination
with magnetic field gradients, selectively disturb nuclei of the
fluid in a voxel (a tiny volume element) anywhere in the sample,
while a second independent coil, positioned at the exit of the
sample, can detect the emerging material. The first coil is
therefore used to tag certain nuclei at a given point in time, while
the second coil is used to record the time of flight of the affected
nuclei as they leave the sample.
Possessing location and velocity of
any portion of the gas allows researchers, in effect, to
look inside the rock and watch its flowing and unfolding. One can
trade off the minimum detectable partial pressure of the target
nuclei (tens of millibar up to one bar) for time resolution (tens of
microseconds to milliseconds) or vice versa.
(Granwehr et al.,
Physical
Review Letters, upcoming article)
A New Kind of Nanophotonic Waveguide
A new kind of nanophotonic waveguide has been created at MIT,
overcoming
several long-standing design obstacles. The resultant device might
lead to single-photon, broadband and more compact optical
transistors, switches, memories, and time-delay devices needed for
optical computing and telecommunications.
If photonics is to keep up with electronics in the effort to produce
smaller, faster, less-power-hungry circuitry, then photon
manipulation will have to be carried out over scales of space, time,
and energy hundreds or thousands of times smaller than is possible
now. One or two of these parameters (space, time, energy) at a time
have been reduced, but until now it has been hard to achieve all
three simultaneously. John Joannopoulos and his MIT colleagues have
succeeded in the following way. To process a photonic signal, they
encrypt it into light waves supported on the interface between a
metal substrate and a layer of insulating material. These waves,
called surface plasmons, can have a propagation wavelength much
smaller than the free-space optical wavelength. This achieves one of
the desired reductions: with a shorter wavelength the spatial
dimension of the device can be smaller.
Furthermore, a
subwavelength plasmon is also a very slow electromagnetic wave. Such
a slower-moving wave spends more time "feeling" the nonlinear
properties of the device materials, and is therefore typified by a
lower device-operational-energy scale, thus achieving another of the
desired reductions. Finally, by stacking up several insulator
layers, the slow plasmon waves occupy a surprisingly large frequency
bandwidth. Since the superposition of waves at a variety of
frequencies can add up to a pulse that is very short in the time
domain, the third of the desired scale reductions is thereby
achieved.
Reducing energy loss is another great virtue of the MIT device. The
plasmons are guided around on the photonic chip by corrugations on
the nano-scale. In plasmonic devices the corrugations have usually
been in the metal layer; this has always led to intractable
propagation losses. However, in the MIT device they reside in the
insulator layer; this, it turns out, allows for a drastic reduction
of the losses by cooling.
(Karaliset al., Physical Review
Letters, 5 August; contact Aristeidis Karalis, aristos@mit.edu)
Possible New Planets in Our Solar System
Possible new planets in our solar system have been spotted
recently. Reservations about claiming new planets arises not from
anything to do with the observations, but with semantics; there is
no universally accepted definition for planet. Even Pluto is not a
planet according to some scientists. The two newest planet
candidates are the latest residents to be discerned in the Kuiper
Belt, the zone of debris material beyond the orbit of Neptune.
Two
earlier specimens go by the name of Sedna
(PNU 677) and Quaoar
(PNU 608).
One of the new objects,
discovered by astronomers at the Sierra Nevada Observatory in Spain,
is called EL61, with an orbital radius of about 51 AU (1 AU, or
astronomical unit, is equal to the Earth-sun distance) and a size
about 2/3 that of Pluto (which itself orbits at a distance of about
32 AU). The other object is called UB313 and was spotted by
astronomers at the Palomar observatory in California and the Gemini
telescope in Hawaii. It orbits at a distance of 97 AU and has an
estimated size larger than Pluto
(see NASA press release).
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