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| Roger Bagula... |
 Posted: Sat Jun 13, 2009 11:36 am |
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http://focus.aps.org/story/v23/st20
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Previous Story / Volume 23 archive
Phys. Rev. Lett. 102, 235504
(issue of 12 June 2009)
Title and Authors
12 June 2009
Stringy Soot
branched and straight soot particles
Phys. Rev. Lett. 102, 235504 (2009)
Shape shifters. The sooty aggregates created in a flame are often highly
branched (top), but some flame conditions create a few that are much
straighter (bottom). Their special properties could affect products like
toner and tires and also have effects in the atmosphere.
The soot from a flame consists mostly of tiny, bush-like particles with
many branches, whose shape is predicted by theory. In the 12 June
Physical Review Letters, however, researchers report that some soot
particles are straighter. These rare, oddly shaped particles have
optical properties that could be important for industrial
flame-generated materials like pigments and printer toner, and they
might also affect the amount of sunlight reflected by the atmosphere.
Soot particles grow inside a flame when tiny, carbon-rich spheres stick
together to form larger, tenuous aggregates. As they grow, the particles
take on a characteristic branched shape because two colliding clusters
are most likely to attach at their protruding "fingers."
These bushy shapes are conveniently described as fractals--geometric
objects whose mass grows as a fractional power of their linear size,
rather than the third power that characterizes ordinary solids like
spheres and cubes. Theory predicts that virtually all clusters should
have a fractal dimension very close to 1.8, and past experiments agree.
But a collaboration led by Hans Moosmüller of the Desert Research
Institute in Reno, Nevada, found many clusters with a much lower
dimension, characteristic of a more rod-like shape.
As a graduate student, Rajan Chakrabarty, now a postdoctoral fellow in
the Reno lab, made soot in a standard premixed oxygen-ethylene flame. He
sprayed the particles with charges from a radioactive source, and then
passed the charged soot through an apparatus that selects particles of a
specific size and charge. Finally, he used an electron microscope to
measure the shapes of the selected particles. Surprisingly,
doubly-charged particles about half a micron in size had a fractal
dimension that ranged from about 1.2 to 1.5, far from the usual value of
1.8. The researchers suggest that the requisite two electrons can more
easily cohabit these small rod-like particles because they can stay
farther apart than on a more compact particle.
But how did these anomalous particles, which they found make up about 3%
of the total, arise in the first place? The researchers speculate that
slightly elongated particles pick up positive and negative charges in
the flame, causing them to align with random electric fields. Moving
along electric field lines, they aggregate to form even longer
particles. In support of this model, the team saw less of the stringy
soot when the flame was hotter, perhaps because the electric field has
less influence when the particles bounce around more rapidly. The
researchers plan to further test their idea by adding an external
electric field, with the hope of making even more of the rod-like particles.
Other researchers, Chakrabarty notes, have found that such elongated
clusters are blacker because they scatter less light than their bushy
cousins. So controlling their numbers within soot could be useful in the
production of many flame-generated products, he says. These products
include the "carbon black" that is added to car tires and used as a
pigment in toner for printers and copiers. Atmospheric soot affects the
amount of solar radiation absorbed by the earth, so the variety of
particle shapes is important to atmospheric scientists as well.
Many previous experiments have found that the entire population of soot
particles has a dimension of 1.8, notes Christopher Sorensen of Kansas
State University in Manhattan, Kansas. The new work is "alerting us to
the fact that the ensemble may have various sub-populations that have
different morphologies," he says. "It's good to be aware that there's a
deeper story than what we've been saying for 20 or 30 years."
--Don Monroe
Don Monroe is a freelance science writer in Murray Hill, New Jersey.
Low Fractal Dimension Cluster Dilute Soot Aggregates From a Premixed Flame
Rajan K. Chakrabarty, Hans Moosmüller, W. Patrick Arnott, Mark A. Garro,
Guoxun Tian, Jay G. Slowik, Eben S. Cross, Jeong-Ho Han, Paul
Davidovits, Timothy B. Onasch, and Douglas R. Worsnop
Phys. Rev. Lett. 102, 235504
(issue of 12 June 2009)
Previous Story / Volume 23 archive
bottom-rule
ISSN 1539-0748 CODEN: PRFHAQ © 2009 by The American Physical Society.
All Rights Reserved.
APS Journals: PROLA PRA PRB PRC PRD PRE PRST-AB PRST-PER PRL RMP VJ |
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