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Soot: Giver and Taker of Light
The complex structure of soot greatly influences the optical effects seen in fires
Christopher R. Shaddix and Timothy C. Williams
rom earliest childhood, people are
fascinated with the flickering yellow glow of candle flames and burning
logs. However, few of us realize, even
in adulthood, that soot—a material that
epitomizes blackness—is behind that
warm light. The 19th-century physicist
Michael Faraday put it well when he
said, “You would hardly think that all
those substances which fly about London, in the form of soot and blacks, are
the very beauty and life of the flame.”
Indeed, if it weren’t for the presence
of clouds of tiny soot particles within
these fires, they would appear blue,
like those on a well-operating gas
stove, which give off considerably less
visible and thermal radiation.
We can vividly illustrate this phenomenon in the laboratory. Ethylene,
for example, burning in air yields
plenty of soot and a bright yellow
flame, whereas the same fuel diluted
with nitrogen to suppress the formation of soot gives a much dimmer
blue flame. The cerulean color arises
from the highly excited products of
the combustion reaction. These molecules emit light in discrete spectral
bands that correspond to the excitation levels of their electrons. One of the
primary molecules is of a class called
radicals because they have unpaired
electrons in their outer shells. This radChristopher R. Shaddix is a principal member of the
technical staff at Sandia National Laboratories in
Livermore, California. He received his Ph.D. in mechanical and aerospace engineering from Princeton
University in 1993. Timothy C. Williams earned
his doctorate in 2000 from the Wolfson School of
Mechanical and Manufacturing Engineering of
Loughborough University in the United Kingdom.
He is a postdoctoral appointee at Sandia. Address
for Shaddix: Combustion Research Facility, Sandia
National Laboratories, 7011 East Avenue, Livermore, CA 94550. Internet: [email protected]
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American Scientist, Volume 95
ical is made from a single carbon and a
single hydrogen, denoted CH*, which
emits light at a wavelength of about
431 nanometers—squarely at the blue
end of the visible spectrum.
In contrast, the light emitted from soot
extends across the visible wavelength
range and into the near-infrared. Cool
soot looks black, but while it’s in a flame,
the heat liberated from the surrounding
combustion reactions makes these particles incandesce like so many tiny lightbulb filaments. Gas molecules cannot
absorb or emit such large amounts of
energy across a range of wavelengths.
As Faraday correctly surmised, it is
the solid nature of soot that gives most
flames their distinctive luminescent
qualities.
A combination of factors explains
why the radiation from soot typically
appears yellow. Part has to do with
way the human eye responds to the
spectral variation in soot emissivity
(the ratio of the light given off to the
light radiated by a perfect black body).
Another reason has to do with the typical temperature of soot (around 1,400
degrees Celsius). Hotter soot appears
whiter and cooler soot will be redder,
just as is true for heated metal. Thus
soot often provides some pleasing optical effects. But these radiant abilities
can also lead accidental fires to grow
swiftly out of control. That is, although
soot is the offspring of flame, it alters
the nature of its creator.
Dante’s Pools
At 6:00 a.m. on Sunday, December 11,
2005, the Buncefield oil-storage depot,
located just outside London, erupted
in a plume of fire and hot gases, the
biggest conflagration that country had
ever seen in peacetime. Forty-three
people were injured by a series of ex-
plosions, and 2,000 were evacuated
from the surrounding area. Twenty
large fuel-storage tanks burned, and it
took firefighters several days to quell
the blaze. Meanwhile, huge clouds of
black smoke billowed into the skies.
This calamity provided a dramatic
demonstration of the complex role of
soot in fires. Photographs of the flames
emanating from the burning gasoline
and aviation fuel showed the bright
yellow and orange colors characteristic
of hot soot particles. From a distance,
however, the flames were dwarfed by
the sooty smoke cloud, which blocked
out the sun in the local area and could
be seen wafting as far as France in satellite images.
Similar catastrophes have struck
other fuel depots around the world at
different times. And devastating fires
have taken place on a smaller scale
when fuel-laden planes and trucks
have crashed. Some noteworthy incidents in recent years have involved
automobile and train tunnels, particularly in the European Alps. In California, just east of San Francisco Bay,
a fuel-truck spill in 1982 turned the
Caldecott tunnel into an inferno.
The attention given to this class of
fires rose after the terrorist attacks of
September 11, 2001, in which large volumes of aviation fuel were deposited
and burned in the World Trade Center
Figure 1. Where there’s smoke, there’s fire.
Indeed, without smoke—or at least without
its main component, soot—fire would look
nothing like we know it. Hot soot within
flames incandesces and creates a warm, yellow glow. But this radiated heat can also feed
energy back into fires. Thus soot further amplifies large fires burning over pools of liquid fuel—such as this one at the Buncefield
oil-storage depot outside London—which
can quickly grow out of control.
© 2007 Sigma Xi, The Scientific Research Society. Reproduction
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AP Images/Hertfordshire Police
F
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2007 May–June
233
Figure 2. Both of these flames are burning ethylene gas. The difference is that the one on the
right has been diluted with nitrogen so that it does not form soot. The brightness of the flame
on the left comes from chains of microscopic soot particles numerous enough to emit a significant amount of heat; they incandesce like so many tiny light-bulb filaments. To obtain the
dimmer image on the right, the camera aperture had to be left open 30 times longer. (Except
where noted, all photographs are courtesy of the authors.)
tendency to form soot and smoke, and
their high levels of radiant heat transfer, all in evidence at Buncefield.
In battling such an emergency, responders have to make decisions that,
if wrong, can cost their lives. When deciding whether to risk putting fire-fighting personnel anywhere near a pool of
burning fuel, it’s critical to know how
The Royal Institution, London, UK/The Bridgeman Art Library
buildings, contributing to their collapse.
The oil-well fires during the first Iraqi
war were another high-profile example.
The fires at Buncefield were of a type
referred to in scientific discussions as
“pool fires,” meaning that the flames
were intimately coupled to pools of
liquid fuel. Pool fires are known for the
difficulty of extinguishing them, their
Figure 3. British physicist Michael Faraday was one of the first scientists to appreciate the role
of soot in flames. A popular lecturer, Faraday often gave talks to packed audiences in which he
would perform dramatic demonstrations, as depicted in this lithograph from 1856. One series
of lectures that Faraday gave for many years was on the chemistry of a burning candle. He was
able to show his audience that a flame without soot produces a very dim glow. Faraday concluded, “It is to this presence of solid particles in the candle-flame that it owes its brilliancy.”
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American Scientist, Volume 95
long the walls of adjacent fuel-storage tanks will last when exposed to
the intense heat. This calculation is,
in part, a radiant heat-transfer issue,
which is fundamentally controlled by
the production of soot and smoke. In
pool fires with hydrocarbon fuels, the
thermal radiation from soot dominates
heat transfer, but conversely, clouds of
cold soot particles in smoke insulate
the surroundings from thermal radiation. Hence it is important for scientists
to understand how exactly soot forms
within fires and becomes smoke.
Combustion requires fuel, which in
the cases of interest is most often made
from long, complex chains of carbon
and hydrogen atoms. When a flame is
lit, the heat breaks apart these hydrocarbons in a process called pyrolysis.
The smaller chunks that result are often
radicals, highly inclined toward chemical reactions, in particular oxidation:
Oxygen combines with the carbon and
hydrogen radicals to produce carbon
dioxide and water, releasing heat in
the process.
However, some of the radicals react with one another, rather than with
oxygen, forming rings of carbon called
polycyclic aromatic hydrocarbons. These
newly formed compounds continue to
grow into carbon-rich lattices and then
into full-fledged particles, which agglomerate into long chains that resemble
strings of beads. As these soot masses
travel upward inside a flame, they react with oxygen molecules, which can
break off pieces and cause the particles
to incandesce more brightly, creating the
flame’s bright yellow glow. Whether or
not the soot will be fully burned in this
way (completely transformed into carbon dioxide and water) before leaving
the flame depends on the type of fire. If
not completely burned, the residual soot
is released as smoke.
Tall, buoyant flames with relatively
lazy mixing of the fuel and air promote
the formation of soot, because there
is ample residence time within these
flames for fuel molecules to pyrolyze
and then recombine. The larger hydrocarbon compounds made in this way
eventually come together and form carbonaceous particles.
The thermal radiation given off by
this soot is important to sustaining
(and initially growing) pool fires. Indeed, the presence of soot allows up to
50 percent of the energy in a flame to
be transferred back to the pool of fuel,
where it acts to vaporize the liquid and
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sustain the reaction. In large fires, thermal radiation from the flames provides
almost all of the energy transferred to
the liquid fuel. Ironically, it is the radiant emission given off by the hot soot
that results in the thermal quenching
of flames at the top and around the periphery of the fire, leading to the release
of the cool, light-absorbing soot. The
radiant heat transfer exceeds 100 kilowatts per square meter for pools 2 meters across but is less for larger pools,
which tend to produce thick layers of
smoke around the fire. (In comparison,
solar radiation on a clear, dry day with
the sun directly overhead is 1 kilowatt
per square meter.)
As important as they are in accidental fires, soot particles also influence
people’s lives in other ways. Commercial combustion products, called carbon blacks, have long been ingredients
in rubbers and inks, and soot was the
original source of the soccer-ball-shaped
carbon molecules called fullerenes, as
well as carbon nanotubes, both of which
are of great interest in the field of nanotechnology. Soot plays the benefactor in
boilers and furnaces that rely on flame
radiation to transfer heat to the walls
(to generate steam, for example), but
the same mechanism makes these particles harmful for internal-combustion
engines, where such heat losses decrease
efficiency and require that high-temperature materials be used.
Soot emissions to the atmosphere
from industrial smokestacks and automobile tailpipes contribute to the
formation of light-absorbing haze and
have recently received attention for
their influence on climate. This connection arises because the more sunlight
that is reflected back out into space, the
cooler the Earth becomes, whereas the
more sunlight absorbed in the lower
atmosphere or on the surface, the more
the planet is warmed. Hence the effect
of soot particles on climate strongly
depends on their optical properties,
specifically how they scatter and absorb sunlight.
Most climatologists believe that atmospheric soot enhances global warming, but some studies have suggested
that soot can have a cooling effect, too,
because these particles can act as nucleation points for molecules of water vapor, allowing more sunlight-reflective
clouds to form. Soot also influences climate because it is deposited on snow
and ice surfaces, darkening them and
thus increasing their rate of melting.
www.americanscientist.org
fuel-pyrolysis zone
soot-inception zone
soot-growth zone
CO2
H2O
soot-oxidation zone
OH*
O2
H2
O*
C2H2
H*
HCO H*
O2
H2
O*
CO
OH*
C2H2
H2O
H*
H2
CO2
CH2*
C2H2
flame
sheet
fuel
air
C4H5*
C3H3*
H2
H*
C4H3*
air
Figure 4. Candles and oil lamps are called diffusion flames because the air and fuel are not
mixed before burning but rather meet and react through diffusion. (In contrast, premixed
flames, such as those in gasoline engines, have the fuel and air combined before ignition.)
Diffusion flames have distinct regions where different chemical reactions take place, as
shown in this schematic by different colors. The surface that encloses the entire flame, called
the flame sheet, is the location of highest temperature. Here, partially oxidized fuel fragments
are consumed in reactions with highly reactive chemicals, called radicals, especially oxygen
atoms (O*) and hydroxyl radicals (OH*). These two radicals are produced by the reaction of
hydrogen atoms (H*) with the O2 present in the air. Fuel, made of complex chains of hydrocarbons, first enters the fuel-pyrolysis zone, where it is heated to high temperatures and decomposes into fragments. Some of the fragments combine to form larger molecules, particularly
carbon-ring structures. Acetylene (C2H2) and some radical hydrocarbon molecules (denoted
with the * symbol) are key players in these chemical reactions. These molecules then flow into
the soot-inception zone, where they grow in size and polymerize. These chains form liquidlike soot-precursor particles (gray spheres), which have no internal structure and do not absorb
or emit visible light. However, as these precursor particles continue into the soot-growth zone,
they give up H2 gas and form solid, light-absorbing particles, which rapidly agglomerate into
clusters. These structures continue to grow through reactions with H* and acetylene. As the
soot aggregates are driven to the top of the flame, the soot-oxidation zone, they are consumed
by reactions with oxygen and hydroxyl radicals, shrinking in size as they give off energy. The
final combustion products, emitted from the flame sheet, are water and carbon dioxide.
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2007 May–June
235
Figure 5. A 5-meter-diameter fire burning aviation fuel at a Sandia National Laboratories test facility was one site of soot collection for the
authors (left). For smaller sources, such as an ethylene slot flame (middle), a small steel tube was used to collect soot at different heights. Aggregates of soot particles were trapped on fibrous grids, a close-up of which shows an area about 1,400 nanometers wide (right).
The absorptivity of cold soot is important in determining not only how
effectively it takes in light, but also
how effectively hot soot emits light, by
virtue of a principle called Kirchoff’s
law: The emissivity of a substance at
thermal equilibrium is always equal to
its absorptivity. Quantifying the optical properties of soot has proven to be
difficult, because it is such a heterogeneous substance. Its atomic bonding
is dependent on the environment in
which it is formed, thereby affecting
its optical properties. (Just think of the
contrast in optical properties between
diamond and graphite, both pure
forms of carbon.) Indeed, the quest for
a better understanding of this subject
has quite a long history.
A Dark Past
An appreciation of soot’s optical properties began, appropriately, in London
early in the industrial age, when the city
was choked with smoke from the widespread burning of coal. It was in this
era that Faraday carried out research on
flames. He soon understood the importance of the optical properties of soot.
Faraday’s knowledge of this subject was
built on the first extensive studies of
combustion initiated by his mentor at the
Royal Institution, Sir Humphry Davy,
when Davy was developing the coal
miner’s safety lamp in 1815. Faraday
was a popular lecturer, and for many
years during the Christmas holidays of
the mid-1800s, he gave a lecture series
for young audiences titled “The Chemical History of a Candle,” in which he
demonstrated that a flame without soot
produces a very weak glow. Faraday
clearly saw the irony: “Is it not beautiful
to think that such a process is going on,
and that such a dirty thing as charcoal
can become so incandescent?”
After Faraday, the optical properties of soot did not receive much atten-
Figure 6. Soot-precursor particles have practically no internal structure and resemble liquid droplets. They do not absorb or emit much light, making them appear faint in images (left). In contrast,
hardened soot particles have well-defined structures and interact strongly with visible and infrared light (right). Both micrographs show an area about 2,000 nanometers wide. (Images courtesy of
Richard A. Dobbins, from Physical and Chemical Aspects of Combustion: A Tribute to Irvin Glassman, edited by Frederick L. Dryer and Robert F. Sawyer, published by Taylor & Francis Books.)
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American Scientist, Volume 95
tion until the early 20th century, when
combustion-related research began to
develop as an active field. A number of
German investigators examined the radiation properties of soot in an attempt
to determine flame temperature and
to understand the contribution these
particles made to the transfer of heat in
furnaces and boilers.
In the United States in the 1930s, Hoyt
C. Hottel, a professor researching heat
transfer in furnaces at the Massachusetts Institute of Technology, improved
on the optical pyrometer, an instrument
much like a thermometer except that it
measures temperature without contacting the surface. The device uses a filter
to determine the amount of energy being radiated from a sample in a single
wavelength, or color, which is compared with a table of known temperature-color relations. Hottel developed
the first two-color device for measuring
the temperature of soot and demonstrated its superiority to conventional
single-color methods. (The advantage
is that one can determine temperature
from the ratio of the two light intensities, whereas a single-color instrument
needs to measure absolute amounts
and is thus difficult to calibrate.) Later
on, Hottel measured the angular distribution of light scattering by soot inside
a flame and in the early 1960s collected
some of the first images of soot particles
by transmission electron microscopy
(TEM), showing a highly agglomerated
structure of primary particles about 25
nanometers in diameter.
At about the same time, Roger C. Millikan, a researcher at General Electric,
measured the spectral variation of soot
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a
b
c
d
e
Figure 7. Laser imaging was used to examine the internal structure of the lower 50 millimeters of test flames, revealing the flame sheet (blue arrows),
the soot layer (yellow arrows) and polycyclic aromatic hydrocarbons that are products of combustion (red arrows). The size of the resulting soot
clusters, shown at bottom, depends on fuel type and flame shape. Shown here are kerosene (a), ethylene (b), methane (c), ethylene slot flame (d) and
methane slot flame (e). The images of the soot samples in a–d are each about 1,250 nanometers wide, whereas e is about 250 nanometers wide.
absorption above a horizontally flat
flame and also analyzed the atomic ratio of hydrogen to carbon in the soot.
He found that with increasing height
above the burner, this ratio decreased, as
did the spectral variation of optical absorption. This observation gave the first
concrete evidence that the absorption
properties of soot varied with position
within a flame and were linked to the
chemical composition of the soot.
In 1969, Adel F. Sarofim, a former
graduate student of Hottel’s who
joined him on the faculty of MIT, published the results of measurements of
the index of refraction of soot collected
from propane- and acetylene-fueled
flames. The index of refraction is the
Holy Grail of material optical properties, allowing the calculation of both
absorption and scattering of various
shapes of the material, including round
particles of a known size.
The index is a measurement of the
path that light will take within a particular material. The value is expressed as
a complex number, with the real component relating to the velocity of light
within the material relative to the speed
of light in vacuum. This component
also indicates how much a ray of light
will be bent, or refracted, when it passes
through the material. The imaginary
term indicates how much of the light is
lost due to absorption. Sarofim reported
the index of refraction of the soot he
measured to be 1.57 – 0.56i.
This result was an improvement
over past attempts to measure the opwww.americanscientist.org
tical properties of soot, and Sarofim
had the convenient result of finding a
similar refractive index for both fuel
sources and for a range of visible wavelengths. Also, a number of succeeding determinations produced values
close to Sarofim’s. As a consequence,
his value for the refractive index was
widely adopted in both combustionand fire-research communities as the
refractive index of soot, and it has
largely retained that status all the way
to the present.
A few investigators pointed out long
ago that the method Sarofim and others
had used to determine the index of refraction was subject to significant errors.
The technique required soot aggregates
to be pressed into a smooth surface, a
virtually impossible task to perform
with hard, agglomerated nanoparticles.
In 1979, Jay Janzen from Phillips Petroleum Company made the prescient
observation that not only were there
unavoidable errors in this approach,
but that the derived refractive-index
values were inconsistent with wellestablished measurements of the
amount of light a given volume of soot
particles extinguish (through absorption and, to a lesser extent, scattering).
Janzen performed measurements of the
spectral extinction of small, uniform
particles of carbon black and calculated
a refractive index that was quite different (2.0 – 1.0i). Unfortunately, this work
was published in a journal that was not
typically read by combustion and fire
researchers, so it went unnoticed.
Seeing the Light
For several years now, we have been
investigating large-scale pool fires,
both experimentally and numerically,
because of the risk they pose during
transport accidents. In the course of
developing and validating our computational models, we’ve acquired a
keener appreciation for soot in determining how fires spread. In particular,
knowledge of the soot concentration,
temperature and optical properties
within these fires is required to quantify the amount of heat transferred.
Over the past 15 years, several investigators have shown through TEM and
optical measurements that when soot
begins to form in a flame, it first develops liquid-like precursor particles, which
have little absorptivity. These tarry particles contain a relatively high amount
of hydrogen and, unlike the more commonly investigated solid soot particles,
they coalesce into a single, larger particle
when they collide. Those larger blobs in
turn lose some of their hydrogen, and at
a critical ratio of hydrogen to carbon of
around 0.2, they solidify into hard carbonaceous particles, which agglomerate
into clusters upon colliding.
Researchers from the University of
Naples have shown that the precursor particles first only absorb light in
the ultraviolet region of the spectrum
and then, as the polycyclic aromatic
hydrocarbons grow in size and lessen
the energy required to put the electrons into an excited state, the optical absorptivity extends into visible
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2007 May–June
237
wavelengths and ultimately into the
infrared. At this point, in other words,
the soot becomes “black,” absorbing
all visible wavelengths.
One unanswered question was
whether the solid carbon particles
formed in this way become denser as
they travel upward in the flame. This
development could lead to variations
in the soot nanostructure and thus its
optical absorptivity. To investigate the
possibility of such soot densification, we
worked with collaborators at the University of Illinois at Chicago, at Drexel
University and at Sandia National Laboratories to collect soot particles onto
grids of loosely packed fibers that can
be examined with a microscope. We extracted samples from different heights in
steady and unsteady laboratory flames
burning ethylene, a simple hydrocarbon
fuel gas, and in a large-scale turbulent
pool fire burning JP-8, a type of fuel used
mostly for powering military jets. We
then analyzed these soot samples using
high-resolution TEM, which resolves the
structure of the resultant particles below
the nanometer level.
We were not able to study soot precursors in the same way, because tar
components associated with these
particles were also deposited on the
collection grids. High-resolution TEM
exposes the samples to high fluxes of
electrons, which caused these tars to
spread out over the soot particles, obscuring their structure.
For mature soot, the TEM images we
and other researchers have obtained
show that, internally, the particles con-
sist of many thin layers of flat carbon
structures arranged in a spiral pattern,
vaguely similar to a rolled-up scroll.
The particles seem to start at one or
several nucleation sites, with growth
curling around each of them. The
build-up is made from layers of thousands of disordered pieces. Soot thus
differs from other forms of carbon,
such as fullerenes or graphite, which
have highly ordered structures. The
presence of hydrogen and occasionally
a little oxygen in soot accounts for at
least some of this irregularity.
To quantify the characteristics of the
carbon layers in our high-resolution
TEM images, we enlisted the help of Árpad Palotás of the University of Miskolc
in Hungary, who had earlier modified
a commercial image-analysis program
for this very purpose. Palotás developed
this technique while he was a graduate
student working under Sarofim. It turns
out that the most important characteristic of soot in determining its optical absorptivity is the spacing of adjacent carbon layers, which is an indirect measure
of the amount of ordering in the particle.
When there is more order, carbon atoms
are more tightly bound and lose some
of their associated hydrogens. As a result, the carbons become more metallike, meaning their electrons require less
energy to move, so they absorb longer
wavelengths of light.
Remarkably, we found that the mean
spacing between the carbon layers was
nominally the same—from 3.47 to 3.57
angstroms—for all of our soot samples.
For comparison, the interlayer spacing
of pure graphite, the most ordered form
of carbon after diamond and fullerenes,
is 3.35 angstroms. The results from this
study imply that the index of refraction
is essentially the same for soot from different flames, once the sooty material
has hardened into solid particles. Thus,
historical studies of the variation in soot
absorptivity as a function of height in
flames can now be understood as reflecting the variable contribution from
precursor particles, which become progressively less abundant as one samples
higher and higher within a flame. At
some height, only solid soot particles
are present in the flame or in the emitted smoke, and the absorptivity of the
soot stays constant.
The results from our investigation
of soot nanostructure begged the longstanding question of just what is the
correct value for the index of refraction
of solid, carbonaceous soot. To get at the
answer, we collaborated with investigators in Sandia’s fire-research program
to measure extinction coefficients (the
exponential factor by which light dims
as it passes through a material), from
which we could derive values for absorptivity and, in turn, the refractive
index. The samples were taken at different heights within laboratory flames
burning methane, ethylene and kerosene (a stand-in for jet fuel). Flames
used were of the standard variety or
were slot flames, where the plume is basically restricted to two dimensions. Slot
flames heat the fuel stream more slowly,
so soot would be expected to form later and mature less rapidly. The use of
Figure 8. A series of consecutive transmission-electron-microscopy images, each about 900 nanometers wide, shows the effect of a high-power
electron beam on newly formed soot and other tarry substances collected on a fibrous grid. The high-power beam is required to obtain highresolution images. At first, the collected soot agglomerate is well defined (left). After 30 seconds (middle) and then 60 seconds of exposure
(right), the soot starts to lose its shape. The amorphous material deposited on the grid appears to have fluidized in the high-power beam and
flowed over the soot aggregate, obscuring it from imaging attempts. It is unclear whether the soot itself also suffered some disassociation from
the beam. But such results make it impossible to analyze soot or precursor particles that form in the lower parts of flames.
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American Scientist, Volume 95
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these flames let us explore the effect of
residence time on soot formation.
Determining the extinction coefficient
entailed extracting soot and the surrounding gases from the flames with a
metal tube, capturing the soot-laden gases in a transparent cell where the dimming of a laser beam passing through
could be measured, and then collecting
the tested soot onto a filter for determination of its mass. To ensure that large
hydrocarbon molecules—which might
condense into solids in the gas sampling
and cooling process—did not influence
the measurement of mass, we passed
methylene chloride, a common solvent
for heavy hydrocarbons, through the
soot samples, which were then placed
in a vacuum (to remove the methylene
chloride and any hydrocarbons present)
before they were dried and reweighed.
For the soot derived from ethylene
and kerosene flames, we found values
for the extinction coefficient that were
consistent with measurements made
by other investigators of soot emitted
from the tops of flames. For the soot extracted from a methane flame, however,
we found a markedly lower value.
We surmised that this disagreement
may be a consequence of a difference
in particle size, which would affect the
amount of light scattered. Individual
soot particles are usually too small to
produce significant amounts of scattering, but as these particles aggregate,
they can grow large enough to scatter
as much as 30 percent of the amount of
light they absorb.
Further measurements showed that
aggregated soot particles derived from
the burning of ethylene and kerosene
are significantly larger than those produced by methane-fueled flames. Methane, having a simpler hydrocarbon
structure than the other fuels, is less inclined to produce soot. With this knowledge of particle sizes, we were able to
calculate the amount of scattering to be
expected. Whereas the larger ethyleneand kerosene-generated soot aggregates
scatter 20 to 30 percent of the amount of
light they absorb, the smaller methaneproduced aggregates scatter only 3 percent. This difference accounts for the
amount of variation we had found in
our measurement of light extinction,
implying that the absorption coefficient
is the same for the soot from all three
types of flames—consistent with our
earlier high-resolution TEM study.
Taking the analysis of our laser measurements one step further, we calculatwww.americanscientist.org
ularly large accidental ones. With this
information, fuel depots might be built
to safer standards, and emergency personnel may be able to make better decisions when serious problems arise. But
we also hope that such studies can aid
in the appreciation of soot, a substance
that can be as brilliant and useful as it
can be dark and destructive.
Bibliography
Figure 9. In a close-up about 33 nanometers
wide, at a magnification of 1.8 million times,
soot shows an ordered internal structure. The
particles seem to start at one or several nucleation points and then build up around those
sites in a spiral pattern.
ed the values of the real and imaginary
components of the index of refraction
that are necessary to yield the absorption coefficient we had measured, using
a relation originally derived by the 19thcentury British physicist Lord Rayleigh
for the scattering and absorption of light
by small particles. In addition, we calculated the index of refraction that would
result in the amount of scattering we
had measured for ethylene soot. The
result was 1.8 – 1.0i, similar to the values
recently reported by two other groups,
and also similar to the number Janzen
arrived at nearly 30 years ago.
A Bright Future
With these results, it appears that we are
finally arriving at a clear understanding of just how black soot really is: Its
characteristic absorptivity is larger, by
about a factor of two, than what many
researchers had long believed. Soot
particles therefore have a greater ability
both to absorb and to emit light than
was previously realized. The data have
brought needed clarity to the understanding of soot optical properties as the
particles evolve in both small, steady
flames and large, erratic pool fires.
Using these new values for how
much light is extinguished as it passes
though a cloud of soot, we can better
estimate the amount of soot in flames
and the temperature of these particles.
Knowing the temperature, concentration and emissivity of soot allows one
to determine how much radiant heat is
transferred from a flame, which in turn
may help investigators to understand
more fully the dynamics of fires, partic-
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of American Scientist Online:
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2007 May–June
239