Optical structure factor measurements of soot particles in a premixed flame

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This paper presents a review of scattering and absorption of light by fractal aggregates. The aggregates are typically diffusion limited cluster aggregates (DLCA) with fractal dimensions of D ' 1:75, which occur frequently in aerosols and colloids, but other types of aggregates are discussed as well. The results of the review are quite general. A scaling description of the scattering of waves forms the basis of our understanding. This description yields the optical structure factor S(q ) of the aggregate, where q is the scattering wavevector, a key quantity. The rigorous approach to the structure factor, the Fourier transform of the density correlation function, is also given and various forms provided in the literature are compared and the best selected. Light scattering is directly related to the structure factor under the assumption of no internal multiple scattering. This so-called Rayleigh -Debye -Gans (RDG) approximation is compared to more rigorous electromagnetic approaches and found to be quite accurate for fractal aggregates. This results despite the presence of internal multiple scattering, which leads to depolarization, and which is also extensively described. For ensembles of aggregates, the effects of aggregate polydispersity on scattering experiments are described and methods for proper analysis are given. A description of optical particle sizing and morphology analysis is given for a complete in situ characterization of the aggregate system. The review relies strongly on straightforward physical reasoning and experimental examples, largely from aerosols, especially those of carbonaceous soot. This paper not only reviews and consolidates, but also presents new results including a scaling analysis of multiple scattering, which leads to depolarization, with a successful comparison to available data and an understanding of structural subtleties; demonstration of the signi cance of a phase shift parameter for evaluating the range of validity of the RDG theory; a Maxwell -Garnet analysis of fractal aggregate scattering; calculations of the albedo and successful comparison to available simulation data; and speculations on multiple scattering and the interior eld. INTRODUCTIONThe purpose of this review is to unify our knowledge of how fractal aggregates scatter (and absorb) light. This is of value for light scattering diagnostics of aerosols and colloids and for understanding the optics of these systems in the environment. The subject has seen able and useful reviews before by Teixeira (1986), Martin and Hurd (1987), Charalampopoulos (1992), and a brief overview by myself (Sorensen 1997). Very recently Fuller and Mackowski (2000) have given an excellent review of the superposition theory of scattering from aggregates. My desire here is to provide an up-to-date and comprehensive review useful for the experimentalist. Any theory that does appear will be based on simple physical arguments rather than detailed mathematical analysis. Many of the general results to be described here apply to nonfractal aggreg...

Section: The Optical Structure Factormentioning

confidence: 99%

Light Scattering by Fractal Aggregates: A Review

Sorensen

2001

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756

“…By moving the burner vertically, measurements were made at different positions along the burner centerline. A correction in the measurement volume at angles other than 90" was made (Chang, 1991;Gangopadhyay et al, 1991). Particle size, morphology, and the agglomeration state were also examined by transmission electron microscopy (TEM, %hilips EM400) with a resolution of about 5 A.…”

Section: G Yang and P Biswasmentioning

confidence: 99%

“…Though there is some variability in the data, the fractal dimension clearly increases as a function of distance. This increase is attributed to the sintering process, assuming that coagulation is considered not to alter the fractal dimension, but only increases R, (Hurd and Flower, 1988;Gangopadhyay et al, 1991). Using the relationship in Fig.…”

Section: Aerosol Science and Technologymentioning

confidence: 99%

Study of the Sintering of Nanosized Titania Agglomerates in Flames UsingIn SituLight Scattering Measurements

Yang

Biswas

1997

OH 4 5 2 2 2 1 -0 0 7 1 ABSTRACT. A premixed flame aerosol reactor was used to produce titania particles by oxidation of titanium isopropoxide vapor. The growth, aggregation of particles, and the agglomerate structure were determined as a function of height in the flame using in situ light scattering and transmission electron microscopy (TEM) measurements. A methodology to determine the sintering characteristic time using light scattering data was established. The light scattering data provided the evolution of the fractal dimension which was then related to the normalized surface area change using a computer simulation. The sintering equation was redeveloped in terms of the normalized surface area, thus not having to account for coagulation effects. Experimental results indicate that isolated titania particles were observed at the high temperatures due to fast sintering. An agglomerate was obtained at downstream locations with an associated change in fractal dimension due to sintering.

“…Considerable effort has been made to study the morphology of soot aggregates that are produced by hydrocarbon flames Heckman, 1967, 1969;Forrest and Witten, 1979;Samson et al, 1987;Bourrat et al, 1988;Zhang et al, 1988;Megaridis and Dobbins, 1990;Gangopadhyay et al, 1991;Charalampopoulos and Chang, 1991;Sorensen et al, 1992aSorensen et al, , 1992bPuri et al, 1993;Cai et al, 1993Cai et al, , 1995Koylu et al, 1992Koylu et al, , 1994aKoylu et al, , 1994bKoylu et al, , 1995. Consequently, it is now well established that these aggregates are composed of small, on the order of a few tens of nanometers, primary particles which are randomly clustered together.…”

Section: Introductionmentioning

confidence: 99%

“…post flame Megaridis & Dobbins (1987) C,H, Diff. Bourrat et al (1988) Carbon Black Zhang et al (1988) CH,/O, premixed Gangopadhyay et al (1991) CH,/O, premixed…”

Section: Introductionmentioning

confidence: 99%

The Morphology of Macroscopic Soot

Sorensen

Feke

1996

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103

ABSTRACT. The morphology of soot collected from a laminar acetylenelair diffusion flame was studied. Collection methods included both thermophoretic and impaction sampling from both the luminous and nonluminous portions of the flow. The soot was viewed with both electron and optical microscopy. Cluster sizes ranged over four orders of magnitude from 50 nm to 400 p m to include some clusters visible to the naked eye. A new method of micrograph analysis, necessary when the clusters were large, was developed to account for the unresolved primary particles. Over this entire size range, the same fractal morphology was found with a fractal dimension of D = 1.8 and, within a rather large uncertainty, the same prefactor k,= 1.7. Thus, the fractal morphology of soot remains constant from clusters of about 10 primary particles per aggregate to macroscopic clusters of over