Av. Neimark et al., EXTENDED CHARACTERIZATION OF COMBUSTION-GENERATED AGGREGATES - SELF-AFFINITY AND LACUNARITIES, Journal of colloid and interface science, 180(2), 1996, pp. 590-597
A large population of combustion-generated soot aggregates (more than
3000 samples) was thermophoretically extracted from a variety of lamin
ar and turbulent flames and analyzed by using transmission electron mi
croscopy (TEM). It was shown that the scaling structural properties of
these fractal aggregates cannot be exclusively characterized by a sin
gle mass fractal dimension. Asymmetric properties of the aggregates we
re considered here by first assuming and then demonstrating their self
-affinity via affinity exponents reflecting different scaling with res
pect to the length and width of the aggregate projections. In addition
to the conventional fractal dimension, D-f, determined by using the g
eometrical mean of the longitudinal and transverse sizes as the charac
teristic length, the affinity exponent, H, and two complementary fract
al dimensions, one longitudinal, D-L = [(1 + H)/2]D-f, and one transve
rse, D-w = [(1 + H)/2H]D-f, were introduced. By fitting the TEM data f
or the entire population of aggregates, the values of D-f = 1.75 and H
= 0.91 were obtained. To classify the density and crossover scales of
aggregates having the same fractal dimensions, lacunarities of the fi
rst and second order were also defined as prefactors in the scaling re
lationships among aggregate mean mass, rms mass and linear sizes. Anal
ysis of the second moment of the mass-size distribution confirmed that
the scaling properties of flame-generated aggregates cannot be consum
mately characterized by a single fractal dimension; it is necessary to
introduce a set of scaling exponents. This more precise description o
f aggregate morphologies in terms of self-affine scaling and lacunarit
ies is not captured by previous idealized cluster-cluster aggregation
models. Current investigations of the reasons for this are expected to
lead to a deeper understanding of the coagulation dynamics, transport
properties, and restructuring kinetics of flame-generated aggregates.
(C) 1996 Academic Press, Inc.