Evaluation of the Fractal Properties of Cluster?Cluster Aggregates

Abstract

ABSTRACT. While the fractal dimension of cluster -cluster aggregates, D f , appears to be a well-established property, the fractal prefactor, kg (also known as structural coef cient), continues to exhibit a large range of possible values. In the present paper, an attempt is made to clarify this issue which leads to conclusive results concerning the value to adopt for the fractal prefactor of simulated aggregates. Starting from a large population of "free" numerically simulated aggregates (i.e., where no restrictions to aggregate formation were imposed) the fractal properties obtained were estimated both using morphological concepts as well as light scattering theories. Furthermore, studies for aggregates having prede ned morphological k g values (namely, k g > 2 and ca. 1) were also performed in order to check the viability of these values. The results obtained for the three different populations of aggregates considered were used to infer, through a best-t analysis, the fractal properties. Our best estimates are k g ¼ 1.27 and D f ¼ 1.82, which are approximately independent of aggregate size and composition. These results are in very good agreement with numerical predictions reported by previous authors. However, experimental results systematically indicate k g > 2. This motivated the present authors to identify possible reasons for these large discrepancies. Present calculations indicate that partial sintering in conjunction with the polydispersity of aggregates (resulting in a cut-off function coef cient, k p , >1) contributes to a systematic increase in k g , possibly justifying the differences between numerical and experimental results.

Investigation of Absorption and Scattering Properties of Soot Aggregates of Different Fractal Dimension at 532 nm Using RDG and GMM

“…It should be noted that there is quite a large uncertainty in the value of prefactor for soot aggregates, ranging from 1.05 to 3.5 in the literature 1396 F. LIU ET AL. Brasil et al 2000). Our choice of k f = 2.3 is somewhat arbitrary, but it is an intermediate value over the range of values reported in the literature and very close to the value of 2.4 determined experimentally by Köylü et al (1995).…”

“…It is generally believed that fractal dimension is an important parameter in the description of fractal aggregates, such as combustion-generated soot particles, and is also a good indicator of the particle aggregation mechanism and aging processes (Adachi et al 2007). Soot particles resulting from combustion or numerically simulated diffusion limited cluster aggregation (DLCA) exhibit fairly open structure with a fractal dimension in the range of 1.4-1.9 (Brasil et al 2000). To date most studies on the radiative properties of fractal soot particles have focused on the evaluation of the accuracy and the range of the validity of the RDG approximation, (Farias et al 1995;Farias et al 1996;Van Hulle et al 2002;Liu and Snelling 2008).…”

Morphology and mobility of synthetic colloidal aggregates

“…Most of the clusters we examined were created with parameters 1 characteristic of aggregates generated by Diffusion Limited Cluster Aggregation -DLCA (1.8, 1.3) [21] or Reaction Limited Cluster Aggregation -RLCA (2.05, 0.94) [22] The agglomeration mechanism for both groups is diffusion, the difference arising from the cluster-monomer sticking probability: it is unity for DLCA clusters and 10 −3 for RLCA [23]. Note that Ref.…”

Friction Coefficient and Mobility Radius of Fractal-Like Aggregates in the Transition Regime

“…The fractal dimension and prefactor (d f , k f ) were (1.8, 1.3) and (2.05, 0.94) that correspond to aggregates generated by diffusion limited cluster aggregation (DLCA) and reaction limited cluster aggregation (RLCA), as recommended by Brasil et al (2000) and Lattuada et al (2003), respectively. The results were averaged over 10 different aggregate realizations for each number of monomers: in total 30 DLCA and 30 RLCA aggregates were analyzed.…”