Comparison of three methods of fractal analysis applied to soot aggregates from wood combustion

The morphological and dynamic properties of fractal-like particles produced from the combustion of wood are studied in this work. Particles with electrical mobility diameters of 200, 275, 350 and 450 nm were deposited on filters and imaged using a high-resolution scanning electron microscope (SEM). The soot particles consisted of aggregated primary spheres with mean radius a(p) = 25.5 +/- 3.5 nm and standard deviation sigma(g) = 1.27 +/- 0.09. The fractal dimension of the aggregates, D-f, was derived from three techniques: (1) D-f = 1.84 +/- 0.05 from the projected surfaces in the SEM images; (2) D-f = 1.83 +/- 0.05 from the relationship between number of primary particles per aggregate N, and radius of gyration R-g; (3) D-f = 1.80 +/- 0.13 from the relationship of mobility and N. Thus, there is agreement between the three techniques with an average D-f of 1.83. The dynamic shape factors of the soot particles were between 1.5 and 2.5 and increasing with N. In the regime 0.2 < Kn < 0.7 (Knudsen number, Kn = 2 lambda/d(mob)), the mobility diameter d(mob) was observed to be proportional to the radius of gyration with a ratio d(mob)/2R(g) = 0.81 +/- 0.07. The specific surface area of the aggregates was determined to be 70 +/- 10 m(2)/g, based on SEM image analysis. (C) 2005 Elsevier Ltd. All rights reserved.