Fractal aerosols

Aerosol particles used for inhalation deposit within the lower respiratory tract mainly by inertial impaction, sedimentation, and diffusion. Loose fractal aerosols were found to settle slower and therefore had more time to increase gravitational coagulation with other floes leading to much more rapid particle growth. This will increase the chance of the aerosol floes settling on the airway walls before reaching the end of the airways. 

Naumann, K.H. COSIMA - a computer program simulating the dynamics of fractal aerosols, J. Aerosol Sci. 34 (2003) 1371-1397. 

K. -H. Naumann, H. Bunz, Aerodynamic properties of fractal aerosol particles. J. Aerosol Sci. 22... 

Since Mandelbrot s original description (1977,1983), fractal geometry has found relevance in a number of scientific disciplines including aerosol technology and science [e.g., see Lovejoy (1982), Meakin (1983), Kaye (1984), Sheaffer (1987), Reist et al. (1989)]. Many applications are covered in some detail by Kaye (1989), so only a brief description of fractals is given here. 

Although DLS is most often used to size solid colloidal particles, the technique has also been applied to characterize aerosols [78,86,87], emulsion droplets [88,89], amphiphilic systems [90-92], and macromolecular solutions [12,16,93]. Another common application is the study of the fractal structure and kinetics of colloidal aggregation [94-102], More information about dynamic light scattering and its applications can be found in Refs. 23. 103 (104), and 105, in reviews, Refs. 11, 13, 36, 37, 49, 50, and 106, and in collections of papers Refs. 12. 14. 16. 93 (107), 105, and 108-114. 

Fractal analysis is also being applied to computer modeling of aerosol agglomeration [76] and the reaction kinetics of pigment formation [77,78]. 

Laser diffraction technique is able to generate the envelope diameter distribution of the aggregates in the aerosol cloud, as well as the surface and mass fractal dimensions as explained in Light Scattering under both Boundary and Surface Fractal Dimensions and Mass Fractal Dimension. However, validation of Eq. (25) by comparison studies between the impaction and laser diffraction techniques is required. 

In a metered dose inhaler, where fine drug particles are suspended in a propellent, stability and aggregation of the suspension are crucial for the performance of the inhaler. In an investigation into the aerosol formulation stability. Span 80 was added to a suspension of lactose to study the effect on deaggregation under shear.f An increase in shear stress was found to decrease aggregate size and boundary fractal dimension (Fig. 9), which was interpreted as a more compact aggregate. 

Wu, M.K. The roughness of aerosol particles surface fractal dimension measured using nitrogen adsorption. Aerosol Sci. Technol. 1996, 25, 392-398. 

Lee, C.T. Chou, C.C.-K. Application of fractal geometry in quantitative characterization of aerosol morphology. Part. Part. Syst. Charact. 1994,11, 436 141. 

Magill, J. Fractal dimension and aerosol particle dynamics. J. Aerosol Sci. 1991, 22 (Suppl. 1), 165-168. 

FIGURE 14.10 Schematic of the general behavior of the structure factor for a fractal aggregate with fractal dimension D, radius of gyration R, and monomer radius a. (From Sorensen, C.M., Aerosol Sci. Tech, 35, 241, 2001. With permission.)... 

Sorensen, C.M., Light scattering from fractal aggregates. A review. Aerosol Sci. Tech., 35, 241, 2001. Cai, J., Ln, N., and Sorensen, C.M., Analysis of fractal clnster morphology parameters stmctnral coefficient and density antocorrelation function cutoff, J. Colloid Interface Sci., 171, 470, 1995. Sorensen, C.M. and Feke, G.D., The morphology of macroscopic soot. Aerosol Sci. Tech., 25, 328, 1996. 

Colbeck, I. (2008). Fractal geometry of aerosol particles. In Biophysical Chemistry of Fractal Structures and Processes in Environmental Systems, Senesi, N. and Wilkinson, K. (eds.), lUPAC Series, John Wiley Sons Ltd, Chichester, UK, Chapter 9. 

Sorensen, C. M. (2001). Light scattering by fractal aggregates a review. Aerosol ScL Technol., 35, 648-687. 

Computer simulations have shown that the value of fractal dimension largely depends on whether the aggregation process is controlled by the diffusion rate of the clusters and single particles or by their chemical reactivity at the time of collision, the latter being mainly controlled by the DLVO forces. This observation, in agreement with experimental work on aerosols and colloids, has led to a new classification of aggregation processes the reaction-limited and diffusion-limited cluster aggregation (RLCA and DLCA respectively) processes. 

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