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Combustion of liquid Al-droplets results in detached heterogeneous flame-smoke envelope.

Interest for "in-situ" diagnostics of nanoparticles and clusters of nanoparticles with overall dimension smaller than 1 micron is sustained by two different kinds of motivation. On one hand, health risks. Correlation between pulmonary diseases and smaller (size 5 - 300 am) particles has been proved by recent epidemiological studies (see for example Donaldson et al., 1998 and references therein cited). The Exposure to PMz5 (particles equal or smaller than 2.5 microns in aerodynamic diameter) has been introduced by the US. Environmental Protection Agency (EP.A.) since 1996. Very recent studies (Sioutas et al., 1999) have focused on the realization and testing of personal particle samplers for particles smaller than 1.0 Ixm (PM1.0). On the other hand, the investigation and the diagnostic methods on nanoparticles are motivated by the material science research with application in the production of catalysts, ceramic composites, pigments, fire retarding materials, conductive pastes, electromagnetic components and electronic devices. Possible routes for production of nanomaterials are combustion, laser ablation, electric discharge, and deposition of particles in hypersonic plasma.

The interest of the scientific community about the agglomeration of aerosol nanoparticles is strongly motivated by the direct implications of this process on the synthesis mechanisms of very different particulate matter, such as soot, silica, titania. In this paper we carry out both experimental and numerical investigation of agglomeration occurring between silicon particles. Aerosol particles are formed by silane pyrolysis in a flow reactor and the coagulation mechanism is observed in the cold downstream zone.

The density fractal and the boundary fractal properties of soot agglomerates from SEM images are evaluated by the application of Nesting Square and Mosaic-Amalgamation methods, respectively. Thermogravimetric experiments are performed on soot samples from ethylene-air and propane-air flames generated by a Bunsen burner with the air inlet blocked. The comparison of the properties of the agglomerates probed directly in flame and of the clusters synthesized via shaking and drying route is performed.

We investigated the shape of soot aggregates as retrieved from light scattering measurements. In particular, the ratios of scattered light intensities at two angles, called the dissymmetry ratios (DRs), will be considered. These are attractive diagnostic tools as they are insensitive to variations in the complex refractive index (m=n+ik) and to the electromagnetic effect of set-interaction that each individual scatterer in the aggregate produces with itself depending on its finite size. The aim of the paper is both to correlate the DRs with the information about the aggregate shape and to establish what is the minimum number of scattering measurements to be performed to characterize the aggregate scattering patterns. The main results can be summarized as follows: (a) an independent evaluation of the primary spherule size is required in order to interpret the scattering data correctly; (b) some of the DRs can share common values for different morphologies when the number of primary spherules per aggregate is sufficiently high; in this respect, we demonstrate the significance of the dissymmetry ratio RVV(10°/90°) for the identification of the prevailing morphology of aggregates at different levels of agglomeration; (c) the information about light intensities scattered at three angles seems to suffice for the recognition of prevailing aggregate morphology.

Laser diagnostics and radiative heat transfer investigations on flame soot particulates appears to be seriously influenced by the a priori uncertainties connected to the lack of knowledge about the physical and chemical properties of the media under test. Both the uncertainty about soot refractive index [1] and [2],[3] morphology are demonstrated to substantially influence the effectiveness of optical in-situ scattering / absorption measurements.