Scattering is one of the most powerful techniques for studying objects with size comparable to experimental radiation wavelength without necessity to sampling them, which is called in-situ characterization as opposed to the analysis of microsamples (ex-situ). Aggregates of nanoparticles are one of the most popular media in science and technology. Combustion, thermal decomposition, laser ablation, and plasma reactors are only a few of the methods used to generate particulates aggregated at the nanoscale. This book describes an ideal walk between scattering methods, experimental techniques, and numerical simulations applied to the study of nanoparticle aggregates. The first part of the book deals with laser light scattering at visible wavelengths. Different approaches using this method are presented, which allows us to gain information about nanoparticle aggregate size, number concentration and morphology without any a priori knowledge about the complex refractive indices of the constituent species. This is particularly relevant for strongly reacting environments in which the chemical composition and microstructure of the nanoparticles are continuously evolving, such as soot in flames. The second part of the book is focused on X-ray scattering. The main differences and analogies with respect to light scattering theory are presented. A general approach is introduced to present the quantities which can be measured in an experiment of Small-Angle X-ray Scattering (SAXS) on gas-phase generated aggregates of nanoparticles. Exemplary SAXS applications are introduced to show how the mechanisms of nucleation and aggregation of particles can be studied with a spatial resolution about two orders of magnitude smaller with respect to laser light scattering methods. The combined use of SAXS with other X-ray techniques, such as WAXS and NEXAFS is discussed. The complementary characters of light and X-ray scattering are emphasized by treating the topic of hierarchical nanoparticle aggregation.
