This paper presents a review of scattering and absorption of light by fractal aggregates. The aggregates are typically diffusion limited cluster aggregates (DLCA) with fractal dimensions of D ' 1:75, which occur frequently in aerosols and colloids, but other types of aggregates are discussed as well. The results of the review are quite general. A scaling description of the scattering of waves forms the basis of our understanding. This description yields the optical structure factor S(q ) of the aggregate, where q is the scattering wavevector, a key quantity. The rigorous approach to the structure factor, the Fourier transform of the density correlation function, is also given and various forms provided in the literature are compared and the best selected. Light scattering is directly related to the structure factor under the assumption of no internal multiple scattering. This so-called Rayleigh -Debye -Gans (RDG) approximation is compared to more rigorous electromagnetic approaches and found to be quite accurate for fractal aggregates. This results despite the presence of internal multiple scattering, which leads to depolarization, and which is also extensively described. For ensembles of aggregates, the effects of aggregate polydispersity on scattering experiments are described and methods for proper analysis are given. A description of optical particle sizing and morphology analysis is given for a complete in situ characterization of the aggregate system. The review relies strongly on straightforward physical reasoning and experimental examples, largely from aerosols, especially those of carbonaceous soot. This paper not only reviews and consolidates, but also presents new results including a scaling analysis of multiple scattering, which leads to depolarization, with a successful comparison to available data and an understanding of structural subtleties; demonstration of the signi cance of a phase shift parameter for evaluating the range of validity of the RDG theory; a Maxwell -Garnet analysis of fractal aggregate scattering; calculations of the albedo and successful comparison to available simulation data; and speculations on multiple scattering and the interior eld. INTRODUCTIONThe purpose of this review is to unify our knowledge of how fractal aggregates scatter (and absorb) light. This is of value for light scattering diagnostics of aerosols and colloids and for understanding the optics of these systems in the environment. The subject has seen able and useful reviews before by Teixeira (1986), Martin and Hurd (1987), Charalampopoulos (1992), and a brief overview by myself (Sorensen 1997). Very recently Fuller and Mackowski (2000) have given an excellent review of the superposition theory of scattering from aggregates. My desire here is to provide an up-to-date and comprehensive review useful for the experimentalist. Any theory that does appear will be based on simple physical arguments rather than detailed mathematical analysis. Many of the general results to be described here apply to nonfractal aggreg...

Particle−particle and particle−substrate interactions cause nanocrystals to self-assemble into superlattice structures upon drying from a colloidal suspension on a solid surface. Rapid dewetting of a volatile solvent, however, can significantly undermine the degree of ordering. We demonstrate here that by increasing the concentration of the nonvolatile dodecanethiol ligand, dewetting can be controlled and gold nanocrystal superlattices can be formed on silicon nitride substrates with long range ordering over several microns. Monolayer and bilayer superlattices can be produced by adjusting the nanocrystal concentration. The superlattice structures are robust and are not perturbed by the final dewetting of the solvent.

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We have measured the density of supercooled water (H2O) in the range −33.41≤T≤−5.23 °C. Our samples were held in glass capillary tubing with an approximate inside diameter of 0.3 mm=300 μ. These samples were prepared by Mossop’s method and could be cooled to their homogeneous nucleation limit before freezing. We compare our density data to other measurements using capillaries and demonstrate what appears to be an excess density in smaller capillaries which is inversely proportional to the capillary inside diameter. The origins of this excess density are unknown, but we show its effect is insignificant on our measurement. The thermal expansivities derived from our data are fit to a power law in temperature relative to a singular temperature. These results are inconclusive due to a poor knowledge of the background expansivity.