Abstract
The concept of scattering contrast is central to (small-angle) scattering of neutrons, X-rays, and visible light, as it defines the visibility to the radiation of the nano- and mesostructures under scrutiny, in their respective solvent or matrix. In this chapter, we start from the scattering process by the elementary scatterers, i.e., nuclei or atoms, which produces secondary, scattered waves, the amplitude of which is given by the scattering length of each scatterer. This allows us to define the scattering length densities, and thus the contrast of each object, molecule, or particle with respect to the matrix for each radiation. With neutrons, the use of selective deuteration, and the subtlety of coherent and incoherent scattering are discussed, as is anomalous scattering for X-rays. The importance of a detailed understanding of the contrast, and how it gives rise to the scattered intensity, is then illustrated by absolute determination of molecular masses, and how the contrast can be adjusted to observe only desired parts of the samples, opening the road to contrast variation experiments. In the last section, we present selected examples from polymers and chain flexibility – including zero-average contrast experiments in nanocomposites -, from biological and self-assembled systems, with optical and SANS studies of microemulsions.