Title: Focus on Imaging Methods in Granular Physics,
Abstract: Granular materials are complex multi-particle ensembles in which macroscopic
properties are largely determined by inter-particle interactions between their
numerous constituents. In order to understand and to predict their macroscopic
physical behavior, it is necessary to analyze the composition and interactions
at the level of individual contacts and grains. To do so requires the ability
to image individual particles and their local configurations to high precision.
A variety of competing and complementary imaging techniques have been developed
for that task. In this introductory paper accompanying the Focus Issue, we
provide an overview of these imaging methods and discuss their advantages and
drawbacks, as well as their limits of application.