Granular materials, such as sand, coffee powder or rice, exhibit a wide range of solid and liquid-like phenomena, some familiar, others remarkable, but almost always poorly understood. For example, a layer of sand can sustain a compressive load, like a solid, but will flow when sheared. Perhaps surprisingly, the distinction between these types of behavior is not sharp: recent experiments have shown that the surface flow which occurs when, e.g., a bucket of sand is tilted, leads to a velocity profile which decays exponentially with depth. Despite a long history of research and obvious technological relevance, the description of granular media is still in its infancy: there are no models that describe, let alone predict, its general properties, even though the dissipative and nonlinear interactions between the individual grains are well-characterized.

Apart from the challenge to describe the collective behavior is such seemingly simple systems, in addition the enormous range of unexpected and exotic behaviors of sand have attracted the attention of physicists: when shaken or stirred, granular matter display glassy behavior, hysteresis, shear banding, pattern formation, memory effects, segregation, convection and compaction. New theoretical concepts, such as jamming, fragility and granular temperature, are hotly debated.