Does the granular matter?

Granular materials, such as sand, gravel, powders, and pharmaceutical pills , are large aggregates of macroscopic, individually solid particles, or "gr ains." Far from being simple materials with simple properties, they display an astounding range of complex behavior that defies their categorization a s solid, liquid, or gas. Just consider how sand can stream through the orif ice of an hourglass yet support one's weight on the beach; how it can form patterns strikingly similar to a liquid when vibrated, yet respond to stirr ing by "unmixing" of large and small grains. Despite much effort, there sti ll is no comprehensive understanding of other forms of matter, like ordinar y fluids or solids. In what way, therefore, is granular matter special, and what makes it so difficult to understand? An emerging interdisciplinary ap proach to answering these questions focuses directly on the material's disc ontinuous granular nature.