A granular material can be defined as any loosely interacting collection of (usually) solid particles.1 Depending on the conditions, a granular material can be best described as a solid, or as a fluid, or as a gas, or in some case not adequately as any of these, which makes this both an interesting and difficult field of study.

The behavior of granular materials is something that is encountered frequently in all aspects of everyday life. Some of the situations include grain silos, trash piles, sand & salt that is used for building and on winter roads, snow (as in avalanches), and mixing of various food and medical supplies, such as pharmaceuticals. A granular material would be generally defined as consisting of discrete units of a solid material that neither chemically interact, nor otherwise strongly cohere together. Thus, the arrangement and motion of these materials is neither that of a solid (in which the various parts are tightly bound together) nor that of a fluid (in which the parts may easily flow over & around other parts). The size of the particles, per se, is not important -- the phenomenology is similar with very fine grains (such as powders) and with rather large 'grains' (such as cannonballs). The possibility of using various sizes of grains allows for some interesting research proposals covering several length scales. For many experiments the size of the container needs to be 'large' compared to the grain size (typically a factor of 100 could be considered as 'large'), so that end effects are indeed restricted only to the ends. This of course means also a large number of particles – easy with things like sand, less so with things like marbles (1003 marbles can be quite massive!)