What are granular materials and why are they interesting?

经常看到有关粒子流的东西，但一直没有搞清楚怎么回事，不知道有什么重要意义？
趁此机会稍作了解，先科普一下吧！

Granular materials are very simple: they are large conglomerations of discrete macroscopic particles.
A granular material is a collection of discrete, solid particles dispersed in a vacuum or interstitial fluid.

Several examples
grains such as corn or wheat flowing from a silo
landslides of boulders and debris
rock and ice collisions in planetary rings
ice floes
dune formation
transport and handling of coal or chemicals in industrial plants
powder metallurgy
powder spray coating
handling of pharmaceuticals

References:
http://jfi.uchicago.edu/granular/introduction.html
http://meweb.ecn.purdue.edu/~psl/background/