Clustering in Granular
Shaking a container filled with granular material creates a granular gas. The key feature of a granular gas (making it fundamentally different from any standard gas) is its tendency to spontaneously separate into dense and dilute regions. This stems from the fact that the collisions between the granules are inelastic. Some energy is dissipated in every collision, which means that a relatively dense region will dissipate more energy, and thus become even denser, resulting in a cluster of slow particles. Vice versa, since the particle number is conserved, a relatively dilute region will become more dilute.
The clustering comes about very clearly in a compartmentalized system, as the pictures above show. Starting with 195 steel balls distributed equally over 5 boxes, it takes about a minute before they have clustered into a single compartment. They do not go into one box directly, but first form a transient state in which two of the boxes are competing for dominance. In the present experiment the cluster ends up in the middle box, but it could have gone into any of the compartments.
Read more about clustering (and declustering!) in a granular gas here
A steel ball dropped onto loose, very fine sand creates a jet exceeding the release height of the ball. Upon impact, sand is blown away in all directions forming a splash. The ball creates a cylindrical void in the sand; the jet is formed when this void collapses. The focused sand pressure pushes the jet straight up into the air. When the jet comes down again, it breaks up into fragments: these are granular clusters.
For sufficiently high impact velocity, air is entrained by the collapsing void, forming an air bubble in the sand. This bubble slowly rises to the surface, and upon reaching it causes a granular eruption. This looks like a boiling liquid, or even a volcano!
A movie of the experiment (at 1000 frames per second) can be downloaded here (1.5 MB).
Read more about granular eruptions here