(All notes are taken from the article: Riddles in the Sand, Fred Guterl, Discover Magazine, 1996)
What is it that sand does?
Scientists began looking into the nature of granular material in the late 1980’s.
Simple, ordinary, modest sand, neglected by engineers and physicists for decades, adopted by well meaning but experimentally challenged mathematicians, defies explanation.
That not even a physicist can explain why sand behaves the way it does seems astonishing. Sand is neither invisibly small nor impossibly distant; observing it requires neither particle accelerators or orbiting telescopes. The interactions of grains of sand are entirely governed by the same Newtonian laws that describe the motion of a bouncing ball or the orbit of the earth about the sun. The odd behaviour of a layer of sand bounced up and down on a tray should, in principle, be entirely knowable and entirely predictable. Why, then, can’t physicist’s simply take a bunch of equations describing the motion of all the individual grains, put them in a very large computer, and wait-for years, if necessary-until it spits out a prediction?
The problem is not one of computation but of knowledge: though sand is acted on by Newtonian forces, we simply don’t know enough about how those forces operate when let loose on a pile of sand.
One grain of sand acts like a ball. When dropped from a great height to the floor, it bounces-pretty high, in fact…drop a sack of sand on the floor and it absorbs the energy of the fall quite well, which means it doesn’t bounce at all.
Because the language of physics does not contain a vocabulary for granularity, engineers must treat granular material as either a liquid or a solid. These approximations work most of the time, but occasionally they lead to disaster. Grain solos, for instance, are designed under the dubious assumption that the grains distribute their weight uniformly, as though they were water molecules. In fact, when the grains come to rest against one another they form intricate, quasi-self supporting structures. That is why adding more grains to the top of a silo often does not increase the pressure delivered to the bottom at all, but rather increases pressure outward against the sides of the silo.
The flow of grain itself can very unpredictably from a trickle to a gush- to the constant annoyance of engineers in the food, mining, and shipping industries. Drop sand a grain at a time and the pile if forms will get higher and higher until, at some critical point, the very next grain causes and avalanche. Sometimes the avalanche occurs almost immediately and constitutes only a few grains sliding down the slope. At other times the grains collect for longer than seems possible, until a great many of them come crashing down at once.