Granular
media are ubiquitous in our daily lives: sand and soil, snow and coal,
coffee and sugar; they are (after water) mankinds most manipulated
materials. Because we are so used to them their behavior appears normal
to us, but from a scientific point of view they are quite unusual. If
you take a handful of sand from the beach, it will rinse through your
fingers like a liquid. But at the same time the sand beneath your feet
behaves like a solid. Most materials can change from a liquid to a
solid, but have to be heated to melt. But not sand!
There are other peculiar effects seen in granular media:
- When a mixture of small and large particles is shaken
vertically, it will in most cases separates with the larger
constituents rising to the top. This phenomenon is called the Brazil
Nut Effect and can easily be demonstrated with granola or a can of
mixed nuts.
- The flow rate of sand through the neck in an hourglass
is independent of the height of the column above. This behavior is very
different from a classical liquid like water.
- When thin layers of brass beads are shaken vertically,
they form different types of surface patterns. See e.g. the so called
oscillons depicted in the image above.
- Imagine a sand pile is built by pouring sand through an
orifice. The largest pressure under the bottom of the pile is not below
the center of the cone, but at a ring with a radius of one third of the
whole radius.
