The Armchair Scientist
Lo
Scienziato in Poltrona
Science & Technology Magazine by Loris
Crudeli
Granular materials are very common in everyday life: salt, sugar, coffee,
talc powder, sand, coal. They are made of common substances but in some
way their behavior is completely different from what we can see in homogeneous
gases, liquids or solids. The sensitivity to temperature is what matters
in this different behavior. You must think about granular materials as
a "substance" whose "atoms" are the grains themselves,
zillion of times heavier and bigger than the atoms or molecules which form
the standard substances. In the case of atoms and molecules the temperature
we normally live in will give so much energy to each minute building block
that it is possible that it will be displaced from its neighbors: below
zero Celsius degrees the molecules of water are strongly tied together,
above zero the molecules roam free and water is liquid. In the case of
a pile of sand the energy required to move one grain from the others is
one million million times higher than what can be produced by thermal effects
alone: the sand will be converted to a solar plasma before the temperature
can be high enough to "move" the grains! From this simple fact
come a lot of interesting implications which have been explored by several
scientists:
Paul Umbanhowar (Texas) described an experiment at last month's American
Physical Society (APS) meeting in St. Louis, in which the effects of air
between the grains and the interaction between the grains and the walls
were minimized by using a wide, shallow container. When the grain sample
is vibrated, numerous patterns form: stripes, hexagons, and baseball stitches.
When the layer thickness is of exactly 17 particles, novel localized
structures appear. According to Umbanhowar, these "oscillons"
are like ripples in a pond but with an important difference; they do not
spread out and they can form bound states.
In the April issue of Physics Today the authors Heinrich Jaeger, Sidney
Nagel, and Robert Behringer explain: Consider sand contained in a tall
cylinder. The pressure in the cylinder does not increase indefinitely
with depth, as it does in ordinary fluids. Rather, the pressure does
not exceed a certain maximum value because the contact forces between grains
and friction between grains and the walls cause part of the weight of the
overlaying material to be imparted to the walls. Another property which
sets granular materials apart from other fluids is the tendency (in
a vibrated tank containing particles of differing sizes) for the larger
objects to float to the top regardless of their density. Part of the
reason for this is that the smaller particles fall into the voids between
the larger particles, making it difficult for the larger ones to move downwards
(especially along the walls) as part of a convective flow.
The properties of granular media in technological issues is clear,
as in packaging, mixing, sifting, mining, and erosion processes, but the
universe itself may exhibit a sort of granular arrangement. One simulation
of granular gases involving a large sample of inelastically interacting
hard disks resulted in the "inelastic collapse" of the system
into a foam of chainlike structures roughly resembling the distribution
of galaxies in the cosmos. The Physics Today authors speculate that the
role of gravity in shepherding galaxies may be analogous to the role played
by container walls in shaping granular fluids into clusters. At the
cosmological level, the "grains" would be stars and galaxies.