[Catalyst 2016] Final

FROM A PILE OF
RICE
TO
AVALANCHE
A BRIEF INTRODUCTION
TO GRANULAR MATERIALS
BY KELLY YAO
Communities living at the foot of the Alps
need a way to predict the occurrence of
avalanches for timely evacuation, but
monitoring the entire Alpine range is
impossible. Fortunately for those near the
Alps, the study of granular materials has
allowed scientists to move mountains into
labs and use small, contained systems
(like piles of rice) to simulate real-world
avalanche conditions. Granular materials,
by definition, are conglomerates of discrete
visible particles that lose kinetic energy
during internal collisions; they are neither
too small to be invisible to the naked eye,
nor too big to be studied as distinct objects. 1
The size of granular material situates them
between common objects and individual
molecules.
While studying extremely small particles,
scientists stumbled upon an unsettling
contradiction: the classical laws governing
the macroscopic universe do not always
apply at microscopic scales. For example,
Niels Bohr sought to apply classical
mechanics to explain the orbits of electrons
around nuclei by comparing them to the
rotation of planets around stars. However,
it was later discovered that an electron
behaves in a much more complicated way
than Bohr had anticipated. At its size, the
electron gained properties that could only
be described through an entirely new set of
laws known as quantum mechanics.
Though granular materials do not exist
at the quantum level, their distinct size
necessitates an analogous departure from
classical thought. A new category of physical
laws must be created to describe the
basic interactions among particles of this
unique size. Intuitively, this makes sense;
anyone who has cooked rice or played
with sand knows that the individual grains
behave more like water than solid objects.
Scientists are intrigued by these materials
because of the variation in their behaviors
in different states of aggregation. More
importantly, since our world consists of
granular materials such as coffee, beans,
dirt, snowflakes, and coal, their study sheds
new light on the prediction of avalanches
and earthquakes.
The physical properties of granular flow vary
with the concentration of grains. At different
concentrations, the grains experience
different magnitudes of stress and dissipate
energy in different ways. Since it is hard
to derive a unifying formula to describe
granular flows of varying concentrations,
physicists use three sets of equations to fit
their states of aggregation, resembling the
gaseous, liquid, and solid phases. When
the material is dilute enough for each
grain to randomly fluctuate and translate,
it acts like a gas. When the concentration
increases, particles collide more frequently
and the material functions as a liquid. Since
these particles do not collide elastically, a
fraction of their kinetic energy dissipates as
heat during each collision. The increased
frequency of inelastic collisions between
grains in the analogous liquid phase
results in increasing energy, dissipation,
and greater stress. Finally, when the
concentration increases to 50% or more,
the material resembles a solid. The grains
experience significant contact, resulting in
predominantly frictional stress and energy
dissipation. 1
Avalanches come in two types, flow and
powder, each of which requires a specific
combination of the gas, liquid, and solid
granular models. In a flow avalanche, the
descending layer consists of densely packed
ice grains. The solid phase of granular
materials best models this, meaning that
friction becomes the chief analytical aspect.
In a powder avalanche, particles of snow do
not stick together and descend in a huge,
white cloud. 2 The fluid and solid models of
granular materials are equally appropriate
here.
Physicists can use these avalanche models
to investigate the phenomena leading
up to a real-world avalanche. They can
simulate the disturbance of a static pile of
snow by constantly adding grains to a pile,
or by perturbing a layer of grains on the
pile’s surface. In an experiment conducted
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CATALYST