[Catalyst 2016] Final
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PHOTO FROM:<br />
USER: ROBIAND ON<br />
WIKIMEDIA COMMONS<br />
by statistical physicists Dr. Daerr and Dr.<br />
Douady, layers of glass beads of 1.8 to<br />
3 mm in diameter were poured onto a<br />
velvet surface, launching two distinct types<br />
of avalanches under different regimes<br />
decided by the tilt angle of the plane and the<br />
thickness of the layer of glass beads. 3<br />
For those of us who are not experts in<br />
avalanches, there are a few key points to<br />
take away from Daerr and Douady. They<br />
found that a critical tilt angle exists for<br />
spontaneous avalanches. When the angle of<br />
the slope remained<br />
under the critical<br />
angle, the size of the<br />
flow did not grow,<br />
even if a perturbation<br />
caused an additional<br />
downfall of grains.<br />
Interestingly, when the angle of the slope<br />
was altered significantly, the snow uphill<br />
from the perturbation point also contributed<br />
to the avalanche. That means that<br />
avalanches can affect higher elevations than<br />
their starting points. Moreover, the study<br />
found that the angle of the remaining slope<br />
after the avalanche was always less than the<br />
original angle of the slope, indicating that<br />
after a huge avalanche, mountains would<br />
remain stable until a change in external<br />
condition occured. 3 Often, a snow mountain<br />
with slopes exceeding the critical angle<br />
can remain static and harmless for days,<br />
because of the cohesion between particles.<br />
Situations become complicated if the<br />
grains are not completely dry, which is<br />
what happens in real snow avalanches. In<br />
these scenarios, physicists must modify<br />
existing formulas and conduct validating<br />
experiments to predict the behaviors of<br />
these systems. Granular materials are not<br />
limited to predicting<br />
GRANULAR MATERIALS<br />
ARE CONGLOMERATES OF DISCRETE<br />
VISIBLE PARTICLES THAT LOSE KINETIC<br />
ENERGY DURING INTERNAL COLLISIONS<br />
avalanches. In<br />
geophysics, scientists<br />
have investigated<br />
the relation of<br />
granular materials<br />
to earthquakes.<br />
For instance, one study used sound waves<br />
and glass beads to study the effects of<br />
earthquake aftershocks. 4 Apart from<br />
traditional modeling with piles of rice<br />
or sand, the understanding of granular<br />
materials under different phases paves the<br />
way for computational modeling of largescale<br />
natural disasters like avalanches and<br />
earthquakes. These studies will not only<br />
help us understand granular materials<br />
themselves, but also help us predict certain<br />
types of natural disasters.<br />
WORKS CITED:<br />
[1] Jaeger, H. M., Nagel, S. R., and Behringer,<br />
R. P. Granular Solids, Liquids and Gases.<br />
Rev. Mod. Phys., 1996, 68, No.4, 1259-1273.<br />
[2] Frankenfield, J. Types of Spring and<br />
Summer Avalanches. http://www.mountainguiding.com/avalanche/info/spring-types.<br />
html (accessed Oct. 29, 2015).<br />
[3] Daerr, A. and Douady, S. Two Types of<br />
Avalanche Behaviour in Granular Media.<br />
Nature, 1999, 399, 241-243.<br />
[4] Johnson P. A., et al. Effects of acoustic<br />
waves on stick–slip in granular media and<br />
implications for earthquakes. Nature, 2008,<br />
451, 57-60.<br />
DESIGN BY Lucy Guo, Vidya Giri<br />
CATALYST 16