Granular material is all around us, but because powders, grains of
sand, gravel, and grains used to be considered low-tech, scientists
have not devoted a great deal of research to them. However, many high-tech
industries such as the railroad, pharmaceutical, polymer and space
industries are now finding they need to understand the physics of
granular materials, and this has proved to be no easy task.
The
CNRS "Laboratoire des milieux désordonnés et hétérogènes"
(LMDH, Disordered and Heterogeneous Media Laboratory) has acquired
expertise in experimentation and modeling the fundamental behavior
of granular materials, for example, their behavior during an avalanche.
The LMDH team has shed light on the transitions between the fundamental
angles that define avalanche behavior: the angle of movement (maximum
inclination just prior to movement), the angle of repose (a smaller
inclination, just after an avalanche), the neutral angle (between
the first two, defining a steady state flow during an avalanche),
and the dynamic angle or maximum angle of stability. This relationship
differs with different morphologies of the granular solid: angular
grains can generally form steeper inclines than spherically shaped
grains; in particular, this phenomenon leads to the segregation observed
in mixtures of granular species, an impediment to industrial processes
for which mixtures must be as homogeneous as possible.
Recently,
the team at LMDH has been studying the physics of fine powders, the
treatment of which gives rise to problems in industry. The behavior
of tiny grains (less than 20 microns) has been shown to be highly
sensitive to ambient fluids, resulting in instability and blockages,
and providing a possible explanation for the first phase of the collections
of volcanic eruptions, such as those observed on the moon Io, which
orbits Jupiter.
