It’s easy to get in a jam. But it’s much harder to explain exactly how or when it started.
still aren’t sure what causes clogs in flowing macroscopic particles,
like corn, coffee beans and coal chunks. But new experiments by Duke
physicist Robert Behringer and his colleagues suggest that when
particles undergo a force called shear strain, they jam sooner than
expected. The results appear in the Dec. 15 issue of Nature.
strain is sort of like cupping sand between your hands, and then,
without changing the width between them, moving one hand forward and the
other hand backward, Behringer said. Not much sand flows between your
hands with a force like this.
flows, including those of nuts, coffee and coal, inherently produce
this type of movement among grains, but the design and engineering for
hoppers and other dispensers that don’t account for it won’t work well,
new work “points out the deficiencies in our current theoretical
framework for when granular materials jam,” said Corey O’Hern, an expert
in granular media at Yale University who was not involved in the new
deeper understanding of this point will lead to the design of new
composite granular matter and also to the development of advanced
materials that could counter weapons of mass destruction, including
amplifiers and other countermeasures for deflecting blast waves, he
past studies, physicists calculating how grains flow estimated their
jamming point without accounting for friction forces among individual
particles. Eliminating friction makes jamming easier to explain
mathematically. It also suggested that just an increase in density would
cause granular materials to jam.
been an uphill battle to convince the scientific community that
friction is important, and that shear causes jamming where it was not
expected. No other experimentalists have really looked at what’s
happening with both friction and shear,” Behringer said.
his new experiments, Behringer and his team controlled the number of
discs placed in a box designed to produce a shear strain. The
researchers applied the shearing strain while allowing the discs to flow
where they wanted.
discs had distinct properties that allowed the team to measure the
force each one experienced due to friction and shearing. The team also
took pictures showing how those forces developed into branched chains,
which spread through many discs and ultimately block their flow. The
images and experiments show that because of friction forces and shear
strain, the discs jammed when they were much farther apart, or at a
lower density, than what had been previously predicted.
not just the number of particles that put them in a jam, it’s also the
strain and the real-world forces, like friction, that cause the back-up,
Behringer said. The discovery could change the design of coal and grain
silos and even the bulk dispensers at Whole Foods.
and shear reveal the richness of possible states of granular matter,
pointing scientists down a road paved with new discoveries, said Daryl
Hess, program director for condensed matter and materials theory at NSF.
Studying these new states of granular matter may also expose deeper
connections between jamming and seemingly unrelated phenomena, from
earthquakes to transformations occurring in other kinds of matter, like
water to ice, he said.