Sand when it’s packed, flows like liquids

Sand blasting

Sand blasting

What do sand and quarks have in common? The answer, according to
physicists in the US, is that they both behave like liquids under
certain circumstances. When Sidney Nagel and colleagues at University
of Chicago fired jets of sand-like granular materials at solid targets,
some of the resulting spray patterns were very similar to what has been
seen when heavy nuclei collide to produce a “quark-gluon plasma”. The
discovery could shed light on why such a plasma appears to behave like
a liquid rather than a gas – something that has puzzled physicists
since the behaviour was first seen in 2005. In their experiments, the team fired jets of tiny glass or
copper beads at a solid cylindrical target and captured the resultant
spray patterns using high-speed photography (Phys. Rev. Lett. 99 188001). They found that after hitting the target, the jet behaves like a liquid by spreading out
in directions perpendicular to incoming jet. The team also discovered
that when they used targets with diameters smaller than the diameter of
the jet, the beads flowed around the target, creating a bell shape on
the other side. It turns out that this is exactly what happens when a
jet of water hits a similar target and such “waterbells” were first
seen in the 19th century . Rebounding particles
These sheets and bells of beads might seem odd if you think of the jet
as a collection of independent particles, which would each strike the
target and just bounce back. However, if the density of the jet is high
enough, incoming particles can collide with rebounding particles in the
region of the target and these collisions would cause the jet to behave
like a liquid. According to Nagel, the incoming stream of particles
creates a pressure on this “liquid”, causing it to squirt out in
directions perpendicular to jet.
The team measured the angle between the initial direction of the jet
and the final trajectories of the particles for a number of different
targets with cross-sectional diameters both larger and smaller than the
jet itself. They found that the relationship between the angle and
target diameter was identical for jets made of glass or copper beads –
and for water jets with high velocities. As a result, Nagel and
colleagues concluded that the sand jets behave as a liquid.
Liquid-like plasmaFurthermore, Nagel and
colleagues believe that this liquid-like behaviour of colliding
particles has been seen before — at the Relativistic Heavy Ion
Collider (RHIC) at Brookhaven National Laboratory in the US. Two years
ago, researchers at the RHIC smashed together pairs of gold nuclei to
create multi-particle “quark-gluon plasma”. Such a plasma is believed
to be present in the early Universe – just before it has cooled enough
for quarks and gluons to combine and form protons and neutrons.
The RHIC researchers were surprised to discover that the plasma behaved
more like a liquid than a gas. This was evident when the nuclei
underwent glancing collisions, which created rugby-ball or almond
-shaped plasma that expanded more rapidly along certain directions.
This came as a surprise because quantum chromodynamics (QCD) – the
theory describing quark-gluon interactions — predicts that the plasma
should behave like a weakly-interacting gas rather than a
strongly-interacting fluid.
Nagel and colleagues tried to simulate the RHIC collisions by firing
sand jets with a rectangular — rather than circular — cross section
at a cylindrical target. This resulted in a liquid that also expanded
in preferred directions. They measured this anisotropy using the same
dimensionless parameter used to characterize the RHIC plasmas and found
them to be very similar.


According to Nagel, the similarities between the two experiments mean
that it could be possible to describe the liquid properties of the
quark-gluon plasma in terms of the very rapid collisions of a dense
collection of hard spheres – just like the particles in the jets of
sand. “All physics is related”, he told “Studying one branch elucidates effects in a seemingly disconnected area of physics”.

Sand could shed light on quark-gluon plasma –


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