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The research of physicist Arshad Kudrolli and his students on the properties of granular matter can provide clues about how substances as diverse as sand and face powder behave. |
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Going with the flow: granular matter
Professor Arshad Kudrolli's research
What do beaches, drugs and cosmetics have in common? They’re composed, all or in part, of grains of material.
Professor Kudrolli, along with graduate student Azadeh Samadani and undergraduate
Apurba Pradhan '00 are interested in how particles behave. And their research may help answer important questions such as: - What is the best way to mix the components of a drug so that it’s
delivered most effectively to the body?
- Why does an avalanche suddenly occur in an area of
peaceful snow?
- What is the effect of weather on beach erosion?
Kudrolli, Samadani and Pradhan studied the behavior of grains in carefully controlled experimental
settings. The first setting used transparent silos filled with small particles. The second
setting consisted of small steel balls rolling in a box. A digital video camera was used
to record the movement of particles. A variety of grain characteristics could be
manipulated to determine how grains mix together (or don’t mix) and move (or
don’t move) in space. Explore here some of their experiments.
How do grains of similar size behave as
they flow out of a hole?
A very thin rectangular "silo" is created out of plates of clear glass separated
by a small space. All sides except the top are closed. There is a small hole in the
bottom. The silo is filled partway with tiny glass beads. See figures 1 and 2. The process
of the beads emptying through the hole, driven by the force of gravity, is captured on video.
At different times in the emptying process, it is observed that some of the beads are
moving and others are not. Also, the surface shape of the bead pile changes. Initially, a
gentle depression is formed, but the beads on the surface don’t slide. Over time,
however, the surface gradually forms a deeper and deeper "v" shape. When the
angle of the sides of the "v" reaches a certain threshold, called the angle of
repose, the beads on the surface start to slide toward the point of the "v".
From then on, the overall height of the surface continues to drop, but its shape stays the
same.
Digital imaging also allows physicists to monitor the flow pattern of beads in the
interior of the pile. There is an area where particles are moving, another area where the
particles are still, and an interface area where beads transition between the two.
What happens to the flow pattern if beads of different sizes are used in the above scenario?
This time two different sizes of beads, each with an identifying color, are mixed as
evenly as possible and then placed in the silo.
The flow of the pile as a whole was seen to be the same as described in the first
experiment. However, the movement of large versus small beads was different. The larger
beads tended to "percolate" up to the surface and then eventually flow down the
center to the opening in the bottom.
Additonal experiments were performed varying the relative sizes of the beads, and the
proportion of larger versus smaller beads.
What happens if you add some liquid to the grain mixture?
(Think of the difference between dry beach sand
and wet beach sand.) Clark physicists are the first to quantify (express using numbers)
these effects.
The introduction of liquid of varying amounts and viscosities into the silo causes grains
to behave differently. (Remember that viscosity describes how thick a liquid
is: honey is more viscose than milk.)
Here’s what was found:
the addition of liquid
- makes the grains clump up
- causes different types of grains to mix more easily; note that when the amount of liquid
increases past a certain point, this effect ceases
- the addition of liquid increases the angle of repose; that is, the sides of the
"v" must get very steep before the surface grains start to move
- when the surface grains move, they don’t flow constantly, but rather, slip a bit,
then stick, then slip, etc.
more viscous liquids
- make different types of grains more likely to mix
- increase the angle of repose
Click to go to the ClarkNews story about undergrads Ryan O'donnell and Eric Frederick who participated in related research with Professor Kudrolli during summer 2001.
Research funded by the National Science Foundation and the
donors of the Petroleum Research Fund.
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Additional Resources
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Figure 1. The silo setup.
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Figure 2: The granular matter (glass beads) empties from an orifice at the bottom.
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Figure 3: Particles size segregate. Larger particles were found at the center.
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Figure 4: The effect of adding a small volume fraction of fluid to bidisperse glass beads. (a) corresponds to the dry case and (d) where 1% by volume of water has been mixed into the granular mixture before pouring. |
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