Phase changes in granular materials

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One of the frontiers of soft matter research is understanding the behavior of granular material. For instance, as the density of a granular material is increased, the particles undergo a transition in which they become "jammed" and form large, spider-like chains. These exhibit many characteristics of a phase transition, such as growing length an time scales. However, unlike other phase transitions, these structures do not become rigid. The structures that form exist only as dynamical correlations. As described by Keys et al. (see #2), these features were actually observed experimentally with a collection of about 1,400 to 1,900 steel spheres (~ 3 mm in radius) that were excited by an upward flow of air, but trapped in a horizontal plane. The motions of each particle were tracked with a camera, in something like a real-world molecular dynamics simulation. Some of their experimental results are shown below:

GranularDiagram.jpg

These results show how various experimentally observed parameters varied over the course of a 20-minute experiment.

  1. The mean-square-displacement rises at varying rates, depending on the whether the beads can travel ballistically or as if trapped in cages.
  2. The average number of particles in each cluster of beads that move together.
  3. The average length of particles in a chain of beads that move together.
  4. "Self-contribution to the four-point susceptibility"
  5. An instantaneous representation of the motion of beads in the system. The top 10% most rapidly moving beads are colored red and are clearly not distributed randomly throughout the area.

For more information see:

  1. Biroli, G., "A new kind of phase transition?" Nature Phys. 3, 222–223 (2007).
  2. Keys, A. S., Abate, A. R., Glotzer, S. C. & Durian, D. J. Nature Phys. 3, 260–264 (2007).