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Physics of Soft Materials

Fluidized granular materials close to the jamming transition exhibit dynamics reminiscent of atoms or molecules close to the glass transition. Due to the highly packed nature of the system, individual grains cannot move without bumping into their neighbors. This causes neighboring grains to move cooperatively, in string-like chains called "dynamical heterogeneities," as shown in this movie:

The stringy motion is clearly visible when when looking at the instantaneous velocity vectors of the particles (upper right) and when using an overlap order parameter, like the persistent area (lower right). However, it is difficult to see when looking at the particles themselves (upper left) which hardly appear to move at all. This is due to the fact that the cooperative motion involves many particles and results in small very displacements -- typically, much smaller than the diameter of a single particle.