We describe a model experiment for dynamic jamming: a two-dimensional collection of initially unjammed disks that are forced into the jammed state by uniaxial compression via a rake. This leads to a stable densification front that travels ahead of the rake, leaving regions behind it jammed. Using disk conservation in conjunction with an upper limit to the packing fraction at jamming onset, we predict the front speed as a function of packing fraction and rake speed. However, we find that the jamming front has a finite width, a feature that cannot be explained by disk conservation alone. This width appears to diverge on approach to jamming, which suggests that it may be related to growing length scales encountered in other jamming studies.

1. •S. R. Waitukaitis, L. K. Roth, V. Vitelli and H. M. Jaeger, “Dynamic jamming fronts,” Europhysics Letters 102 , 44001 (2013). pdf file
   

Leah Roth, from St. Olaf College in Minneapolis, MN, was a REU student with our group during summer 2012 and now is a grad student with us.

The figures below are from the paper and give more detail about the set-up and the results.  Fig. 1 is a view of the experiment from the top.

The two movies show the experiment filmed from above, with individual particles colored to indicate their velocity (left; green = 0m/s, red = 2.5 cm/s as in Fig. 1 above) and their local packing fraction (right).