Jamming. One emphasis of our work is to understand the properties of disordered materials. Such materials have many common features that are different from their crystalline counterparts.  One example is of particular note.  By varying some external parameter, these materials can become structurally arrested - that is they jam.  We are interested in understanding what controls the onset of rigidity in a wide variety of situations.  Do all jammed systems have a common set of inherent properties?  If so, can we learn about the nature of glasses (where the ability to flow has been lost when the temperature is dropped to too low a value) by studying the jamming that occurs in a granular material such as a sand pile as it suddenly stops flowing?  In an effort to deal with diverse phenomena where systems become stuck in a region far from equilibrium (e.g., at the glass transition and in clogged granular materials flowing - unsuccessfully - through a pipe), we have been investigating, in collaboration with Professor Andrea Liu at The University of Pennsylvania, whether there can be a more general way of looking at these systems in terms of a Jamming Phase Diagram.  Such a concept would relate the physics of granular materials with those of glasses.

Glass Transition. We have been studying this transition using a variety of techniques including neutron diffraction, specific heat spectroscopy, computer simulation, dielectric susceptibility, and shear modulus.  We have managed to produce a master curve onto which all the dielectric data from all samples over 15 decades in frequency can be scaled. Such scaling has important implications for the nature of the glass transition.

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Active Research Areas:

  1. •“Normal modes in model jammed systems in three dimensions,” L. E. Silbert, A. J. Liu and S. R. Nagel, Phys. Rev. E 79, 021308 (2009).

  2. •“Energy transport in jammed sphere packings,” N. Xu, V. Vitelli, M. Wyart, A. J. Liu, S. R. Nagel, Phys. Rev. Lett. 102, 038001 (2009).

  3. •"Excitations of Ellipsoid Packings near Jamming," Z. Zeravcic, N. Xu, A. J. Liu, S. R. Nagel and W. van Saarloos arXiv:0904.1558v1

  4. •“Excess Vibrational Modes and the Boson Peak in Model Glasses,” N. Xu, M. Wyart, A.  J. Liu and S. R. Nagel, Phys. Rev. Lett. 98, 175502 (2007).

  5. •“The compressible spin glass: Simulation results,” A. H. Marshall, Phys. Rev. B 75, 054414 (2007)..

  6. •“Structural signatures of the unjamming transition at zero temperature,” L. E. Silbert, A.  J. Liu, and S. R. Nagel, Phys. Rev. E 73, 041304 1-8 (2006).

  7. •“Vibrations and diverging length scales near the unjamming transition,” L. E. Silbert, A.  J. Liu and S. R. Nagel, Phys. Rev. Lett. 95, 098301  1-4 (2005).

  8. •“Structural signature of jamming in granular media,” E. I. Corwin, H. M. Jaeger, and S.  R. Nagel, Nature 435, 1075-1078 (2005).

  9. •“Effects of compression on the vibrational modes of marginally jammed solids,” M.  Wyart, L. E. Silbert, S. R. Nagel and T. A. Witten, Phys. Rev. E 72, 051306 1-11 (2005).

  10. •“Numerical studies of the compressible Ising spin glass,” A. H. Marshall, S. R. Nagel and B. Chakraborty, Europhysics Lett. 74, 699-705 (2006)

A jammed packing of ellipsoids showing the amplitude of each particle's angular motion in a low-frequency normal mode.

Displacement field showing cluster formation at four densities during the jamming of tapioca pearls.  The scale bar in (a) is 5 cm.