Memory Effects in Granular Material
  We present a combined experimental and theoretical study of memory effects in vibration-induced compaction of granular materials. In particular, the response of the system to an abrupt change in shaking intensity is measured. At short times after the perturbation a granular analog of aging in glasses is observed. Using a simple two-state model, we are able to explain this short-time response.   We also discuss the possibility for the system to obey an approximate pseudo-fluctuation-dissipation theorem relationship and relate our work to earlier experimental and theoretical studies of the problem. 
Christophe Josserand, Alexei Tkachenko, Daniel M. Mueth and Heinrich   M. Jaeger (James Franck Institute, U. of Chicago)