Nonmonotonic reversible branch in four model granular beds subjected to   vertical vibration
  We present results from four independent models of a granular assembly subjected to tapping. We find that the steady-state packing fraction as a function of the tapping intensity is nonmonotonic. In particular, for high tapping intensities, we observe an increase of the packing fraction with tapping strength. This finding challenges the current understanding of compaction of granular media since the steady-state packing fraction is believed to decrease monotonically with increasing tapping intensity. We propose an explanation of our results based on the properties of the arches formed by the particles. 
Luis A. Pugnaloni, Martin Mizrahi, Carlos M. Carlevaro, Fernando   Vericat