Simple model for reverse buoyancy in a vibrated granular system G. Gutiérrez, O. Pozo, L. I. Reyes, R. Paredes V., J. F. Drake and E. Ott Europhys. Lett., 67 3 (2004) 369-375 Published online: 01 July 2004 DOI: 10.1209/epl/i2003-10300-3