Vertically shaken column of spheres. Onset of fluidization

The onset of surface fluidization of granular material in a vertically vibr ated container, z = A cos (wt), is studied experimentally. Recently, for a column of spheres it has been theoretically found (see T. Poschel, T. Schwa ger, C. Saluena, Phys. Rev. E 62, 1361 (2000)) that the particles lose cont act if a certain condition for the acceleration amplitude <(z)double over d ot> = Aw(2)/g = f(w) holds. This result is in disagreement with other findi ngs where the criterion <(z)double over dot> = <(z)double over dot>(crit) = const was found to be the criterion of fluidization. We show that for a co lumn of spheres a critical acceleration is not a proper criterion for fluid ization and compare the results with theory.