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The onset of surface fluidization of granular material in a verticallyvibrated container, z=A cos(\omega t), is studied experimentally. Recently, fora column of spheres it has been theoretically found that the particles losecontact if a certain condition for the acceleration amplitude \ddot{z} \equivA\omega^2/g = f(\omega) holds. This result is in disagreement with otherfindings where the criterion \ddot{z}=\ddot{z}_{crit}=const. was found to bethe criterion of fluidization. We show that for a column of spheres a criticalacceleration is not a proper criterion for fluidization and compare the resultswith theory.

Vertically shaken column of spheres. Onset of fluidization