Vertically shaken column of spheres. Onset of fluidization
  The onset of surface fluidization of granular material in a vertically vibrated container, z=A cos(\omega t), is studied experimentally. Recently, for a column of spheres it has been theoretically found that the particles lose contact if a certain condition for the acceleration amplitude \ddot{z} \equiv A\omega^2/g = f(\omega) holds. This result is in disagreement with other findings where the criterion \ddot{z}=\ddot{z}_{crit}=const. was found to be the criterion of fluidization. We show that for a column of spheres a critical acceleration is not a proper criterion for fluidization and compare the results with theory. 
Simon Renard, Thomas Schwager, Thorsten Poeschel, and Clara Saluena