  We experimentally investigate the fluidization of a granular material subject
to mechanical vibrations by monitoring the angular velocity of a vane suspended
in the medium and driven by an external motor. On increasing the frequency we
observe a re-entrant transition, as a jammed system first enters a fluidized
state, where the vane rotates with high constant velocity, and then returns to
a frictional state, where the vane velocity is much lower. While the
fluidization frequency is material independent, the viscosity recovery
frequency shows a clear dependence on the material, that we rationalize by
relating this frequency to the balance between dissipative and inertial forces
in the system. Molecular dynamics simulations well reproduce the experimental
data, confirming the suggested theoretical picture.
