Fluidized States of Vibrated Granular Media Studied by Mechanical Spectroscopy

We investigate the jamming transition observed in vibrated granular systems composed of millimeter size glass beads. When a granular system is submitted to vibrations with decreasing intensity, it evolves in a way similar to glass-forming liquids: from a low viscosity, liquid-like state, it evolves into an amorphous jammed state. This evolution is observed by the means of an immersed oscillator acting as a torsion pendulum in forced mode. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency and of the vibration intensity. Focusing on the strongly vibrated states, we observe that there are two different dynamic regions. The first is a high fluidization regime, where the internal friction is found to be proportional to the ratio between the pulsation and the vibration intensity: tan(delta)=omega/Gamma. In this region, the system shows an apparent viscous friction eta=1/Gamma. In the second, low fluidization, regime, we observe a more complex behavior, and the measured internal friction appears to be well described by a relation of the form: tan(delta)=(omega/Gamma(2))(alpha) In this second case, the key role is played by a critical breakaway stress, sigma(cr), needed to break the network of chains of forces that form between the grains. Finally, if vibration intensities are still reduced, we also observe that onset of jamming is clearly distinguishable: we observe a sharp increase in the apparent dynamic modulus together with a peak in internal friction. This transition presents important similarities to those observed in glasses, and it leads to the second (low vibrations) regime, where the key role is played by the square root of the vibration intensity.