ABSTRACT

The frequency response of a granular medium is measured by a forced torsion pendulum method, with forcing frequency fp in the range 10−4 Hz to 5 Hz, while external snaking at high-frequency fs, in the range 50 Hz to 200 Hz, mimics thermal fluctuations. The intensity of shaking, T, is below the fluidization limit. A loss factor peak is observed in the pendulum response as a function of Γ or fp. In a plot of ln(fp) against 1/Γ, the position of the peak follows an Arrhenius-like behavior over five order of magnitude in fp. The data are described as an activated process involving a Boltzmann-like factor, exp(-τ j/τ), with τ j a characteristic unjamming time, and τ =Γ1/2/2πfs a temperature-like parameter. τ is seen as the average time during which the shaken grains have some freedom to rearrange and, possibly, unjam. The loss factor peak appears as a jamming transition, which separates a jammed phase where the pendulum is stuck into the granular material, from a fluid-like phase where the pendulum slides dissipatively by successive stick-slip events.