Mobility of a Sphere in Vibrated Granular Media

We report the results of the first measurements of the mobility/friction coefficients of a sphere dragged horizontally through a vertically vibrated granular medium. The friction coefficient depends on the state of excitation of the medium and drops rapidly when the acceleration of the vibrated cell exceeds that of gravity. We propose a simple model involving fluidization/solidification events during a period of vibration of the cell. We argue that the observed mobility is proportional to the fraction of the time spent in the "fluid" state and discuss the origin of the observed dependence on the various parameters.