FRICTION IN VIBRATED POWDERS - A MECHANISM FOR MEMORY

This work is a phenomenological attempt to characterise the dynamic response of a granular pile to vibration. We model the effect of vibration on a powder as random noise in its interior, and write down an equation for the relaxation of the compactivity parameter X, which characterises the packing of the pile. This is shown to settle to a value which reflects the balance between the amplitude of the noise and gravity. In addition, we formulate a phenomenological coefficient of friction which depends on the instantaneous packing, and thereby put forward a plausible mechanism for the way in which the history of the powder determines its final state.