Sheared, not shaken: contact numbers are history dependent

Granular media gain mechanical stability by the contacts between individual particles. Predicting the average number of contacts per particle as a function of shape, volume fraction, and friction coefficient has been an important test case for the various theoretical approaches. At present, experimental data for tapped sphere packings is in good agreement with the mean field type approach of Song et al. (2008). Here we present measurements of sphere packing prepared by gyratory shear, a method which avoids the formation of shear bands. We find that the number of contacts as a function of volume fraction does depend strongly on the way the packing was prepared.