Field Driven Charging Dynamics of a Fluidized Granular Bed
  A simplified model has previously described the inductive charging of colliding identical grains in the presence of an external electric field. Here we extend that model by including heterogeneous surface charge distributions, grain rotations and electrostatic interactions between grains. We find from this more realistic model that strong heterogeneities in charging can occur in agitated granular beds, and we predict that shielding due to these heterogeneities can dramatically alter the charging rate in such beds. 
R. Yoshimatsu, N.A.M. Ara\'ujo, T. Shinbrot, and H.J. Herrmann
physics.comp-ph cond-mat.soft