  Powders and grains exhibit unpredictable jamming-to-flow transitions that
manifest themselves on geophysical scales in catastrophic slip events such as
landslides and earthquakes, and on laboratory/industrial scales in profound
processing difficulties. Over the past few years, insight into these
transitions has been provided by new evidence that slip events may accompanied,
or even preceded, by electrical effects. In the present work, we quantify the
correlation between slip and the separation of electrical charges, using an
archetypal granular material: photoelastic polymers. We measure a strong
correlation between material displacement, acoustic emissions, and voltage. We
find that the generation of voltage is associated with surface, rather than
bulk properties of the granular materials. While voltage precursors are only
occasionally observed in this system, there is some asymmetry in the
cross-correlation between the slip and voltage signals that indicates
differences between the pre-slip and post-slip dynamics.
