  In clouds of suspended particles (grains, droplets, spheres, crystals, etc.),
collisions electrify the particles and the clouds, producing large electric
potential differences over large scales. This is seen most spectacularly in the
atmosphere as lighting in thunderstorms, thundersnow, dust storms, and volcanic
ash plumes where multi-million-volt potential differences over scales of
kilometers can be produced, but it is a general phenomenon in granular systems
as a whole. The electrification process is not well understood, especially for
electrification of insulating particles of the same material. To investigate
the relative importances of particle properties (material, size, etc.) and
collective phenomena (behaviors of systems at large scales not easily predicted
from local dynamics) in granular and atmospheric electrification, we used a
table-top experiment that mechanically shakes particles inside a cell where we
measure the macroscopic electric field between the electrically conducting end
plates. The measured electric fields are a result of capacitive coupling and
direct charge transfer between the particles and the plates. Using a diverse
range of mono-material particle sets (plastics, ceramic, glass, and metals), we
found that all our particle materials electrify and show similar dynamics with
long time-scale temporal variation and an electric field amplitude that depends
on the particle quantity in a complex way. These results suggest that while
particle properties do matter like previous investigations have shown,
macroscopic electrification of solids is relatively material agnostic and large
scale collective phenomena play a major role.
