Figure 1: Typical snapshot of simulation showing the charging of an agitated granular bed.
From: Self-charging of identical grains in the absence of an external field

Here we use χe = η = 1 as defined in text, and show time after 5 seconds. Grains are colored according to their net charge defined as: (blue: negative, red: positive); with grains with an absolute net charge magnitude below 20 pC in transparent grey.