If you rub two different insulating materials against each other, they will charge up: one will become negatively, the other positively charged. This is one of the elementary lecture demonstrations in classes on electrostatics.  But what if you rub two pieces of the very same material against each other? No net charge should build up on either piece, since the likelihood of transferring charge from either one to the other is the same. However,....

Observations of flowing granular matter have suggested that same-material tribocharging depends on particle size, rendering large grains positive and small ones negative. Models assuming the transfer of trapped electrons can explain this, but so far have not been validated. Tracking individual grains in an electric field, we show quantitatively that charge is transferred based on size between materially identical grains. However, the surface density of trapped electrons, measured independently by thermoluminescence techniques, is orders of magnitude too small to account for the scale of charge transferred. This suggests that another negatively charged species, such as ions, is responsible.

1. •Scott R. Waitukaitis, Victor Lee, James M. Pierson, Steven L. Forman, and Heinrich M. Jaeger, “Size-dependent same-material tribocharging in insulating grains”, Phys. Rev. Lett. 112, 218001 (2014). pdf file
   

To measure the net charge on individual particles, the experiments use the ‘free fall videography’ technique introduced earlier by our group (see entry from 3/4/13). The plot above shows the distributions of small and large grains in the sample, measured by the falling camera.  The plot below shows the measured distributions of the horizontal acceleration (red = large grains, blue = small grains). Since we applied a horizontal electric field, this acceleration is directly proportional to the grains’ charge.  Note that the small grains have an average charge that is negative, while the large grains’ average charge is positive.