  Observations of flowing granular matter have suggested that same-material
tribocharging de- pends 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.
