Size-dependent same-material tribocharging in insulating grains

TL;DR

This study demonstrates that size-dependent tribocharging in insulating grains involves charge transfer based on size, but the actual charge transfer likely involves ions rather than trapped electrons, challenging existing models.

Contribution

The paper provides experimental evidence that size-dependent tribocharging occurs via charge transfer between identical grains, but suggests ions may be responsible instead of trapped electrons.

Findings

Charge transfer depends on particle size among identical grains.

Surface trapped electron density is too low to explain observed charges.

Ions, not trapped electrons, are likely responsible for tribocharging.

Abstract

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.