Tribocharging of patchy particles

When granular materials flow, particles charge significantly resulting in large electric fields between them and a strong alteration of the material’s flowing properties. The charging of granular materials arises from a transfer of charges at contact between the particles as a consequence of the triboelectric effect. It has been shown that many parameters such as ambient humidity or particle size influence the charging of granular materials making this effect complex to understand. The existence of positively and negatively charged patches at the surface of the objects in contact has been postulated to interpret same-materials tribocharging and show good reproducibility. However, this model has never been applied to granular materials. We performed DEM numerical simulations of patchy particles and obtained charge transfers with our model which are consistent with experimental results. We also show that this model correctly reproduces the specific charging of bi-disperse granular materials where small particles charge negatively while large particles charge positively. Our results thus provide another evidence that the existence of patches at the surface of objects could explain same-materials tribocharging. They also strengthen the current hypothesis on the role of water as we compare our model to the transfer of ions from wet to dry patches on the particles and make the connection with ambient humidity.