Influence of humidity on the tribo-electric charging and segregation in shaken granular media

TL;DR

This study investigates how humidity influences tribo-electric charging and segregation in shaken polymer granulates, revealing a strong dependence on humidity levels and material properties, which can be used to control particle segregation.

Contribution

It provides the first detailed analysis of humidity effects on tribo-charging in granular media and demonstrates how humidity control can influence segregation behavior.

Findings

Low humidity (<30% RH) causes high charge accumulation.

High humidity (>80% RH) results in negligible charge.

Charge sign correlates with contact angle and hydrophilicity.

Abstract

We study the effect of humidity on the charge accumulation of polymer granulates shaken vertically in a stainless steel container. The setup allows to control the humidity level from 5 % to 100 %RH while performing automated charge measurements in a Faraday cup directly connected to the shaking container. We find that samples of approximately 2000 polymer spheres become highly charged at low humidity levels (<30 %RH), but acquire almost no charge for humidity levels above 80 %RH. The transition between these two regimes does depend on the material, as does the sign of the charge. For the latter we find a correlation with the contact angle of the polymer with only very hydrophilic particles attaining positive charges. We show that this humidity dependence of tribo-charging can be used to control segregation in shaken binary mixtures.