  During pneumatic conveying, powder electrifies rapidly due to the high flow
velocities. In our experiments, the particles even charge if the conveying duct
is made of the same material, which might be caused by triboelectrification
between two asymmetric contact surfaces. Surprisingly, we found the airflow
rate to determine the polarity of the overall powder charge. This study
investigates the charging of microscale PMMA particles in turbulent flows
passing through a square PMMA duct. The particles are spherical and
monodisperse. A Faraday at the duct outlet measured the total charge of the
particles. At low flow velocities, the particles charged negatively after
passing through the duct. However, the powder's overall charge switched to a
positive polarity when increasing the flow velocity.
