Polarity change in the tribo-charge of particles with and without an initial charge

Polarity change in the electrostatic charge of particles impacting on a rotating metal target has been investigated. Experimental results are discussed from the viewpoint of the relationship between the initial charge of particles and the polarity of the tribo-charge. Experimental apparatus, where the initial charge is hardly generated before impacting on the rotating metal target, has been developed using a unit of coaxial dual tubes. It is found that the current generated from the target decreases with time and the tendency depends on the impact angle θ. In particular, the polarity of the current generated by the tribo-charge changes from positive to negative for θ > 75°. A sharper decrease in the current occurs for a higher normal component of impact velocity. A particle feeding system that can change the initial charge of particles has been contrived and it is set between the glass hopper and the unit of the coaxial dual tubes. This equipment enable us to obtain the fact that for θ = 48° (no polarity change), the current decreases linearly with increasing the initial charge, while for θ = 86° (polarity change occurs) it increases with increasing the initial charge and then approaches a certain constant value for a higher initial charge. Both minor and major axes of scars, which are formed by impacting particles on a target, are measured for the case that the polarity change occurs and the apparent area of each scar is calculated as an ellipse. From the results, this is found that the area increases with increasing the impact angle and the tribo-charge increases with the area. Therefore, this suggests that the apparent contact area controls the tribo-charge for the case where the polarity change occurs.