Measurement of single particle impact charging under an external electric field

Electrostatic charges of granular materials always generate under a strong electric field for both nature and industry. To understand the impact charging of a single particle in such situations is therefore essential. However, the traditional Faraday cup approach has difficulty in determining the impact charge with the electric field. Here, we develop Millikan's method to measure the transferred charge of individual particle collision. The target plate lies beneath the vertical electrodes, and a high-speed camera records the particle movement. The single particle passes through the uniform electric field before and after colliding with the target. We can calculate the impact charge of the particle according to its initial charge and final charge, as well as the impact speed and impact angle. In addition, the electric field strength on the particle above the impact point is determined with the finite element method. The immunity to electric field influence on charge measurement and the ability to obtain relevant parameters make this a powerful tool to characterize charging processes under the electric field.