Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:
Approach 1: Impact Charging Experiment:
To apply the previous version of “impact charging experiment,” in which rather bigger particles were used as samples, to smaller particle sizes, the sensitivity of the charge measurement and trajectory control of the accelerated particles were required to be improved. In the first year project, we carried our the improvements, and new rig, which is available for hundreds micro- meter particles, was manufactured successfully. In the second year, actual experiments were performed, but we encountered with data scattering, while the order of magnitude of the data agreed well with that predicted by “charge relaxation model” which we have proposed as a scheme determining the amount of the impact charge.
In the third year, we tried to improve the rig with some points which could cause the scattering. In a point, particle trajectory control, was improved successfully, and the data accumulation was improved. However, the data scattering did not disappear: a question to be answered here was if there was a reasonable cause of this data scattering. Some discussions were done, and as its result, a localization of the initial charge was supposed. A top and a rear side localization against the contact point, gave two limitations. The scattering data were indeed held within the range. As a conclusion, the charge relaxation model works in the range of the particle size from 100 to 300 micro-meter. For the future study, a successive impact experiment for electrostatic charging of particles would be recommended.
Approach 2: Development of the Method of Electrostatic Adhesive Force Measurement:
This is the completely new approach to determine the amount of charge transferred onto a par- ticle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target.
In the third year project, we have actually launched AFM study. Now the experiments are still pilot status, but a force curve due to electrostatic charge was measured successfully for a 35 micro-meter particle. The amount of the electrostatic charge was estimated in the order of 0.1fC, and at this moment the sensitivity is enough for the order of magnitude of the amount of charge, which corresponds 1000 electrons (elementary charges). In the next year project, actual and detailed discussion will be available.