Particles are often electrostatically charged by frictional contact during powder-handling operations. This phenomenon is called 'triboelectric charging' or 'contact electrification'. The charged particles cause problems such as particle deposition and adhesion. In addition, if particles are excessively charged, an electrostatic discharge may occur, which can pose a risk of fire and explosion hazards; thus, to mitigate the adverse effects, it is important to elucidate the underlying triboelectric charging mechanisms. The electrostatics is, on the other hand, very useful in a number of applications that have been developed using the principles. In this review, the basic concepts and theories of charge transfer between solid surfaces are summarized, and chemical factors depending on materials and environmental effects are described. To theoretically analyze the process of particle charging, relevant models are discussed. Using the models, particle charging by repeated impacts on a wall is formulated. To experimentally evaluate particle charging, measurement and characterization methods are outlined. Furthermore, important applications and computer simulations are described.

The phenomenon of electrostatic charge generation and its effects on granular flow behavior in a pneumatic conveying system was studied. The main parameters used for quantitative characterization of the phenomenon were the induced current, particle charge density, and equivalent current of the charged granular flow. These were measured using a digital electrometer, Faraday cage, and modular parametric current transformer, respectively. Three different flow patterns corresponding to different electrostatic effects within the pneumatic conveying system were observed, and these were named the disperse flow, half-ring flow, and ring flow patterns. It was found that the induced current, particle charge density, and equivalent current increased with decreasing flow rates. Electrostatic effects generally become stronger with time, and this may lead to clustering behavior occurring even in the disperse flow regime. The effects of several factors such as pipe wall material, particle composition, relative humidity of the conveying air used, and the presence of an antistatic agent in the system were investigated and found to be important in determining the electrostatic charge generation characteristics and granular flow patterns observed.

Impact electrification between an elastic sphere and a metal plate has been studied experimentally. To find out how charge transfers between the contact bodies, the voltage profiles at the impact are measured under various experimental conditions using a digital oscilloscope, and simultaneously the contact deformation of the sphere is visualized with a high-speed camera. The initial charge on the sphere and the transferred charge are obtained from the integrated voltage with respect to the elapsed time of the impact process. The variation of the electrification by repeated impacts is analysed by taking account of the initial charge and charge relaxation with elapsed time. Furthermore, the relationship between the transferred charge and the contact area as a function of the impact velocity is investigated based on the electrification theory and Hertz analysis of elastic contact deformation.

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