Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

Abstract

Processes based on electrostatic projection are used extensively in industry, e.g., for mineral separations, electrophotography, or manufacturing of coated abrasives, such as sandpaper. Despite decades of engineering practice, there are still unanswered questions. In this paper, we present a comprehensive experimental study of the projection process of more than 1500 individual spherical alumina particles with a nominal size of 500 μm, captured by high-speed video imaging and digital image analysis. Based on flight trajectories of approximately 1100 projected particles, we determine the acquired charge and dynamics as a function of the relative humidity (RH) and the electric field intensity and compare the results with classical theories. For RH levels of 50% and above, more than 85% of distributed particles are projected, even when the electric field intensity is at its minimum level. This suggests that, beyond a critical value of the electric field intensity, the RH plays a more critical role in the projection process. We also observe that the charging time is reduced dramatically for RH levels of 50% and above, possibly due to the buildup of thin water films around the distributed particles, which can facilitate charge transfer. In contrast, projected particles at the 30% RH level exhibit excessive amounts of electric charge, between 2 and 4 times than that of the saturation value, which might be attributed to triboelectric charging effects. Finally, the physics of electrostatic projection is compared and contrasted with those of induced-charge electrokinetic phenomena, which share similar field-square scaling, as the applied electric field acts on its own induced charge to cause particle motion.

Charging properties and particles dynamics of Chang’E-5 lunar sample in an external electric field

“…For atmosphere conditions, after poweringup, particles resting on the lower electrode would acquire charges of the same sign from the lower electrode, and would be immediately levitated under the effect of the upward Coulomb force, which is similar to those described in Ref. [30]. The upward Coulomb force would be enhanced with the increasing eld strength.…”

“…The data of charges for each individual particle, which were believed to be the inductive charges, can be obtained by post-processing of the recorded pictures of particles' trajectories, as the method described in Ref. [30]. The detailed data processing method for particles' inductive charges is provided in the next section.…”

Light and Thermally Induced Charge Transfer and Ejection of Micro‐/Nanoparticles from Ferroelectric Crystal Surfaces

“…In this model, the transferred carriers would be OH − and H + ions, and strong experimental evidence supporting it has been reported. [37,38,40,41] Indeed, such mechanism could certainly account for the similarities between metallic and dielectric particles found in this work, as well as the essentially invariant behavior regardless of the sign of the PV charge. Relatedly, several studies have already analyzed the adsorption of water on LN surfaces [42][43][44] and other ferroelectrics.…”

“…However, when dielectric materials are involved, this mechanism is expected to be hindered by the availability of transferable electrons and/or suitable electronic states, and there is an ongoing discussion in the literature about the dominant mechanism in this situation. [37,38] The complexity of this issue is aggravated in our case, where the PV ferroelectric substrate is an insulator in the dark whose photoconductivity increases with light intensity.…”