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 $500\mu \mathrm{m}$, 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.

中文翻译：

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高速视频成像揭示静电投射的物理原理

基于静电投射的方法在工业中被广泛使用，例如，用于矿物分离，电子照相术或诸如砂纸的涂覆磨料的制造。尽管有数十年的工程实践经验，但仍然存在未解决的问题。在本文中，我们对标称尺寸为1500的1500多个球形氧化铝颗粒的投影过程进行了全面的实验研究。$500\mu \mathrm{米}$，由高速视频成像和数字图像分析捕获。基于大约1100个投影粒子的飞行轨迹，我们确定所获得的电荷和动力学是相对湿度（RH）和电场强度的函数，并将结果与​​经典理论进行比较。对于50％或更高的相对湿度水平，即使电场强度处于最小水平，也将投影出超过85％的分布颗粒。这表明，除了电场强度的临界值外，RH在投影过程中起着更为关键的作用。我们还观察到，相对湿度50％或更高时，充电时间显着减少，这可能是由于分布的颗粒周围形成了薄水膜，这可以促进电荷转移。相反，相对湿度为30％时，投射的粒子显示出过量的电荷，是饱和值的2到4倍，这可能归因于摩擦带电效应。最后，将静电投射的物理性质与感应电荷的电动现象进行了比较和对比，感应电荷的电动现象具有相似的场平方比例，因为施加的电场作用于其自身的感应电荷以引起粒子运动。