  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 projection process of more than 1500 individual spherical
alumina particles with a nominal size of 500 $\mu$m, captured by high-speed
video imaging and digital image analysis. Based on flight trajectories of
approximately 1100 projected particles, we determined the acquired charge and
dynamics as a function of relative humidity (RH) and electric field intensity
and compared the results with classical theories. For RH levels of 50\% and
above, more than 85\% of disposed particles were projected, even when the
electric field intensity was at its minimum level. This suggests that, beyond a
critical value of electric field intensity, relative humidity plays a more
critical role in the projection process. We also observed that the charging
time is reduced dramatically for RH levels of 50\% and above, possibly due to
the build-up of thin water films around the particles which can facilitate
charge transfer. In contrast, projected particles at 30\% RH level exhibited an
excessive amount of electric charge, between two to four times than that of
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 field acts on its own induced charge to
cause particle motion.
