Minimum dc electric field requirements for removing powder layers from a conductive surface

An electrostatic powder dispenser was constructed to dispense particles without the use of a carrier gas. This device consisted of two contoured, outer stainless-steel plates that were electrically grounded and a flat, inner copper grid that was electrified and contained a central powder reservoir. Experiments were performed to investigate the levitation of various powders from the reservoir in the presence of an applied dc electric field. Eleven materials including metals, oxides, and conductively coated oxides were studied under vacuum and atmospheric conditions. The electric field required to remove particles from the powder reservoir was found to be a function of particle density and size. An equation was developed that predicted the minimum voltage necessary to remove conductive particles larger than 10 μm in diameter from a conductive surface in a vacuum environment: E=[4.85(ρD)1/2 +0.362]×105 , where E is the field strength (V/m), ρ is the particle density (kg/m3 ), and D is the mass median particle diameter (m). For particles in this size range, gravitational and electrostatic forces appeared to dominate, whereas for particles with a mass median diameter less than 10 μm, adhesive forces appeared to dominate. This equation was also found to hold for the removal of glass beads in air. A semiquantitative model was developed that was consistent with experimental results. This model calculated the force and charge induced on the particles in an electric field while taking into account the neighboring particles.