Charging and Levitation of Particles Using UV Irradiation and Electric Field

The charging and levitation of particles using UV irradiation in an upward electric field were investigated. Glass beads with a mass median diameter of 61 μm were used for the experiments. The particle layers formed on a glass plate were irradiated by UV light with wavelengths ranging from 240 to 400 nm. The particles in the top layer were positively charged by photoemission and levitated by the electrostatic forces. The fluxes and motions of the levitated particles were analyzed by digital processing of the images obtained with a high-speed camera. The charge of each levitated particle was determined by fitting the results calculated using the equation of motion to the experimentally obtained particle motion. The charge of the particles beginning to be levitated was determined by the force balance and thus was not affected by the UV irradiance. The positive charge of the levitated particles could be increased by continuous photoemission from the particles and decreased by the capture of photoelectrons emitted by other particles. Approximately 40% of the levitated particles descended because of the negative charge clouds formed by the photoemission from the particle layers and the levitated particles. Furthermore, in a second set of experiments, an upward electric field was applied after UV irradiation. In these experiments, the particles in the top layer were levitated after applying the electric field. These particles could be gain more excess charges than the particles for which the levitation was governed by the force balance.