Particles can be lifted in electric fields when the electrostatic forces overcome the force due to gravity. For conducting particles, the lift-off occurs when the electric field causes charge to be transferred from a grounded surface to the particle, and the charged particle is then lifted in the electric field; this mechanism occurs with either polarity of the electric field. For perfectly insulating particles, this charge transfer cannot occur, but the particles that are already charged can be lifted by an electric field of the appropriate polarity (but not the other polarity). Experiments were carried out, under ambient and environmentally controlled conditions, on the lift-off of particles composed of different materials, and the results for the threshold electric fields necessary for lift-off were compared with predictions based on the conducting and insulating particle models. Results for an aluminum particle are in agreement with the conducting particle model. In the case of insulating particles, the lifting of polytetrafluoroethylene (PTFE) particles is consistent with the insulating particle model, but Nylon® and soda-lime glass exhibit humidity-dependent behavior. At low humidity, Nylon® and soda-lime glass particles are lifted in accordance with the insulating particle model, while at high humidity, the lifting behavior surprisingly follows the conducting particle model. It is suggested that at high humidity, the hydrophilic nature of the Nylon® and soda-lime glass particle surfaces leads to a conducting surface layer of water that facilitates charge transfer similar to a metal particle.
electrostatic charge; humidity; lift-off
