Permanent electric dipoles on gas-suspended particles and the production of filamentary aggregates

The shape of an aggregate of particles is the result of the forces acting in its formation, and in turn strongly influences the properties of the aggregate. In this paper we investigate the forces acting on particles in the gas phase to form extended chains. Electrical charge on the particles is normally invoked to explain these chains, in the form of charges of opposite sign on different particles, and dipoles induced by imposed electric fields. We emphasize another possibility; permanent electric dipoles composed of separated persistent charges of opposite sign on the same particle. We discuss (a) contact charging for micron sized dielectric particles and (b) chemical charging during growth of oxide layers on metal nanoparticles, noting experimental evidence supporting the existence of dipoles. A mapping of different aggregate forms from dipolar particles has been attempted, including magnetic and electrical dipoles, taking account of concentration, dipole strength and time, all in dimensionless form. Turbulent diffusion is found important with flow situations. Aggregation of dipole particle chains into ball-like objects is mentioned with respect to ball lightning.