The electrospray: Fundamentals and applications

Abstract

The principal features of a particular class of electrostatic sprays are reviewed as they emerge from experimental studies relying mostly on the complementary use of Phase Doppler Anemometry (PDA) and flash shadowgraph. The sprays are all operated in the cone-jet mode in which a liquid is pumped through a metal capillary, at the exit of which it is accelerated through a conical meniscus terminating into a microjet. The acceleration is caused by the electric field established once a voltage differential is applied between the capillary and a counter-electrode. The jet breaks up farther downstream into fine, charged droplets that disperse into an (electro)spray. Key aspects highlighted are: the mechanism of electrostatic/inertia separation by which monodispersity of the droplet size distribution is ensured; the parameters controlling the size and charge of the droplets and some relevant scaling laws; the disruptive phenomenon limiting the maximum charge that a droplet can stably retain; the electric field within the spray and the relative importance of the external field applied between the electrodes and the internal one due to the droplet Coulombic repulsion. The review concludes with a brief description of some applications in the areas of mass-spectrometry (electrospray ionization), combustion, targeted drug inhalation, aerosol science, materials synthesis, deposition technologies and electric propulsion.