In-situ charge reduction of electrospray plumes by unipolar corona ions

Abstract

Obtaining uniformly-sized microparticles by electrospray drying poses challenges due to the high electric charge of the droplets, which often leads to Coulombic fissions, short lifetimes, and charge buildup on the collection. In this study, we have conducted numerical simulations to investigate the use of unipolar ions for achieving extensive droplet discharging in electrosprays, aiming to prevent Coulombic fissions and attain high efficiency in extracting the electrically discharged aerosol. Our mathematical model incorporates Lagrangian tracking of the droplets, their evaporation till forming a solid residue, Coulomb fission, Eulerian descriptions of ion and progeny droplet transports, and droplet discharge kinetics through ion capture. We have identified an effective system configuration comprising coaxially a capillary electrode supporting the Taylor cone, a nearby ring electrode, and an Earth-grounded plate with a central orifice emitting ions. The electrode potentials are selected to sustain the Taylor cone. Air flows coaxially with the electrospray, passing through the ring hole, and exiting the system through a circular slit in the plate. We have simulated two electrospray systems with identical droplet size distributions but different volatilities of the solvents (acetone and methanol). In these systems, the ion cloud rapidly expands from the plate orifice, driven by strong electrical self-repulsion, subsequently flooding the entire spray while traveling towards the ring. The droplets are electrostatically repelled from the ring and are encouraged by the airflow to pass through the ring hole. Most of the droplets are discharged downstream of the ring, before they undergo their first Coulomb fission. Importantly, when the ring is thin and short, it attracts the majority of excess ions. The remaining ions pass through the ring towards the Taylor cone, but are intercepted by the spray, thus preserving the cone stability. Consequently, our simulations demonstrate that the configuration with axial symmetry comprising a ring and an orifice plate between the spraying and corona tips is a robust strategy to obtain stable and efficient in-situ discharging and extraction of electrospray plumes.