Droplet-resolved transport and charge neutralization in bipolar electrosprays

Electrohydrodynamic atomization (EHDA) enables precise control of droplet size but inherently produces highly charged droplets, limiting its applicability in charge-sensitive processes such as pulmonary drug delivery. Bipolar EHDA has emerged as a promising strategy for in-flight charge neutralization through the interaction of oppositely charged plumes; however, the underlying transport mechanisms remain poorly understood. In this work, a droplet-resolved numerical framework is developed to investigate the coupled transport and evolution of droplet trajectories, size, and charge during plume interaction. The formulation combines Eulerian electrostatics with Lagrangian droplet tracking, incorporating inter-droplet Coulomb interactions and a conservation-based collision–coalescence model capable of resolving multi-particle mergers. The results reveal that plume-level neutrality does not imply droplet-level neutralization. Only a fraction of droplets undergo effective charge cancellation, while many escape without interaction, and others experience multiple coalescences, forming larger droplets with residual charge. The ambient electric field is shown to play a dual role: it enhances collision probability by focusing plumes, yet promotes sequential mergers that reduce overall neutralization efficiency and increase size polydispersity. These competing effects lead to a complex, non-monotonic dependence of neutralization performance on operating conditions. This study provides a transport-based understanding of in-flight neutralization in bipolar EHDA systems and highlights the intrinsic coupling between droplet size evolution and charge redistribution. The findings offer critical insights for the design of charge-controlled aerosol systems across biomedical, chemical, and environmental applications.