A numerical study of electro-flow-focused micro-jets

Purpose This paper aims to investigate liquid micro-jet formation in electro-flow focusing (EFF), a technique that combines electrostatic and hydrodynamic focusing, and to quantify the influence of electric, flow and liquid properties on jet morphology, stability and breakup dynamics. Design/methodology/approach A numerical model coupling incompressible two-phase Navier–Stokes equations with electrostatic forces and charge transport is developed and experimentally validated. A comprehensive parametric study is performed over a wide range of operating and material parameters, including Reynolds numbers Re = 15.6–191.1, Weber numbers We = 0.1–37.9, capillary numbers Ca = 0.01–0.45 and an electrostatic influence parameter χ = 0.33–10. Findings EFF produces jets that are thinner, longer and more stable than those produced by FF or electrospray alone. Gas pressure contributes to electric-field stabilisation by removing free charges in the gas phase, while the applied voltage enhances interfacial focusing. Liquid properties, such as electrical conductivity, viscosity and surface tension, are shown to modulate jet morphology and breakup dynamics significantly. A previously unreported spatially stable jetting regime is identified, with fixed droplet size and position, forming a periodic pattern relevant for synchronised droplet delivery in serial femtosecond crystallography. The results agree with established experimental scaling relations. Originality/value This work provides the first comprehensive numerical characterisation of EFF. It identifies a novel spatially stable jetting regime, establishes the influence of operational parameters on jet diameter and length and offers physically grounded design guidelines for optimising micro-jet generators across a broad range of technological applications.