Jet thinning and charge transport mechanism in the transition region of cone-jet electrospray

Cone-jet electrospray has been extensively studied and applied in industrial fields. In this work, we investigate the evolution of the jet and charge transport in the transition region from cone to jetting combining theory, visualization, and simulation. A two-dimensional model is developed, yielding a scaling law for the jet diameter as d∼z−0.25, primarily governed by the interaction between surface charge and the axial electric field. Experimental results for ethylene glycol and anhydrous ethanol confirm the power-law (d∼za) decay of the jet diameter with axial distance (z). The exponent a ranges from −0.35 to −0.21 for ethylene glycol and from −0.28 to −0.22 for anhydrous ethanol, closely aligning with the theoretical value of a = −0.25. The jet diameter also exhibits a nonlinear dependence on flow rate, consistent with the scaling relation d/d0∼(Q/Q0)c. Numerical results further validate these findings and elucidate the underlying current transport mechanisms. For ethylene glycol, the charge transport transitions from conduction-dominated to co-dominated by conduction and convection, whereas for anhydrous ethanol, with its higher conductivity and lower viscosity, conduction current remains dominant throughout the transition region. Additionally, the surface charge distribution was measured, revealing that the transition region essentially represents a dynamic equilibrium between electrostatic stress, viscous stress, and surface tension. This work establishes a universal scaling law for jet thinning in the transition region and provides essential insights into the formation mechanism of the steady cone-jet.