On the quasi-ionic emission regime of electrified ionic liquids from capillary sources, and its transition into the purely ionic regime

Abstract Negatively electrified liquid cone jets supported on capillary tubes with 30–36 $ \,\unicode{x03BC} \text{m}$ tip diameters are investigated in vacuo with four ionic liquids (ILs) selected for their high electrical conductivity and low viscosity. All four use the same cation 1-ethyl-3-methylimidazolium $^+$ ( $ \text{EMI}^+$ ), paired with the four anions $\textrm{SCN}^-$ , $\text{N}(\text{CN})_2^-$ , $\text{C}(\text{CN})_3^-$ and $ \text{BF}_4^-$ . Purely ionic (PI) emissions are not unambiguously achieved with any of the four, but are closely approached by all under a broad range of conditions. In this unusual quasi-ionic (QI) regime, drops contribute minimally to the current ( $\sim$ 0.5 %–3 %) but substantially to the mass flow. A sharp QI $\rightarrow$ PI transition below a critical liquid flow rate has been demonstrated for capillary emitters by Caballero-Perez et al. (2025) J. Propul. Power, for 1-butyl-3-methylimidazolium-C(CN) $_3$ (BMI-C(CN) $_3$ ) by using 15 $\unicode{x03BC}$ m capillary tips able to stabilise unusually small liquid flow rates. None of their other 3 ILs achieves the QI regime, indicating the singularity of BMI-C(CN) $_3$ and our four ILs. We focus on the peculiarities of the QI regime, the likely mechanism for the QI $\rightarrow$ PI transition and argue that ILs reaching the QI regime will probably also attain the PI regime when sprayed from sufficiently small capillary tips. Paradoxically, while high conductivity and low viscosity appear to favour the QI mode, for a liquid operating in this regime, inverting these properties by lowering the emitter temperature appears to better approach the PI regime.