Analysis of the Effects of Ionic Liquid Properties on Electrospray Thruster Performance

Ionic liquids (ILs) have proven extremely useful for a wide variety of roles, including as propellants for electrospray thrusters (ETs), due to their unique physical and chemical properties, as well as the potential tunability of those properties, through chemical engineering. However, there is a lack of literature exploring the effects of IL properties on ET operation. This paper presents experimental results investigating key physical properties of the common ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMI-TFO), EAN, and Bmpyr-DCA not provided by manufacturers or reported in the literature, namely, their electrochemical stability windows (ESWs) and contact angles. Cyclic voltammetry experiments were employed to define the ESW of each IL, which is necessary for long-term ET operation while avoiding chemical breakdown. Contact-angle measurements were also conducted to study the wettability of the ILs on glass surfaces to be used for ET thruster substrates [Howell, J. H.; . J. Electrost. 2023, 122, 103799]. In addition, an analytical discussion is presented using established parametric relationships and scaling laws to examine the effects of relevant IL physical properties, such as surface tension and ion molecular weights, on ET performance. The results demonstrate the relative impact of IL properties on important ET figures of merit such as thrust density, power density, and specific impulse, which provide key insights into the future development of novel ILs specifically tailored for use as ET propellants.