Wetting Properties of EMIIm & its Relevance to Electrospray Design

Recent advances in the development of highly conductive ionic liquids have made them of interest for use as propellant in spacecraft propulsion systems. Electrospray thrusters apply strong electrostatic fields to an ionic liquid in order to extract and accelerate charged particles/droplets, producing thrust. The behavior of these ionic liquids as they pass through the components of an electrospray system can have a significant effect on thruster operation. The wetting and adhesion behavior between the ionic liquid propellant and solid materials can be characterized using the surface tension and contact or wetting angle formed when a liquid droplet comes in contact with a solid surface. Ideally this angle is a function of the interactions between the solids surface energy, the surface tension of the liquid and the interactions of both with the surrounding medium. Deviation from ideal contact angle behavior can indicate surface inconsistencies, environmental effects or contamination of the solid and liquid. Contact angle and surface tension measurements are presented for the ionic liquid propellant 1-Ethyl-3-methylimidazolium bis(triuoromethylsulfonyl)imide, called EMIIm or EtMeImTf2N, with respect to various substrate materials and environmental conditions. Analysis of these measurements determines optimum materials and operating conditions for current and future electrospray thruster designs. The original document contains color images.