On the design of electrospray emitters and their microfluidic behavior

Abstract

Propellant transport to the emission site of electrospray thrusters determines the operational regime of the propulsion unit. The focus of this research is on electrospray emitters composed of a porous substrate through which the fuel is passively fed. The hydraulic impedance of the propellant from the fuel reserves to the emission site is critical in the determination of the emission characteristics of the thruster. An impedance value of <1.5 x 10¹⁷ kgm⁻⁴s⁻¹ or higher is required to achieve pure ionic emission. A gap in the literature exists in for the analysis of high-fidelity modeling of the total hydraulic impedance of emitters. Estimates of the internal and external wetting characteristics are combined to approximate the total hydraulic impedance through the porous lattice of borosilicate glass, carbon aerogel, Varapor100, and fused silica emitters.

Sensitivity analysis of the governing equations reveals that the choice of material properties, i.e., pore size, is the driving factor of the impedance. Secondary factors are driven by the geometrical features of the emitter tips. The state-of-the-art emitter tips are fabricated via a laser ablation process. Characterization of the ablation properties was performed on the carbon aerogel and the Varapor100 substrates. Analysis of the internally fabricated carbon aerogel identified correlations between the lattice structure and the fabricated emitter tips. Furthermore, significant variations in the ablation characteristics were noted between batches of the carbon aerogel. The fabrication of the Varapor100 test articles was designed to decouple the highly coupled free parameters of the laser ablation process. The relative effects of multiple variables are quantified in the context of the Varapor100 substrate.

Strategies to limit the by-production of debris and a fused layer on the Varapor100 emitter tips are presented. The Varapor100 and fused silica emitter arrays are tested for Retarding Potential Analysis and Time of Flight Mass Spectrometry measurements to determine the emitted beam composition. Both substrates produced pure ionic emissions. Approximately half of the beam is found at the applied potential for both materials. However, significant fragmentation was observed in the acceleration region of the Varapor100 emitters, reducing thruster efficiency. From the literature carbon aerogel emissions have been reported to operate in the pure ionic regime, with the ionic liquid EMI-BF₄, and the mixed ionic-droplet mode, for low viscosity EMI-CF₃BF₃, consistent with impedance modeling results.