Novel Materials & Additive Manufacturing for Electrospray Propulsion
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davis-ruthd-sm-aeroastro-2026-thesis.pdf
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Author(s)
Davis, Ruth A.
Advisor(s)
Wardle, Brian L.
Date Issued
February 2026
Publisher
Massachusetts Institute of Technology
Abstract
Electric propulsion technologies can meet many emerging needs in space propulsion, with advantages of high specific impulse and increased fuel efficiency. Electrospray propulsion in particular offers low-thrust propulsion at high efficiency, and is scalable to larger-thrust applications through the operation of many individual electrospray emitters in parallel. Electrospray emitters are microscale, needle-like structures that facilitate microfluidic transport of ionically conductive propellant and emit ions or charged droplets from their tip due to an
applied electric field. While electrospray propulsion is promising, common manufacturing methods result in limited lifetime with failure mechanisms associated with the propellant flow path and the geometry of each individual emitter. Commonly experienced issues include flooding of ionic liquid (IL) propellant at the emitter tip and misaligned menisci, which often result in off-axis emission and shorting of the device. The manufacturing of electrospray geometries with micron-scale resolution, or better, and increased repeatability could mitigate these issues. This thesis investigates polymer electrospray emitters, a material set not commonly used for this purpose. First, the additive manufacturing (AM) technique of two-photon polymerization (TPP) is employed to create physically porous thermoset polymer electrospray geometries with microscale resolution. The surface energy of the TPP polymers, important for wetting by the IL, is found to be lower than most traditional electrospray materials by 1-2 orders of magnitude. This did not impede creating working emitters, but atomic layer deposition (ALD) was investigated as one way to alter the printed
polymers’ surface energy. Various electrospray emitters were designed based on electrospray principles and performance predictions. After manufacturing, the emitter designs were characterized by collecting emitted current and intercepted current through voltage ramping in
positive and negative polarity. Initial experiments indicated the importance of the polymer surface energy and wetting characteristics of the emitter geometry, with non-axial emission experienced for each designed emitter in the first design iteration. A second design iteration
resulted in improved outcomes, with both higher emitted currents and lower startup voltages observed. An emitter incorporating 8 micron scale porosity and a 15 micron diameter central capillary exhibited a startup voltage of ∼1600 V and emitted more than 1 μA in current,
performance on par with traditional electrospray emitters, and operated in both polarities for up to ten minutes without current degradation.
Second, this thesis also investigates the potential of solid polymer electrolyte (SPE) emitters, which are not physically porous but rather conduct ions through their free volume, potentially circumventing many of the extant issues. A procedure is established for casting SPE electrospray emitter geometries from silicone mold negatives created from solid, TPP-printed emitter positives. Two novel, high-performance SPEs, sulfonated poly ether ether ketone (sPEEK) and an Epoxy SPE, are synthesized and cast into electrospray emitters. A casting procedure was iteratively developed for both single emitters and arrays, with an array of 100 emitters demonstrated for the sPEEK SPE. Ionic conductivity of the SPEs in thin film form, after soaking in IL, is measured through electrochemical impedance spectroscopy
(EIS), showing that the Epoxy SPE has higher ionic conductivity compared to the sPEEK SPE, with both being 1-2 orders of magnitude less than the SPE NafionTM used in prior work. However, the new SPEs allow for ionic conductivity tuning based on synthesis parameters and possess superior mechanical properties, which could prevent degradation and increase lifetime. Future ion emission testing under vacuum is required to adequately assess and compare the performance of each SPE.
The manufacturing and experimental testing results for the AM electrospray emitters suggest the potential of TPP to improve electrospray performance. These methods and materials could enable the design of emitters for operation in the most beneficial mode for
a specific mission application. SPE emitters are further promising, and incorporating TPP into the silicone mold manufacturing process opens up the design space for SPE emitters by enabling tuning of the emitter geometry towards even greater performance. The work presented in this thesis provides a basis for utilizing novel materials and AM processes to advance the design and synthesis of polymer-based electrospray emitters with enhanced performance and attributes.
MIT Department
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics
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