Integrated out-of-plane nanoelectrospray thruster arrays for spacecraft propulsion

Nanoelectrosprays, well known for their use in sample injection for the mass spectrometry of large biomolecules, can also be used in other applications such as spacecraft propulsion. The thrust generated by a single electrospray emitter is well below 1 μN, which is several orders of magnitude below the required thrust for planned formation flying missions. This paper presents the process flow and the microfabrication of large 2D arrays of out-of-plane nanoelectrospray capillary emitters with integrated extractor electrodes as well as electrospray results. The capillaries, 70 μm high and with 24 μm inner diameter, are etched from one silicon-on-insulator wafer. The extractor electrodes are from another silicon-on-insulator wafer. Both parts are passively aligned to within 2 μm, centering each capillary under one extractor electrode, thus ensuring highly uniform emitter characteristics over large arrays. Low hydraulic impedance has been a major problem in out-of-plane electrospray designs in the past, which is solved here by adding a post-processing step in which the capillaries are filled with 5 μm silica microspheres fixed in place by silanization. Finally, this paper reports on successful spray tests carried out under vacuum conditions with single and arrays of capillaries spraying the ionic liquid EMI-Tf2 N demonstrating the operation of our nanoelectrospray thrusters in an ionic mode.