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-Tf 2 N demonstrating the operation of our nanoelectrospray thrusters in an ionic mode.

Hydraulic impedance is a critical parameter for the operation of electrospray emitters, and for preventing flooding when spraying from arrays of emitters. Controlling flow rate by tuning the flow impedance allows accessing different operating modes, such as droplet, ionic, or pulsating. We report on a method to tailor the hydraulic impedance of micromachined capillary out-of-plane emitters with integrated extractor electrodes by filling them with silica microspheres. Spraying the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMI-BF 4 ͒, we demonstrate the ability to tune from droplet emission to pure ion emission depending on microbead diameter, obtaining stable emission from single emitters and from arrays of 19 emitters. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3117191͔ Large arrays of nanoelectrospray emitters capable of operating in pure ionic mode using ionic liquids are of great interest for space micropropulsion and as ion sources for future focused ion beam applications. The feasibility of large arrays of microfabricated silicon out-of-plane arrays of capillaries has been shown previously, 1,2 but their operation has been difficult due to low hydraulic impedance, identified as a key parameter in the stability of electrosprays, [3][4][5] and as a key parameter in determining ion to droplet ratio when spraying ionic liquids. Recent studies with the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMI-BF 4 ͒ and 1-ethyl-3-methylimidazolium bis͑trifluorom-ethylsulfonyl͒imide ͑EMI-Tf 2 N, also referred to as EMI-Im͒, show that when sprayed from a capillary under vacuum conditions, a purely ionic regime ͑PIR͒ is only reached at low flow rates ͑Ӷ1 nL/ s͒ while at higher flow rates mixed droplet-ion emission is observed. [6][7][8][9] We have developed a method to tailor the hydraulic impedance in microfabricated silicon capillary emitters by creating a "porous" structure inside the capillary, thus combining the advantages of internally wetted capillaries with flow rate matching of externally wetted emitters. Similar to the method described by Valaskovic and Ehrenfeld 10 for loading capillaries with diameters below 300 m with particulate materials, in our case without applying any pressure, the modification of the hydraulic impedance is achieved by introducing silica microspheres into the capillaries and fixing them by means of a silanization step using silicon tetrachloride ͑SiCl 4 ͒ gas. In addition, microfabricated extractor electrodes have been directly integrated onto the capillary arrays allowing for homogeneous spray conditions across the array. The capillaries have an inner diameter of 24 m and a height of 70 m. Their fabrication has been described in detail in a companion paper. 11Retarding potential measurements with single integrated emitters spraying EMI-Tf 2 N, reported below, show operation in droplet mode without microspheres and ionic mode with them. Time-of-flight ͑TOF͒ spectra with arrays of 19 emitters with 5 m microspheres spraying EMI-BF 4 demonstrate th...

This paper reports on an accurate and rapid method to compute the onset voltage of a single or an array of electrospray emitters with complex geometries and on the correlation of the simulation with experimental data. This method permits the exact determination of the onset voltage based only on the surface tension of the sprayed liquid and on the emitter geometry. The approach starts by determining the voltage at which electrostatic forces and surface tension forces are equal for a sharpening conic surface at the tip of a capillary as a function of the apex radius of the liquid. By tracing the curve of this computed equilibrium voltage as a function of the apex radius, the onset voltage for a liquid surface with the Taylor half-angle of 49.3°or larger can be determined. For smaller cone half-angles the method is only applicable to ionic sprays as an approximate knowledge of the critical field for ion emission is necessary. The combination of analytical models and finite element tools used to compute the necessary parameters is described. The method is validated on a complex microelectromechanical system emitter geometry as well as on a linear array of electrospray emitters. Finally an empirical model of the behavior of the electric field near the apex of a conic surface with asymptotes at a fixed half-angle is proposed, which allows establishing a simple method for onset voltage determination.

This paper presents the fabrication and operation of an integrated nano-electrospray thruster consisting of an array of microfabricated silicon capillary emitters and microfabricated silicon extractor electrodes. Based on previous work in which we showed operation of single microfabricated capillaries [1], the improved thruster presented here allows simultaneously operation of arrays of emitters. In addition, we control the hydraulic impedance of the capillaries by filling them with silica beads, thus tailoring the flow rate in order to spray either in droplet regime or in ionic regime for two ionic liquids. Operation in both modes is confirmed by mass spectrometry and retarding potential analysis. In ion regime, a specific impulse of 3500 s is obtained at 1.2 kV for the ionic liquid EMI-BF 4 .

This paper reports on the design, fabrication and test of an integrated colloid micropropulsion system for spacecraft attitude control using the ionic liquid EMI-BF 4 as fuel. The principle of operation of the thruster is identical to electrospray ionization. The objective of the project was to demonstrate the feasibility and operability of arrays of microfabricated capillary emitters with individual extractor electrodes. This design approach results in an identical electric field distribution from one capillary to the other, avoids crosstalk and therefore allows for a more finely modulated thrust control. Spraying tests with different thruster configurations were conducted under vacuum conditions.Tests were performed with different thruster configurations and starting voltages around 700V were observed.