Renato Krpoun

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.

Tailoring the hydraulic impedance of out-of-plane micromachined electrospray sources with integrated electrodes

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-BF4), 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.

A method to determine the onset voltage of single and arrays of electrospray emitters

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...

Micromachined electric propulsion using ionic liquids as fuel

MEMS-based electric propulsion (EP) presents several advantages over conventional EP approaches, including scalability, redundancy, fine thrust control, low thrust noise, and reduced operation voltage. Small spacecraft (1-100 kg) in particular stand to benefit from MEMS-based propulsion. The design, microfabrication and performance of an electrospray thruster with integrated individual extractor electrodes are reported. Micro-machining technologies were employed to achieve large emitter density, simple integration and high structural repeatability.

Onset voltage modeling of micromachined colloid thrusters

The onset voltage of a colloid thruster is an important parameter in the design of micromachined thrusters. Because dimensions in MEMS thrusters are in the range of several tens of micrometers and because extractor electrodes can be only 5 µm from the capillary emitters, the aspect ratios for MEMS thrusters are much lower than the one of individual stainless steel or silica glass needles. Therefore the results obtained with established analytical models, that do not consider any parasitic structures, deviate significantly from experimental results on MEMS thrusters. This paper presents a new method to determine the onset voltage of micromachined colloid thrusters by combining electrostatic FEM analysis, including a model of the formation of the Taylor cone, with the equilibrium equation between electrostatic and surface tension stress. The correlation between onset voltage results obtained using the method presented in this paper and experiments are in very good agreement and validate the method for this type of microfabricated thruster. Nomenclature E Electric field, V/m Bi,p Bernstein basis function C Parametric function, representing conic section Pi Vectors defining conic section ri Principal radii of curvature, m ra Radius of curvature at the apex of the forming Taylor cone, m t Variable parameter w Variable parameter γ Surface tension, N/m δik Kronecker-delta � 0 Permittivity of free space, F/m κ Proportionality factor, 1/m σE Electrostatic stress, N/m 2 σγ Surface tension stress, N/m 2 Subscript i, k, p Variable numbers