Publication

Microfabrication of Capillary Electrospray Emitters and ToF Characterization of the Emitted Beam

Abstract

Microfabrication, assembly, and characterization of internally fed arrays of electrospray emitters for spacecraft propulsion are discussed. Several different emitter geometries were fabricated, and the sprayed beams from these emitters are characterized for their ion/droplet composition. It is shown that with smaller inner diameters, ionic mode of operation can be achieved more easily, due to the increase in the hydraulic impedance. Up to 2000 s Isp was measured from capillaries with 5 μm inner diameter and 100 μm with 750V extraction voltage using EMI-BF4. Due to the small dimensions of the microfabricated capillaries, the onset and ionic mode operation voltages can be significantly lower thanmacroscopic emitters. The major failure mode for the emitters was the liquid overflow from the capillary tip, which can be resolved through coatings that wet the ionic liquids differently. A detailed study of the wetting behavior of two ionic liquids is also presented for various materials compatible with silicon microfabrication technology.

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Related concepts (34)
Electrospray ionization
Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different from other ionization processes (e.g. matrix-assisted laser desorption/ionization (MALDI)) since it may produce multiple-charged ions, effectively extending the mass range of the analyser to accommodate the kDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments.
Ion source
An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines. Electron ionization Electron ionization is widely used in mass spectrometry, particularly for organic molecules. The gas phase reaction producing electron ionization is M{} + e^- -> M^{+\bullet}{} + 2e^- where M is the atom or molecule being ionized, e^- is the electron, and M^{+\bullet} is the resulting ion.
Electrospray
The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter (usually a glass or metallic capillary). Ideally the liquid reaching the emitter tip forms a Taylor cone, which emits a liquid jet through its apex. Varicose waves on the surface of the jet lead to the formation of small and highly charged liquid droplets, which are radially dispersed due to Coulomb repulsion.
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