Electrospray from an Ionic Liquid Ferrofluid utilizing the Rosensweig Instability

Abstract

A new type of electrospray technology that could be used for space propulsion was developed at Michigan Technological University. This thruster utilized an ionic liquid ferrofluid that was synthesized by suspending magnetic nanoparticles in an ionic liquid carrier solution so that the resulting fluid is superparamagnetic. The magnetic properties of the fluid were exploited to create self-assembling static arrays of surface peaks which were then amplified with an applied electric field until ion current was emitted from the array. The current and voltage profile of the emitting array was measured and its ability to self-heal after a damaging event was observed. I. IntroductionTypical electrospray thruster concepts utilize linear "blade like" emitters, 2-D arrays of tips, or 2-D arrays of micro-capillaries to generate large electric fields and ion/droplet emission from an electrostatic Taylor cone. Linear arrays are manufactured from thin sheets of metal, such as porous tungsten, and sharp peaks are etched into the surface. The arrays are aligned with a slit in the extraction electrode and the propellant (ionic liquid) is passively pumped through the porous tungsten. 1, 2 This type of electrospray emitter can have good packing density along the length of the array, but leaves a sizeable gap between consecutive arrays. Legge and Lozano were able to demonstrate 2 tips per mm packing density along the length of the array. 2 A similar strategy is the tungsten crown emitter, which is a porous tungsten structure shaped like a king's crown with a circular array of 28 emitters. [3][4][5] The crown emitter used indium as its propellant.Two-dimensional arrays are typically etched from silicon with regularly spaced discrete needles. 6 For this type of arrangement, the propellant is typically externally applied to the surface of the silicon where capillary flow causes a uniform layer of propellant to coat the substrate and emitter tips. In order for the propellant to easily wet to the silicon surface, a surface treatment is usually necessary, such as making black silicon. 7 These arrays are then assembled with an extraction electrode and alignment/retention mechanisms. 8 The third type of electrospray thruster is the capillary thruster. These types of thrusters generally contain the fluid in a hollow tube or needle and the electric field is created at the exit of the capillary. 9 One such device is the capillary array of 19 emitters built by Krpoun and Shea. 10 In this device each of the capillaries are etched out of silicon and then filled with silica spheres to provide a more uniform hydraulic resistance within each emitter. A micromanufactured extraction electrode was placed and aligned using ruby balls for alignment and electrical isolation. Another version of a capillary thruster was developed by Busek. 11 Each of Busek's thruster heads contained an array of 9 individually manufactured emitters assembled into an array. A third type of thruster array which does not cleanly fit into any of these groups is the porous tungst...

Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

“…capillaries [3,4,25,26], silicon tips roughened through etches or CNT growth [1,17], electrochemical etching of porous metals [16,27], use of Rosensweig instabilities in ionic liquid ferrofluids [28] and laser ablation of porous glass [5]. Arrays of externally wetted [17] and porous [2,5] ILIS have, encouragingly, been demonstrated to support a few hundred μA per cm 2 of highly ionic emissions.…”

Species measurements in the beam of an ionic liquid ferrofluid electrospray source

“…Though suitable for peak formation via the Rosensweig instability, these ILs were less-suitable for electrospray due to their relatively high vapor pressures and viscosities. 7,8 A collaboration between Jain and Hawkett, and Meyer and King led to an ILFF using EMIM-NTf 2 as the carrier liquid; 8 EMIM-NTf 2 is less viscous than EAN, has near zero vapor pressure, and has extensive use in the electrospray arena.…”

“…Research on the electrospray of ferrofluids includes emission experiments to demonstrate the ILFF electrospray technology, 7,8 and wavelength measurement experiments of non-conducting oil-based ferrofluids to determine the peak density as a function of the B∇B fields. 8…”

“…4,5 More recently ferrofluids were proposed as propellant for electrospray space propulsion devices. 7,8 When a ferrofluid is subjected to a strong magnetic field, the domains of the individual particles align with the field lines. Furthermore, perturbations on the surface of the fluid cause localized concentrations of the magnetic field.…”

Characterization of an Ionic Liquid Ferrofluid Electrospray Emission Pattern

“…In 2012, Meyer and King were the first to demonstrate that electrospray emission can be achieved using a ferrofluid with Ethylammonium Nitrate (EAN) as the carrier fluid [5]. Emission has since been demonstrated using 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) based ferrofluid [6].…”