Matthew S. Alexander

Electrospray Performance of Microfabricated Colloid Thruster Arrays

Microfabricated emitters have been produced by deep reactive ion etch technology. To demonstrate their suitability as components of integrated colloid thruster systems, we have evaluated the electrospray performance of individual and arrays of these microfabricated emitters and compared them to that of conventional stainless-steel emitters. We show that, after accounting for electrostatic differences caused by changes in physical geometry, the spray current dependence on flow rate for microemitters demonstrates similar scaling behavior to that of conventional single stainless-steel emitters. The spray current per nozzle is found to be independent of array size and the total spray current to depend simply on the number of nozzles in the emitter array. We have also found that in a triangular array of microemitters there is no significant geometry-induced shielding or field reduction between emitters. We report on the electrospray performance of the room- temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate with conventional emitters, which appears to be a promising new colloid thruster propellant.

Voltage effects on the volumetric flow rate in cone-jet mode electrospraying

Electrospray data collected demonstrate the detailed dependence of volumetric flow rate through an electrospray system upon the applied voltage. The sensitivity of nominal flow rate to applied voltage was found to be higher for lower nominal flow rates. For a volumetric flow rate ∼4nL∕s a 25% change in flow rate per kilovolt was recorded over a cone-jet mode stability range spanning ∼1.5kV. This volumetric flow rate voltage sensitivity holds particular significance for potential colloid electrospray propulsion systems, which operate at or near minimum flow rate conditions. Analysis is presented to show that the change in flow rate due to change in voltage cannot be ascribed to evaporation from the meniscus as has been suggested by others.

The Sensitivity of Volumetric Flow Rate to Applied Voltage in Cone-Jet Mode Electrospray and the Influence of Solution Properties and Emitter Geometry

A high accuracy online flow rate measurement system has been used to demonstrate the effect of applied voltage, Vapp, on the volumetric flow rate, Q, through an electrospray system. Several solutions of the organic solvents ethylene and triethylene glycols doped with sodium iodide to give varying conductivities in the range of 0.0025–0.23S∕m have been sprayed. It was established for the first time that solution conductivity has no appreciable effect on the sensitivity of flow rate to applied voltage in the cone-jet mode of electrospray. However, it appears that even when the hydraulic resistance is taken into account, the sensitivity of flow rate as controlled by the applied voltage is additionally related to the emitter exit geometry. These findings are of particular importance to both spacecraft propulsion and electrospray mass spectrometry technologies and suggest careful emitter geometry design considerations will lead to greater control over electrospray properties.

Effect of emitter geometry on flow rate sensitivity to voltage in cone jet mode electrospray

The effect of voltage on flow rate within cone jet mode electrospraying has been investigated, with particular emphasis on the effect of emitter geometry. A set of experiments investigated the effect of the outer and inner diameters on the flow rate relationship to voltage, in cone jet mode electrospray. This was accomplished by the use of a high fidelity flow meter, capable of measuring changes in flow rate to a fraction of a nanolitre per second.It has been previously demonstrated that there are two separate parameters that influence the flow rate sensitivity to voltage; the hydraulic resistance of the flow system, and the outer diameter of the emitter. By a simple derivation, the second of these two is explained by the variation of theoretical electric pressure with voltage, as the outer diameter is varied.Good agreement is found between experimental and theoretical results, suggesting the simple theory reasonably explains the physics of the situation.As well as elucidating the physics involved in electrospray—suggesting the electric field is an important controlling parameter within cone jet mode electrospray—the theoretical and experimental agreement has important implications for variable throttling of thrust in colloid thrusters, and could bring about better optimization of performance in other electrospray-employing fields.

Voltage-Modulated Flow Rate for Precise Thrust Control in Colloid Electrospray Propulsion

We describe a feature of electrospray operation that allows precise control of flow rate through an electrospray emitter by use of the extraction voltage. The effect of extractor voltage on the propellant flow rate through an electrospray emitter has been determined for triethylene glycol and ethylene glycol solutions doped with varying levels of sodium iodide and the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate using an in-line high-accuracy flow measurement system. In these experiments, a nominally fixed flow rate, obtained by providing a fixed supply pressure, is observed to be influenced by the applied voltage during stable cone-jet electrospray production. The relative sensitivity of flow rate to applied voltage was found to be higher as the nominal flow rate decreased. This method of flow rate control holds particular significance for colloidal electrospray thruster systems, which operate at or near minimum flow rate conditions.

Pulsating electrospray modes at the liquid-liquid interface

Unforced liquid-liquid electrosprays have been demonstrated for water/heptane and ethylene glycol/heptane couples which exhibit stable high frequency current oscillations in the kilohertz range. Using in-line flow sensor measurements, the controlled ejection of subpicoliter volumes into an immiscible liquid phase was shown possible.

Fabrication and Operation of Microfabricated Emitters as Components for a Colloid Thruster

An integrated design for a colloid thruster, using microfabricated components, is described. This thruster is suitable as a propulsion unit for both a small satellite and a conventional satellite. One key component of thethruster is a nano-emitter array. A description of the fabrication processes that are used for the manufacture of these nano-emitters is provided. We report the first quantitative results for the electrospray current as a function of both flow rate and applied voltage for these emitters. Electrospray test results comparing the performance of these new emitters with conventional stainless-steel capillaries reveal that when allowance is made for the electrostatic configuration differences the spray current demonstrates qualitatively similar flow-rate dependence. Importantly, our electrospray current measurements from differently sized emitters show, for the first time, that this current is sensitive to the geometry of the emitter itself. This result is used to highlight the role of emitter design in the efficiency of a colloid thruster.

Fabrication and Operation of Micro-fabricated Colloid Thruster Arrays

Fabrication processes for nano-emitters suitable for a colloid thruster are described. Electrospray results from these emitters shows that the electrospray current is sensitive to the emitter geometry.

Electrospray pulsation: A diagnostic to understand cone-jet stability and minimum flow

Detailed current and flow rate measurements obtained during pulsation mode nano-electrospray of ethylene glycol solutions are presented. High temporal resolution current measurements reveal for the first time that current is observed at the electrode immersed in the electrospray fluid, even when there is no observed spray. This current, believed to be the charging current associated with electrode and meniscus polarization, is dependent upon the voltage applied to the electrospray system. Quantitatively, the total charge transfer that is observed during the non-spray period of the pulsation approximates to that required to obtain electrostatic equilibrium in the conical meniscus, and is relatively independent of the fluid conductivity. A linear relationship exists between this charging current and the observed pulsation frequency. Pulse shape, indicated by the parameters of spray current rise time, fall time, and spray current pulse duration, are observed to be independent of frequency for a given solutio...

The role of molar conductivity in electrospray cone-jet mode current scaling

A high accuracy online flow rate measurement system has been used to determine the current flow rate scaling relationships for solutions of organic solvents doped with sodium iodide and for the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate over a range of conductivities from 0.0025−1.3S∕m. The current flow rate trends for these solutions were found to exhibit a power law relationship similar to that described by previous researchers, where I (the electrospray current) is proportional to Q (the volumetric flow rate) to some power n. However, the exponent n of the current flow rate trends was found to differ from the theoretical predictions reported in the literature. A study including data from literature revealed the exponent of the current flow rate trends to be sensitive to the molar conductivity of the sprayed solution.A high accuracy online flow rate measurement system has been used to determine the current flow rate scaling relationships for solutions of organic solvents doped with sodium iodide and for the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate over a range of conductivities from 0.0025−1.3S∕m. The current flow rate trends for these solutions were found to exhibit a power law relationship similar to that described by previous researchers, where I (the electrospray current) is proportional to Q (the volumetric flow rate) to some power n. However, the exponent n of the current flow rate trends was found to differ from the theoretical predictions reported in the literature. A study including data from literature revealed the exponent of the current flow rate trends to be sensitive to the molar conductivity of the sprayed solution.