John P. W. Stark

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.

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.

Fully voltage-controlled electrohydrodynamic jet printing of conductive silver tracks with a sub-100 μm linewidth

Silver microtracks with excellent electrical functionality were created by electrohydrodynamic jet (e-jet) printing of commercial metallo-organic ink. Novel e-jet printing was performed in a fully voltage-controlled fashion. By using a 20 μm nozzle and reducing the printing distance to 50 μm, metallic tracks with a sub-100 μm linewidth were successfully achieved on Si substrates. The physical properties of the printed tracks were characterized by means of scanning electron microscopy, atomic force microscopy, energy-dispersive x-ray spectrum analysis, and electrical measurements. A low resistivity in the range (2–4)×10−8 Ω m, 1.7–2.4 times of the theoretical value of silver, was obtained for the printed microtracks. A uniform fine track with a 35 μm feature size was produced by pulsed jet printing operating at low voltage, and a drop-on-demand capability of ∼7 pl/drop was estimated.

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.

A low-cost approach for measuring electrical conductivity and relative permittivity of liquids by triangular waveform voltage at low frequencies

We present a novel low-cost experimental method for measuring the electrical conductivity and relative permittivity of liquids at low frequencies. Based on the different relationship between the resistive and capacitive currents with an applied voltage, the resistance and capacitance of fluid test cell can be obtained independently from different time regions of a triangular waveform voltage (TWV). The electrical conductivity and relative permittivity of test liquids can be derived following calibration of the cell constant. This novel technique avoids measuring current–voltage phase shift angles that are sometimes exceedingly small, requiring high-precision instrumentation. The new method also eliminates the need for interpretation of the cell in terms of an equivalent circuit to obtain the cell resistance and capacitance from the measured cell impedance. It can be seen from numerical demonstration of a real TWV that the measured cell resistance and capacitance by the TWV technique is merely determined by the liquid under test and insensitive to the waveform of the applied TWV. The TWV technique is validated by measuring sample liquids at low frequencies, whose properties are well known.

Measurement of low frequency relative permittivity of room temperature molten salts by triangular waveform voltage

We present a low-cost experimental method for measuring cell resistance and capacitance independently. In this technique a triangular waveform voltage excitation is adopted. Conductivity calibration is straightforward. However, it is demonstrated that a specific calibration procedure needs to be followed to obtain correct relative permittivity values for highly conductive liquids. It is also required to measure conductivity in order to measure relative permittivity for highly conductive liquids. Using this technique, the relative permittivities of room temperature molten salt 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) were measured. To our knowledge, this is the first ever directly measured low frequency relative permittivity value of highly conducting EMImBF4 and BMImBF4.

Microfabricated electrospray emitter arrays with integrated extractor and accelerator electrodes for the propulsion of small spacecraft

Microfabricated electrospray thrusters could revolutionize the spacecraft industry by providing efficient propulsion capabilities to micro and nano satellites (1–100 kg). We present the modeling, design, fabrication and characterization of a new generation of devices, for the first time integrating in the fabrication process individual accelerator electrodes capable of focusing and accelerating the emitted sprays. Integrating these electrodes is a key milestone in the development of this technology; in addition to increasing the critical performance metrics of thrust, specific impulse and propulsive efficiency, the accelerators enable a number of new system features such as power tuning and thrust vectoring and balancing. Through microfabrication, we produced high density arrays (213 emitters cm−2) of capillary emitters, assembling them at wafer-level with an extractor/accelerator electrode pair separated by micro-sandblasted glass. Through IV measurements, we could confirm that acceleration could be decoupled from the extraction of the spray—an important element towards the flexibility of this technology. We present the largest reported internally fed microfabricated arrays operation, with 127 emitters spraying in parallel, for a total beam of 10–30 µA composed by 95% of ions. Effective beam focusing was also demonstrated, with plume half-angles being reduced from approximately 30° to 15° with 2000 V acceleration. Based on these results, we predict, with 3000 V acceleration, thrust per emitter of 38.4 nN, specific impulse of 1103 s and a propulsive efficiency of 22% with <1 mW/emitter power consumption.

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 effects on the nanoelectrospray characteristics in fully voltage-controlled atomisation of gold nanocolloids.

The voltage effects on fully voltage-controlled nanoelectrospraying of aqueous gold nanocolloids were investigated. In the nanoelectrospray using 30 μm nozzle, with an increasing of voltage two stable spray regimes of pulsating and cone-jet mode were clearly observed by a combination of current measurement and fast imaging technology. In this nanoelectrospray uniquely determined by the electrical field voltage the current-voltage characteristics were analysed and evaluated based on an equivalent circuit method. At high field in cone-jet regime all the equivalent resistances derived by fitting appear close to a value of 0.53 ± 0.03 GΩ, showing independence to the conductivity of the nanocolloids. In low field pulsating regime a high pulsation frequency up to 100 kHz with relatively stable current pulse was exhibited in all the gold nanocolloids. A linear relationship between the DC components in the pulsating current and the voltage was found and the DC equivalent resistance obtained from the fitting varies between 0.90 and 1.47 GΩ. A strong correlation between the pulsating properties and the conductivity of the colloids was identified.