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

Abstract

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-BF 4 ͒, 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. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3117191͔ Large arrays of nanoelectrospray emitters capable of operating in pure ionic mode using ionic liquids are of great interest for space micropropulsion and as ion sources for future focused ion beam applications. The feasibility of large arrays of microfabricated silicon out-of-plane arrays of capillaries has been shown previously, 1,2 but their operation has been difficult due to low hydraulic impedance, identified as a key parameter in the stability of electrosprays, [3][4][5] and as a key parameter in determining ion to droplet ratio when spraying ionic liquids. Recent studies with the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMI-BF 4 ͒ and 1-ethyl-3-methylimidazolium bis͑trifluorom-ethylsulfonyl͒imide ͑EMI-Tf 2 N, also referred to as EMI-Im͒, show that when sprayed from a capillary under vacuum conditions, a purely ionic regime ͑PIR͒ is only reached at low flow rates ͑Ӷ1 nL/ s͒ while at higher flow rates mixed droplet-ion emission is observed. [6][7][8][9] We have developed a method to tailor the hydraulic impedance in microfabricated silicon capillary emitters by creating a "porous" structure inside the capillary, thus combining the advantages of internally wetted capillaries with flow rate matching of externally wetted emitters. Similar to the method described by Valaskovic and Ehrenfeld 10 for loading capillaries with diameters below 300 m with particulate materials, in our case without applying any pressure, the modification of the hydraulic impedance is achieved by introducing silica microspheres into the capillaries and fixing them by means of a silanization step using silicon tetrachloride ͑SiCl 4 ͒ gas. In addition, microfabricated extractor electrodes have been directly integrated onto the capillary arrays allowing for homogeneous spray conditions across the array. The capillaries have an inner diameter of 24 m and a height of 70 m. Their fabrication has been described in detail in a companion paper. 11Retarding potential measurements with single integrated emitters spraying EMI-Tf 2 N, reported below, show operation in droplet mode without microspheres and ionic mode with them. Time-of-flight ͑TOF͒ spectra with arrays of 19 emitters with 5 m microspheres spraying EMI-BF 4 demonstrate th...

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

“…Relatively large fluctuations in φ contributed to the large propagated errors in polydispersive efficiency reported. Regardless, the comparatively large polydispersive efficiencies measured when emitting EMI-BF 4 are consistent with emissions within or close to the PIR [7,16,41]. Conversely the larger spread in constituent masses and possible droplet tail, after removing offsets, observed from both EMI-Im sources contributed to lower efficiency.…”

“…Low polydispersive efficiencies (<50%) can result from only a few percent of large droplets within an otherwise ionic beam [40]; greatly penalizing the power efficiency when targeting a high specific impulse through the PIR. This relatively severe loss mechanism has led to significant focus on achieving the PIR [26,41], or more recently and with promising results, obtaining extremely small (∼ few kDa) monodisperse droplet beams [20]. In this regard, ILISs which achieve the PIR through externally wetted [7] or porous [16] electrospray emitters are a promising avenue for high efficiency, high specific impulse devices.…”

Fragmentation in Time-of-Flight Spectrometry-Based Calculations of Ionic Electrospray Thruster Performance

“…Consistent with typical ILIS [2,4,7,[10][11][12], Fig. 1 assumes a beam primarily composed of n 0 and n 1 ions along with a small but not insignificant proportion of n 2 ions.…”

“…Ion fragmentation in the context of ILIS has been previously examined using retarding potential analyzers (RPAs) [4,[10][11][12][13][14]. Those studies have identified both discrete populations corresponding to breakup in field free regions and distributed populations.…”

Wafer-level integrated electrospray emitters for a pumpless microthruster system operating in high efficiency ion-mode

“…Electrospray thrusters operating in purely ionic mode outperform any other rivaling thruster technology in this regard, but they suffer from a fundamental trade-off between the I sp and the thrust. Our group has previously developed an array of electrospray emitters using microfabrication that has the potential to combine the very high I sp of this method with sizable thrust [1,2]. Moreover, ion mode operation from an internally wetted capillary type electrospray system was proven for the first time to our knowledge by tailoring the capillary hydraulic impedance by silica bead filling [2].…”