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.

Ice is an important but poorly understood atmospheric reaction medium. Reactions in ice and at air−ice interfaces are often modeled using rate constants measured in liquid aqueous solution, despite evidence that reactivity in these two media can be very different. This approach may be valid at high ionic strengths (e.g., in sea ice) as a result of the formation of liquid brine. However, recent experiments indicate uneven solute distribution at ice surfaces, suggesting that liquid water does not completely wet ice surfaces at environmentally relevant solute concentrations. We have investigated the distribution of liquid solution, solid ice, and solid salt (NaCl•2H 2 O, "hydrohalite") at the surface of frozen aqueous sodium chloride (NaCl) solutions and frozen seawater using Raman microscopy. At temperatures above the eutectic temperature (−21.1 °C), the ice surfaces were incompletely wetted, except occasionally at the highest temperatures (approximately −5 °C). Liquid water at the surface took the form of either isolated patches or channels, depending upon the salt concentration and sample temperature; liquid fractions ranged from approximately 11 to 85%. Three-dimensional ("volume") maps showed similar liquid fractions and channel widths at all depths investigated (up to 100 μm) as well as at the surface for each sample composition. Below −21.1 °C, no liquid was observed in any sample. Instead, hydrohalite was observed with surface coverages ranging from 13 to 100% depending upon the salt concentration; surface coverage was independent of temperature between −30 and −22 °C. Accounting for the presence of two distinct reaction environments at the surface of salty ice might improve predictions of physical and chemical processes in snow-covered regions.

Organic solutes in snow and ice can be distributed heterogeneously throughout the ice bulk and across the ice surface. This may affect air-surface interactions and heterogeneous reactions in snow-covered regions.

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Sea ice presents a complex reaction medium containing poorly understood distributions of solutes, ice, and sometimes liquid brine. We studied the three-dimensional distribution of liquid brine and dissolved organic matter using confocal Raman microscopy at environmentally relevant sodium chloride (NaCl) and dissolved carbon concentrations in frozen ternary solutions containing NaCl and humic acid (HA). While HA is predominantly excluded to the ice surface in the absence of salt, it coexists with liquid brine in the micrometer scale channels throughout the entire ice sample when salt is present. HA increased the extent of liquid coverage of the ice surface at each temperature studied, but the total amount of liquid water at the surface did not increase. This suggests that HA promotes spreading of liquid water at the surface of frozen salt solutions, but does not increase the extent of surface melting. HA suppressed the phase transition from solid ice + liquid brine to solid ice + solid hydrohalite (NaCl•2H 2 O) when samples were cooled. Phase-transition temperatures ranged from (−26.4 ± 0.6) °C (with 0.003 mg/L HA) to (−33.7 ± 0.5) °C (with 30 mg/L HA), compared to (−22.3 ± 0.5) °C in the absence of HA. The phase transition in the reverse direction (as the samples were warmed) occurred at (−21.8 ± 0.5) °C, very close to the Eutectic temperature (−21.1 °C) at all HA concentrations and in the absence of HA.

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Heterogeneous processes can control atmospheric composition. Snow and ice present important, but poorly understood, reaction media that can greatly alter the composition of air in the cryosphere in polar and temperate regions. Atmospheric scientists struggle to reconcile model predictions with field observations in snow-covered regions due to experimental challenges associated with monitoring reactions at air-ice interfaces, and debate regarding reaction kinetics and mechanisms has persisted for over a decade. In this work, we use wavelength-resolved fluorescence microscopy to determine the distribution and chemical speciation of the pollutant anthracene at the surfaces of environmentally relevant frozen surfaces. We show that anthracene adsorbs to frozen surfaces in monomeric form, but that following lateral diffusion, molecules ultimately reside within brine channels at saltwater ice surfaces, and in micron-sized clusters at freshwater ice surfaces; emission profiles indicate extensive self-association. We also measure anthracene photodegradation kinetics in aqueous solution and artificial snow prepared from frozen freshwater and saltwater solutions and use the micro-spectroscopic observations to explain the rate constants measured in different environments. These results resolve long-standing debates and will improve predictions of pollutant fate in the cryosphere. The techniques used can be applied to numerous surfaces within and beyond the atmospheric sciences.