Scalable Microfabrication of Multi-Emitter Arrays in Silicon for a Compact Microfluidic Electrospray Propulsion System.

The recent proliferation of SmallSats and their use in increasingly demanding applications require the development of onboard electric propulsion compatible with the power, mass, and volume constraints of these spacecraft. Electrospray propulsion is a promising technology for SmallSats due to its unique high efficiency and scalability across the wide power range of these platforms, for example, from a few watts available in a CubeSat to a few hundred watts in a MiniSat. The implementation of electrospray propulsion requires the use of microfabrication techniques to create compact arrays of thousands of electrospray emitters. This article demonstrates the microfabrication of multi-emitter electrospray sources of a scalable size for electrospray propulsion. In particular, a microfabrication and assembly process is developed and demonstrated by fabricating sources with arrays of 1, 64, and 256 emitters. The electrospray sources are tested in a relevant environment for space propulsion (inside a vacuum chamber), exhibiting excellent propulsive performance (e.g., absence of beam impingement in the extractor electrode, absence of hysteresis in the beam current versus propellant flow rate characteristic, proper operation in the cone-jet electrospraying mode, etc.) and nearly coincident output per emitter. Several design elements contribute to this performance: the even distribution of the propellant among all emitters made possible by the implementation of a network of microfluidic channels in the backside of the emitter array; the small dead volume of the network of microfluidic channels; the accurate alignment between the emitters and extractor orifices; and the use of a pipe-flow configuration to drive the propellant through closed conduits, which protects the propellant.

Study of the electrostatic jet initiation in near-field electrospinning.

HYPOTHESIS: The electrostatic initiation of a jet from the meniscus of a polymeric solution is a key step in near-field electrospinning (NFES), however this process is not sufficiently understood to determine a criterion for the critical emitter voltage triggering the jet, nor to optimize the electrodes. It is expected that the jet initiation in NFES is similar to that in cone-jet electrosprays, and can be described with a first principles model. EXPERIMENTS: The electrostatic jet initiation of an SU-8 polymeric solution is studied with two different electrode geometries to quantify the initiation parameters and illustrate the optimization of the electric field. A first-principles model is developed to predict and analyze the initiation. FINDINGS: Two jet emission mechanisms are identified: one in which the jet is ejected from a free-standing conical meniscus; and one resulting from the contact of the meniscus with the collector, as it evolves from a spherical to a conical shape. Both are triggered by a critical emitter potential. The former produces the thinnest jets with a diameter that depends on the properties of the fluid, while the latter is an alternative to existing mechanical initiation methods. The model reproduces well the experimental phenomena including the critical voltage.