Electrospray is a process to ionize electrically conductive liquids that relies on strong electric fields; charged particles are emitted from sharp tips that serve as field enhancers to increase the electrostatic pressure on the surface of the liquid, overcome the effects of surface tension, and facilitate the localization of emission sites. Ions can be emitted from the liquid surface if the liquid is highly conductive and the emitter flowrate is low. Previous research demonstrated successful operation of massive arrays of monolithic batch-microfabricated planar electrospray arrays with an integrated extractor electrode using ionic liquids EMI-BF₄and EMI-Im ^{[1] [2]} – liquids of great importance for efficient nanosatellite propulsion. The current work aims to build upon the previous electrospray array designs by increasing the density of the emitter tips, increasing the output current by custom-engineering suitable nanofluidic structures for flow control, and improving the ion optics to gain control of the plume divergence and exit velocity.

The basic version of the MEMS electrospray array consists of an emitter die and an extractor die (shown in Figure 1), both made of silicon and fabricated using deep reactive ion etching. The two dies are held together using a MEMS high-voltage packaging technology based on microfabricated springs that allows precision packaging of the two components with less than 1% beam interception ^{[3] [4]} . The emitter die contains dense arrays of sharp emitter tips with as many as 1,900 emitters in 1 cm².  A voltage applied between the emitter die and the extractor electrode creates the electric field necessary to ionize the ionic liquid (see Figure 2). A nanostructured material transports the liquid from the base of the emitters to the emitter tips. The present research focuses on engineering the nanofluidic structure to attain higher emitter current while maintaining good array emission uniformity and on developing batch microfabricated advanced ion optics to control the electrospray plume.

1. L. F. Velásquez-García, A. I. Akinwande, and M. Martínez-Sánchez, “A planar array of micro-fabricated electrospray emitters for thruster applications,” Journal of Microelectromechanical Systems, vol. 15, no. 5, pp. 1272-1280, Oct. 2006.
2. B. Gassend, L. F. Velásquez-García, A. I. Akinwande, and M. Martínez-Sánchez, “A microfabricated planar electrospray array ionic liquid ion source with integrated extractor,” Journal of Microelectromechanical Systems, vol. 18, no. 3, pp. 679-694, June 2009.
3. B. Gassend, L. F. Velásquez-García, and A. I. Akinwande, “Precision in-plane hand assembly of bulk-microfabricated components for high voltage MEMS arrays applications,” Journal of Microelectromechanical Systems, vol. 18, no. 2, pp. 332-326, 2009.
4. L. F. Velásquez-García, A. I. Akinwande, and M. Martínez-Sánchez, “Precision hand assembly of MEMS subsystems using DRIE-patterned deflection spring structures: An example of an out-of-plane substrate assembly,” Journal of Microelectromechanical Systems, vol. 16, no. 3, pp. 598–612, 2007.

Electrospinning is a process in which a membrane-like web of thin fibers can be produced using high electrostatic fields and polar liquids with high viscosity. It is the only known technique that can generate continuous fibers with controlled morphology in the 10-500 nm diameter range and has tremendous versatility as it can create non-woven or well-aligned mats of polymer, ceramic, semiconductor, and/or metallic fibers using the same hardware. Electrospinning is also capable of conformally coating 3D complex shapes with ultrathin layers that have complex multi-layered structure and thickness variation across the surface. In particular, polymer electrospun fibers have been proposed to develop multi-stack functional fiber mats for protective gear, because they show high breathability, elasticity, and filtration efficiency. In addition, electrospun fibers made of the appropriate materials could also be used in flexible electronics (graphene) and in structural reinforcement against mechanical trauma. However, the production of electrospun nanofibers has very low throughput due to the small fiber diameter, which limits their applications to high-end products. In this project we are investigating the development of high-throughput electrospun nanofibers using batch-microfabricated arrays of externally fed electrospinning emitters. Externally-fed emitters are attractive, because they do not require high pressure drops as internally-fed emitters do.  Also, they do not clog and can process liquids that bubble.

An aspect of this project is looking into the physics of wicking to optimize the fluidic micro/nanostructures that control the emitter flow rate. For solids with intrinsic contact angles below some critical value determined by roughness geometry, it becomes energetically favorable for a droplet to completely impregnate the roughness and spread through it ^[1] . This process of hemi-wicking has been described in pillar arrays of varying shapes and sizes ^{[2] [3]} . For externally-fed electrospinning, we must ensure a sufficient and steady flow rate of polymer solution to avoid broken or irregular fibers. We are theoretically and experimentally investigating optimal morphologies of both the micro/nano fluid control structures and the emitter geometry to attain good array emission uniformity.

1. D. Quéré, “Wetting and roughness,” Annu. Rev. Mater. Res., vol. 38, pp. 71-99, April 2008.
2. L. Courbin, J. C. Bird, M. Reyssat, and H. A. Stone, “Dynamics of wetting: From inertial spreading to viscous imbibition,” Journal of Physics: Condensed Matter, vol. 21, p. 464127, 2009.
3. C. Ishino, M. Reyssat, E. Reyssat, K. Okumura, and D. Quéré, “Wicking within forests of micropillars,” Europhys. Lett., vol. 79, p. 56005, 2007.