Various materials have been explored for the manufacturing of electrospray emitters, such as porous carbon and porous glass. These materials deliver propellant to the apex region via the transportation of ionic liquid through capilary forces. However, the shapes of the emitters produced with these materials do not result in macroscopically-smooth geometries as shown in the figure below. In fact, due to their porous nature the apex region of the emitter is not always regularly defined resulting in off-axis ion plumes, which reduces the efficiency of the thruster. The goal for this project is to investigate the use of Nafion as an alternative material for electrospray emtters in order to allevaite this shortcoming of porous materials.
Nafion is a polymer material whos chemistry consists of long fluorocarbon chains with sulphonate sites. These chains are flexible such that they can be packaged into desirable geometries of interest, thereby resulting in parts that are characterized by smooth surfaces. The hydrophilic sulphonate sites and hydrophobic fluorocarbon chains work together to "store" liquid within the material's structure, which can then be transported via a combination of induced electric and osmotic pressures within these emitters. Also, Nafion is modeled to consist of ionic clusters that promote transport with sufficient material hydration, a feature that aids in better transportation of ionic liquid. As a conductive polymer, Nafion promises to deliver desirable advantages when used as an alternative material for electrospray emitters.
Attempts have been made in the manufacturing and testing of Nafion-based emitters for electrospray propulsion. Our prior work shows that manufacturing and testing of these tips is possible. In fact, it is quite clear that Nafion-based electrospray emitters attain desirable hydraulic impedance and emit relatively high currents. Current research has been focused on developing a reliable and predictable manufacturing method to allow for reproducible structures. The following are pictorial depictions of some of the manufacturing results of Nafion-based ion emitting surfaces, ranging from single emitters to arrays. The next steps are to test and characterize these structures via analysis of current-voltage curves, examination of ion-plume energies, and beam spreading characteristics among others.
A. Adams, "An Investigation of Nafion Electrospray Emitter Tips," Master's thesis, Massachusetts Institute of Technology, Cambridge, MA, 2021