Propulsion science

The physics underlying electrospray propulsion are studied using a variety of different methods and tools. This includes theoretical modeling as well as experimental investigations, and can range from studies of single emission sites to characterization of full emitter arrays. Understanding the physical processes involved in ionic liquid electrospray emission and beam acceleration characterizes emitter performance and is used as design guideline for future thruster development. 

Research at SPL is currently focusing on the following areas:

Advanced propellants and ion cluster fragmentation

The performance of ion electrospray thrusters depends significantly on the properties of the ionic liquid propellant [1]. The relationship between the composition and properties of the ionic liquid and ion electrospray thruster performance is not well understood. The goal of this research is to develop a model that can be used to predict the performance of ionic liquid ion sources (ILIS) solely based on the composition of ionic liquids.

Ionic liquid ion sources are the basis of ion electrospray thrusters. They consist of a sharp tip coated with ionic liquid placed a few hundred microns from a metal plate with a hole in it, called an extractor. When 1-2 kilovolts are applied between the tip and the extractor, the ionic liquid surface collapses into a structure called a Taylor cone [2]. The electric field at the tip of the Taylor cone can exceed the field strength required to evaporate ions from the liquid meniscus. When this happens, ions are emitted from the tip and are accelerated through the aperture in the extractor, producing an ion beam. The ion beam typically consists of single ions as well as clusters of ions. The ion clusters can break up midflight through a process called fragmentation, which has an important effect on the thruster performance. The image below shows a single emitter ILIS with an ion cluster breaking up midflight.

The beam properties that strongly control the performance of electrospray thrusters are the composition of the beam and the fragmentation rates of solvated ions, which vary with the ionic liquid [3]. Experimental characterization and modeling of ILIS beams will be used to develop a method of predicting ILIS performance. Time of flight mass spectrometry and retarding potential analysis will be used to measure the composition and fragmentation rates of ILIS beams. The major experimental challenges are obtaining repeatable and consistent measurements as well as measurements that accurately represent the true beam properties. By integrating the results from experiments and theoretical models, the relationship between the ionic liquid composition and the ILIS beam properties will be determined and a model to predict the performance of ionic liquids in ion electrospray thrusters will be developed.

For further information, please contact Catherine Miller. Catherine Miller is funded by the NASA Space Technology Research Fellowship.

 

Molecular dynamics simulations of emission

Molecular dynamic simulations are performed to study electrospray emission, as well as effects happening during the acceleration of the ion beam, such as fragmentation of emitted ion clusters. 

Snapshots in time during emission from a droplet of EMI-BF4, containing 26,112 atoms, under an horizontal electric field. In this sequence, an emitted negative tetramer fragments into a negative dimer, a negative monomer, and a positive dimer. While the negative ions are accelerated to the left, the positive ion either returns to the liquid bulk or collides with subsequently emitted ions, from [4]
Video of a molecular dynamic simulation of emission of positive monomers and dimers from a small droplet of 343 molecules of EMI-BF4

For further information, please contact Tom Coles

 

Modeling emission from dielectric menisci 

Motivation

Electrospray atomization is a field emission process that occurs as a result of the breakdown of an electrically stressed meniscus.  For many leaky dielectrics, e.g., ionic liquids, immersion in a strong electric field causes the fluid to adopt a conventional cone-jet configuration involving a large conical body that is capped by a thin jet protruding from the apex.  The jet produces a train of charged droplets of similar size that propagate under the influence of the surrounding field.

In practice it is well-known that the size of the electrosprayed droplets, as well as the diameter of the parent protuberance extending from the meniscus tip, varies as a function of the prevailing electrolytic flow rate.  Specifically, the droplets and the jet become smaller as the flow is decreased, although empirical findings suggest that a limit exists for this basic behavior.  With electrolytes of low electrical conductivity, for example, exceedingly tenuous flow invariably leads to an instability in which the conical meniscus is no longer permitted to be stationary.  For electrolytes of somewhat high electrical conductivity, on the other hand, the instability may be circumnavigated altogether, resulting in complete extinction of the jet and emission of discrete molecular ions through the kinetics of field-induced evaporation.  While sources of such ions are of great technological interest, their basis is not yet fully clear.  The purpose of this work is to elucidate critical aspects using fundamental models.

Methods

We have developed a basic model that is intended to capture the essential behavior of an idealized leaky dielectric meniscus.  For computational simplicity, this meniscus protrudes from a plate that is both conductive and perfectly hydrophobic by way of a small feeding hole that intersects its surface.  An arbitrary electric field may be applied by biasing a parallel and very distant electrode with respect to the plate, such that the intervening field is asymptotically uniform very far from the meniscus.  A simple relaxation algorithm is used thereafter to identify axisymmetric morphologies which satisfy an appropriate balance of mechanical stresses.

Results

Investigation of the parameter space has revealed two families of mechanical solutions: one that is relevant to relatively low values of the applied electric field and another that exists in a high field regime.  For the former, the solutions involve menisci that are “egg-shaped” in the sense that the surface curvature is globally very uniform.  Such structures are unlikely to shed charge unless they are exceedingly small.  In contrast, the second family is characterized by extremely sharp liquid tips that do appear to support evaporation even when the meniscus is somewhat large, e.g., several microns in radius.  The figure below delineates representative shapes for these families alongside an archetypal Taylor cone for reference.

Representative members of the two solution families plotted in dimensionless cylindrical space. The height and radius of the interface points have been scaled by the radius of the hole in the plate. Notice that the low-field solutions are relatively blunt while their high-field counterparts may be very sharp. In both cases there are significant deviations from Taylor’s classical cone morphology.

For further information, please contact David Krejci

 

Beam simulation

A SPL developed Particle-in-Cell (PIC) code is used to simulate the behavior of the emitted ion beam. Different effects related to the emission of highly densified emitters, increased emission currents and intersecting ion beams can be studied. 

PIC simulation of electrospray ion beam after acceleration, effects in x- and y- axis are amplified

For further information, please contact David Krejci

 

Electrochemistry of Room Temperature Ionic Liquids

Structure and potential drop of classical double layer models, from [5]

In ionic liquid devices, the electrochemical system behavior is governed by interfacial properties, in particular the electrical double layer (EDL) which exhibits properties distinct from traditional aqueous electrolytes. A direct consequence of the layered structure is a differential capacitance curve, and the voltage drop across this double layer can lead to decomposition of the liquid when surpassing the electrochemical window, a limit above which electrochemical reaction start. To maintain propellant stability during extended periods of charge emission, for example when firing an electrospray thruster, knolwedge of the behavior of the double layer in complex systems with many parameters, such as geometry, ion species and emission polarities, becomes important. At SPL, research on modelling and experimental characterization of double layers is performed.

Ionic liquid double layer charging characteristics (bottom) for the SPL electrospray thruster at nominal emission as response to electrospray emission (top), from [5]

For further information, please contact David Krejci

 

References

[1] Legge, R., & Lozano, P. (2011). Electrospray Propulsion Based on Emitters Microfabricated in Porous Metals. Journal Of Propulsion and Power, 27 (2), 485-495. doi:10.2514/1.50037

[2] Taylor, G. (1964). Disintegration of Water Drops in an Electric Field. Proceedings of the Royal Society of London, 280 (1382). 383-397.

[3] Miller, C. (2015). On the Stability of Complex Ions in Ionic Liquid Ion Sources (Master’s thesis). Retrieved from DSpace@MIT. (mit.002344831)

[4] Coles, T., Fedkiew, T., Lozano, P. (2012). Investigating Ion Fragmentation in Electrospray Thruster Beams, 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & exhibit, AIAA 2012-3793, Atlanta, GA.

[5] Masuyama, K. (2016). Electrochemistry of Room Temperature Ionic Liquids with Applications to Electrospray Propulsion. PhD thesis, MIT, Cambridge, MA.