Project Overview

We are studying the production and energetic impact of electrosprayed nanodroplets and molecular ions, covering continuously the projectile size range from 1 nm up to hundreds of nm. The 1-20 nm size range is of great interest to achieve variable Isp from 100 s to >1000 s in drop-based electrical propulsion, while pure molecular ion emission enables higher Isp.

Electrospray atomization in the cone-jet mode
	Charged nanodroplets are generated by electrospraying a liquid in the cone-jet mode. This figure shows a cone-jet operating in a vacuum. The main features are, from left to right: a metallic emitter; a conical liquid meniscus (Taylor cone) attached to the emitter base; a slender jet emanating from the apex of the Taylor cone (not visible because of the small jet radius); and a conical beam of droplets, produced by the breakup of the jet. A cone-jet is typically set at the tip of a conducting tube (emitter) facing a second electrode (extractor). Liquid is fed through the inner channel of the emitter, and a voltage difference between the electrodes applied. If the flow rate and emitter potential are within certain limits, a stable cone-jet develops, and the fluid is atomized
into charged droplets. The droplet diameter can be controlled down to a few nanometers, by adjusting the physical properties of the fluid (mainly its electrical conductivity, surface tension, viscosity, dielectric constant and density) and its flow rate. In addition, when the radii of curvature of the jet and drops reach values of tens of nanometers, the electric field on the liquid surface becomes large enough to field evaporate molecular ions.

We have demonstrated that electrosprayed nanodroplets are extraordinary sputtering projectiles (their sputtering rates exceed those of the state of the art ion beam technology by a factor between 100 and 600). Their impact on crystalline substrates produces new and unique phenomena such as surface amorphization and texturing of surfaces with controllable roughness. We have also investigated the emission of molecular ions from ionic liquids, many of which contain reactive species and display substantial etching. In addition, electrospray sources are ideal for focused beam applications. These findings lead to new opportunities in the fields of electric propulsion (achievement of improved and variable Isp, thrust and power density and propulsive efficiency, quantification of the lifetime of electrospray thrusters), MEMS and IC fabrication (broad-beam and focused-beam nanodroplet and ion sources for high speed beam milling and microfabrication, reactive nanodroplet and ion etching, polishing of large and curved mirrors), surface processing (patterning of crystalline surfaces with amorphous layers, patterning of a textured surface with controllable roughness, strengthening of materials for increased thruster life, microscopy), and secondary ion mass spectrometry (SIMS) of organic surfaces. The goals of this proposal are to gain a detailed, first-principles understanding of the production of nanoprojectiles, their interaction with surfaces, and the investigation of the surface processing and propulsive applications outlined above. The extremely high mass fluxes of nanodroplet beams, coupled with novel projectile/surface interaction modes, will make nanodroplet beam a transformational technology for surface processing. The development of electrosprayed nanodroplets and molecular ions for electric propulsion (made possible by AFOSR investment), and the proposed spinoff into surface engineering, echoes the development of the Kaufman ion engine and its subsequent use for ion beam processes. We anticipate similarly that, by virtue of focusing on both propulsion and materials processing, the present investigation will have a much greater impact than if focused strictly on electrical propulsion.

This project offers the opportunity to study systematically, for the first time, the generation and impact of projectiles with sizes between approximately 1 nm and 1 micron. The impact of projectiles larger than one micron (commonly referred to as the hypervelocity impact problem), and that of atomic and small molecular ions (i.e. sizes below 1 nm), have been well studied and will not be pursued here. Conversely, the very substantial intermediate size range remains unexplored due to the absence of projectiles, except for our own initial studies, and for a most interesting prior research line on cluster ion beams covering the 1-4 nm size range. We thus expect that the study of a large and unexplored parametric window in the projectile/surface collision problem will produce important discoveries, and have a significant scientific impact. For example, previous research has shown that:

• The sputtering rates of nanodroplet beams outperform the state of the art broad-beam ion sources by a factor between 100 and 600, a transformational finding for MEMS and IC fabrication of inert substrates such as SiC. This observation is directly relevant to current efforts in drop-based propulsion, as its potentially devastating effect on device life had been previously unknown.
• The impact of nanodroplets on crystalline Si and the associated shock compression amorphatize the target’s surface, a phenomenon never observed with macroscopic projectiles.

Electrospray-based propulsion has been supported by the AFOSR over recent years. Small drops and ions having high charge to mass ratio can be electrostatically accelerated to very high Isp, but produce little thrust for given power. Larger drops having smaller charge to mass ratio produce larger thrust, though at lower Isp. Controlling the charge to mass ratio hence provides the freedom to tune Isp for a given single thruster-propellant combination. It is therefore possible to compromise between the desire to consume little fuel at the expense of a slow acceleration, and the occasional need to achieve high thrust at the expense of a less efficient use of propellant. In either case controlling the production of nanodrops and ions is at the heart of developing improved electrospray propellants, and studying this problem will be an important thrust of this project.

Ion beams are extensively used in surface processes such as sputtering, sputtered deposition, ion implantation, reactive ion etching, polishing, etc. Despite its ubiquity, ion beam technology presents two major drawbacks for many applications: a) very high projectile energy (typical energies per ion are in the hundreds to thousands of eV, while surface modification with reduced damage requires energies of a few eV); and b) low beam mass flux, and therefore low surface processing rates (this is a consequence of the projectiles’ high charge to mass ratio which translates into beams with high space charge). Cluster ion beams were developed to overcome these two problems, and have become an important tool for surface processing and SIMS. Cluster ions have much lower charge to mass ratios than atomic ions, and therefore their beam mass fluxes at the space charge limit are orders of magnitude higher than for atomic ion beams. Furthermore, although cluster ions are much more massive than atomic ions, they are still relatively small compared to nanodrops. Unfortunately, the benefits of cluster ions have been restricted to projectiles formed by vapor condensation in hypersonic free jets, and suffer from a number of practical limitations in terms not only of size (<4 nm), but also of low energy and restricted range of projectile materials, high gas pumping capacity requirement, etc. This project will overcome the shortcomings of ion and cluster ion beams by using electrosprayed nanodroplets, which have very low charge to mass ratio and therefore beams with extremely high mass fluxes. In addition, unlike cluster ions the electrospray beam is emitted from a point source: this enables the electrostatic focusing of electrosprayed beams on a submicrometric spot, an essential property for precision micromachining and 3-D profiling of organic surfaces via SIM.

This fundamental research program will experimentally investigate and model a broad range of problems: the generation of optimal electrosprayed projectiles for propulsion and surface processing; colloid thruster erosion associated with nanodroplet and ion impact; beam modeling for maximization of thrust; nanodroplet impact on crystalline surfaces with varying concentration of defects (both dislocations and grain boundaries); molecular dynamics and multiscale modeling of the impact; amorphization of crystalline surfaces, including those of metallic bond coatings prior to high-temperature exposure for making a thermally grown oxide; the role of reactive ion and droplet projectiles on surface modification; the fragmentation of molecular ions and its impact on thruster efficiency, etc. To tackle this multidisciplinary problem we have assembled a team of experts on electrospray atomization and colloid thruster technology, materials and surface science, molecular dynamics and multiscale modeling, and surface spectrometry. The key investigators and their main expertise related to this project are:

• Manuel Gamero-Castaño (UCI) is an expert on electrospray atomization. He was originally in charge of developing the colloid thrusters for NASA’s ST7 mission, and has continued working on advancing colloid thruster technology for LISA. He recently discovered that energetic nanodroplets sputter hard crystalline solids at very high rates, and has started a novel line of research on the use of nanodroplet beams for surface processing. Other activities relevant to this effort include theoretical and experimental work on the electrohydrodynamics of electrosprays, the expansion of electrospray beams, and electrospray ionization.
• Juan Fernández de la Mora (Yale) has worked extensively on electrospray propulsion as well as many aspects of the fluid dynamics of electrosprays, on electrospray ionization and electrospray mass spectrometry. His group has introduced most of the electrospray propellants currently used both in propulsion and surface modification applications. He has published several dozen articles on the subject, including a review article in the Annual Review of Fluid Dynamics.
• Paulo Lozano (MIT) is an expert on electrosprays working in the pure ionic regime and has published more than 50 papers related to the characterization of ionic liquid ion sources, electric propulsion and interaction of molecular ion beams with materials. Relevant contributions to this research include the first detailed measurements of the energy structure of molecular ion beams from ionic liquids, the introduction of the externally wetted configuration for ionic liquids, the development of microfabrication processes to build dense electrospray arrays on porous metals, the confirmation of chemical enhancement in molecular ion sputtering on semiconductors, the observation of solvated molecular ion fragmentation and a preliminary determination of the focusing properties of molecular ion beams.
• Markus J. Buehler (MIT) is a leading expert in computational materials science, and one of the most decorated young scientists in the US. Before joining MIT, he was the Director of the Multiscale Modeling and Software Integration, Materials and Process Simulation Center at the California Institute of Technology. He has published extensively in Nature and other high-impact journals like Nature Materials and Physical Review Letters. His expertise in molecular dynamics and multiscale modeling is most important for understanding the physics of the varied phenomena pursued in this proposal.
• Alessandro Gomez (Yale) is a well known expert in electrospray physics, including electrospray combustion. This work has led to his development of practical 2D arrays of electrosoprays that have been used successfully for handheld energy generators at atmospheric pressure. He has recently extended this multiplexing work to produce thrust in vacuum.
• Daniel Mumm (UCI) is a recognized expert in surface engineering and materials science. He has conducted research on microstructure-property relations, new materials synthesis and microstructure evolution in inorganic materials and multi-layer structures, and on the application of this knowledge to the design of coatings, high temperature electrochemical systems, structural materials and composites. Of particular interest is the dynamics of interfaces under service environments and microstructural instabilities. Current research interests include thermal barrier coatings, solid oxide fuel cell systems, hydrogen storage materials, and metal oxidation processes.
• Jian-Guo Zheng (UCI) is a Project Scientist and the Director of Operations of the Laboratory for Electron and X-ray Instrumentation at UCI’s Calit2 center. He is an expert in transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. His work and guidance is key for the analysis of surfaces bombarded by energetic nanoparticles.