WEBVTT

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Mark Kushner: Welcome to the first MIPSI seminar for the 2026 fall semester. Before we begin with the seminar.

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Mark Kushner: I'd like to announce the outcome of this past summer's most competitive and interesting event, and that was the competition for the design of the new Lipsy mug.

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Mark Kushner: So it was time to design a new Nipsey mug.

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Mark Kushner: And we held a competition for the image to place on the mug. And the entries were judged by the MIPSE community. I think you folks voted, and voted well.

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Mark Kushner: And the winner was Landon Tafoya, whose image is now emblazoned on the new MIPSI mug.

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Mark Kushner: And this is an image from an MHD simulation of a wire array Z tension plosion. Landon received, hopefully he's online. Landon, are you with us?

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Mark Kushner: So, you have your mug, and you've… But…

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Landon Reese Tafoya: Yep, I got it. Here we go.

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Mark Kushner: He received as a prize for designing the mug a mug. No, I see, it's very appropriate. And thank you very much for, your efforts. He's now sitting, apparently, in his car in Los Alamos, as internship. So, thank you very much, Senator.

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Mark Kushner: It is my pleasure to introduce Professor David Goh of Notre Dame University as today's seminar speaker.

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Mark Kushner: David received his BS in Mechanical Engineering from Notre Dame, after which he worked for GE Aviation, and this I didn't know, designing turbines. Yeah, yeah.

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Mark Kushner: And while he was at TE, he received his MS in Aerospace engineering from the University of Cincinnati. This was followed by his PhD in mechanical engineering from Purdue University, after which he joined the faculty at Notre Dame University.

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Mark Kushner: Dave is now the Viella Hagg Professor at Aerospace and Mechanical Engineering, former chair of the department, and is currently Vice President and Associate Provost for Academic Strategy.

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Mark Kushner: EPA has a very broad research portfolio covering platinum science and engineering, heat transfer, fluid dynamics, and chemical analysis, which, in addition to producing the appropriate scholarly publications, has also resulted in 10 patents and several licensed technologies.

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Mark Kushner: David has received several recognitions for his work, including the NSF Career Award, the FOSR Young Investigator Award, the AEEE NPSS Society Early Achievement Award.

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Mark Kushner: Electrostatic Society, American Rising Star, the Visconta Fellowship, and he is an ASME Fellow, as well as receiving the Outstanding Mechanical Engineering Award from Purdue University, and several awards for teaching and service from Notre Dame.

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Mark Kushner: And on top of all that, David is a former president of the Electrostatic Society of America.

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Mark Kushner: The title of this talk this afternoon is Non-Thermal Plasmas for Revolutionizing Goods Production, but we are going to present today that a prize which far exceeds all those other plasmaids, and that is…

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Mark Kushner: So, thank you very much for making the trek to Ann Arbor.

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Mark Kushner: Perfect. Excellent. Thank you so much.

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Mark Kushner: So, thank you, Mark. I will share my screen now.

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Mark Kushner: All right. So congratulations, Landon, from me, and thank you to all of you joining us online. Actually.

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Mark Kushner: saw my collaborator from down in Chile, Felipe Velosa's on, so that's kind of fun joining. And thank you, Mark, so much for inviting me. I've known Mark for almost the majority of my entire faculty career, and he's been a great supporter and an inspiring leader of the entire plasma community, and I think Mimcy is a wonderful example of that. My students come here, all the time.

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Mark Kushner: So, I'm going to be talking about non-thermal plasmas for revolutionizing goods production, let's see…

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Mark Kushner: This was working, let's make sure it is working.

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Mark Kushner: There we go. Just in case I,

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Mark Kushner: Don't get to it. I just want to make sure I do all of my acknowledgements. Lots of people have held me in this work. Those in red, you'll see the exact production of results there, as well as funding from the Army, the Department of Energy, National Energy Technology Lab, and the AFOSR as well.

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Mark Kushner: Okay, as was noted, I do work in the broad area of thermal fluid transport, so there's some fluid mechanics.

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Mark Kushner: some energy, some heat transfer, but really, it's the charge, it's the non-thermal plasma and gas discharges that make up the majority of my research, especially right now, my active research. And I do a mix of theory and simulations, experiments, and then also technology development. So my approach, my philosophy has always been I…

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Mark Kushner: I ask the questions that I personally find interesting, and I kind of try to figure out what's the right tool, whether it's theory or simulations or experiments, to help me get towards an answer. And so that's allowed me to have a very varied and, in my opinion, very interesting career.

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Mark Kushner: As you know, this is great, it's a plasma conference, or a plasma community, so I don't have to go over this in detail, but plasma is an ionized gas. There's really… we sometimes call it the forced state of matter. I'm not a huge fan of that, because that's really an equilibrium-type perspective. But we know that, really, we can classify these into two areas. One is thermal or equilibrium plasmas.

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Mark Kushner: And then the other is non-thermal or non-equilibrium, where in non-equilibrium.

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Mark Kushner: what's happening is the energy of the electrons is going to be much greater, the energy of the ions, and the energy of the ions is roughly the energy of the neutrons, okay? These can also be called cold plasmas, because they're both

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Mark Kushner: cold relative to thermal plasmas, but also because they can be cold to the touch. They can even be at room temperature, as I show here today. And that's going to be relevant as we come back, to talk about some of our results.

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Mark Kushner: I always like to show this cartoon.

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Mark Kushner: Right? Two of the most successful plasmic technologies out there today are the fluorescent light bulb and the technologies that enabled this computer and that phone, and basically every microchip that is, currently used today, whether it's an etching technology, a sputtering technology.

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Mark Kushner: But if I were to Google plasma.

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Mark Kushner: I'd probably get 5 or 6 places where I could go donate my blood and get money right now, right? So the general public doesn't really understand what

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Mark Kushner: how important

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Mark Kushner: non-thermal plasmas have really been to their everyday quality of life, right? They've experienced it very visually and viscerally in multiple ways. And I think what's really interesting is that we have the opportunity now, with some of the emerging areas, to really show them, sort of, plasmas in action. And so here's some examples. One's plasma dentistry.

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Mark Kushner: Plasma disinfection of fruits and vegetables, or plasma agriculture, it could be called, or even plasmas coming in contact with skin for plasma wound healing.

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Mark Kushner: One way to think about this is in terms of what I call goods production. And so typically, if we're going to be making something, something here in this room.

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Mark Kushner: Right? It's gonna start from some sort of natural resource. Here I show mining, or it could be natural gas, or it could be oil, it could be wood even, and we're gonna take that, we're gonna bring that to some centralized processing facility, and usually, with heat.

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Mark Kushner: process it, distill it down into usable components that then become the building blocks of fuels and medicines and pharmaceuticals, fertilizers, plastics and metals, anything really like this. And the question, or the proposition, rather, that the plasma community has been making

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Mark Kushner: is that we don't necessarily need that high temperature component in the middle. Non-thermal plasmas provide so much opportunity to replace it

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Mark Kushner: Such that you can now have something that's more electrically driven, and if it's electrically driven, it can actually be run by renewable energy, and it doesn't promote…

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Mark Kushner: produce as much CO2. And while I know some of those ideas and phrases are verboten right now.

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Mark Kushner: The real thing is that it creates a more resilient

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Mark Kushner: energy and goods production economy, right? Where you're not relying on, say, foreign resources to help drive that, as I'll talk about in a little bit.

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Mark Kushner: here later. Now, I've not… I was not the person who came up with this idea. I want to point to an actual workshop, and so this is a slide borrowed from Professor Kushner several years ago that promoted this idea to the National Science Foundation of a future based on electricity through non-thermal plasmas. And there's been lots of people studying lots of different areas.

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Mark Kushner: The area I'm going to talk about specifically today

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Mark Kushner: This is gonna be on plasma-liquid interactions.

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Mark Kushner: and how they can drive this. So I'm gonna narrow this talk from all the different, sort of.

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Mark Kushner: threads of research I have to really focus on plasma liquids. Now, plasma liquids go back centuries. They were used as far back as the 17 and 1800s in systems, arc-based systems, to include the discovery of ozone, the discovery of argon, but they were also used industrially

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Mark Kushner: for the production of fertilizer. So, Brooklyn Eyde in the late 1800s used that, found that if you have an arc in contact with water, produces nitrate and nitrite, which are great fertilizers, and it became, a standard, pseudo-standard technology. Now, it was ultimately replaced by Haber-Bosch, which was more energy efficient, but

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Mark Kushner: plasmas in contact with water, and in contact with liquids broadly, have had industrial application, right? So an electrified future is possible if we address some of the shortcomings that were happening at that time.

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Mark Kushner: Plasma-liquid interactions, it's been fascinating to watch this field grow over the last 15 years or so. I was giving a talk earlier this summer, and just for fun, I googled plasma Liquid Review in Google Scholar, and 6,000 kits showed up. Now, not all 6,000, those are going to be perfect articles.

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Mark Kushner: But there's not only so many reviews on plasma and liquid physics and chemistry, but just in application spaces. There are tons of application spaces, like I said, in agriculture, in medicine, in other areas, that it's become a very popular, area. And if you just want to focus on the fundamentals.

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Mark Kushner: There are plenty of reviews, including by Professor Kushner here, out there just in the last 6 or 7 years alone. So it's a very interesting and hot field, and one that I think has a ton of potential.

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Mark Kushner: When we talk about a plasma in contact with a liquid, what we're talking about is typically a non-thermal plasma, so it's going to be electrically driven.

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Mark Kushner: Most of the time, could certainly be microwave, or something like that. And it's going to be in one of these six configurations. And so, one is you could have it actually occur inside the liquid. You've submerged your electrodes, you apply high enough voltage on a short enough timescale, you actually will generate a plasma in the liquid, largely in the vapor formed by evaporation near the electrode.

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Mark Kushner: You could have it above the liquid, generated in a non-contact way, like a plasma jet, and then that plasma jet propagates and it happens to make contact with the liquid. You could have the liquid as part of the circuit, so one electrode is up here, and then the liquid itself is grounded, so it's now part of the circuit, so that's in contact.

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Mark Kushner: You could also have two electrodes on the surface, so now you have a surface discharge, similar to a surface discharge you might have on, say, a dielectric material.

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Mark Kushner: And then, if bulk solutions aren't your thing, you can have aerosols, and have a plasma that interacts with the aerosols, or invert that, have bubbles with plasma inside, and liquids,

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Mark Kushner: surrounding it. And so there's lots of different configurations, lots of ways to generate these, and I'm not going to overview all of them today. And I'm going to narrow my conversation to two plasma liquid applications.

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Mark Kushner: that I find interesting and show potential opportunities for revolutionizing goods production. One is what we call plasma electrochemistry. There's other names for it, but that's the one that we've been using for a while. And then plasma-driven material processing.

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Mark Kushner: And so, let's start with plasma electrochemistry. And in this configuration, we're going to be talking about plasmas in contact with the liquid. The liquid is part of the circuit. And…

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Mark Kushner: If you think of a conventional electrochemical cell, or electrolytic cell, you have an electrode submerged, two electrodes submerged, connected to some power source. And that power source is going to drive electricity through the liquid, which is part of the circuit, and there needs to be a charge transfer at each electrode to continue that circuit. And so you'll have a reduction reaction, where electrons go into the liquid, and an oxidation reaction

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Mark Kushner: Where electrons come out.

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Mark Kushner: of the liquid.

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Mark Kushner: Plasma electrochemistry simply says, let's replace one of these, or both if you're really into it, with a plasma, a non-thermal plasma, so now your power supply is going to drive an electrode suspended over the liquid, and you form a plasma inside. What's interesting about this

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Mark Kushner: is that you now are driving a reduction or an oxidation reaction and a charge transfer process. Again, the circuit needs to be completed at a gas-liquid interface, not at a metal-liquid interface. You don't need any absorption.

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Mark Kushner: on the liquids… on the surface, and if you're interested in chemistry, you actually can do this in what I would call a non-local way. I can do something at this interface that affects chemistry deeper in, as opposed to conventional electrochemistry, where it all occurs on the surface, the material surface itself.

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Mark Kushner: So let's look at this first configuration here, where the plasma is acting as a cathode in the electrolytic cell, so electrons are being directed inside of the liquid.

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Mark Kushner: And so here, I'm going to show you a video of just of that exact same cell, and I turn on my plasma, and I have a pH-sensitive dye. And then that pH-sensitive dye, as I change the local pH,

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Mark Kushner: it changes color. And what you see is that as I come in contact with the liquid solution, I get, A, a change in color, and that's because I'm electrolyzing that water, I'm evolving hydrogen gas.

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Mark Kushner: Right? And I get this sort of vigorous mixing that's occurring, right? Nothing else is happening. There's no stir bar, there's no student in the background shaking the table, right? This is all just induced naturally by the interaction at the surface. There's a nice electrostatic interaction that causes electrostatic pressure as well, and it's very rapid. That was real time. I didn't speed that up.

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Mark Kushner: In any way.

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Mark Kushner: Chemically, the key species that my group has looked at historically is the same species we look at in the plasma phase, which is the electron. And so, if we have electrons being directed in

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Mark Kushner: Towards the solution, they will actually become free electrons in the solution, what we call solvated electrons, or in water, aqueous electrons.

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Mark Kushner: the water, or any polarizable liquid, will rearrange itself to form dipoles, and that creates, essentially, a little well, potential well, similar to a 1S orbital.

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Mark Kushner: We call this typically the cavity model. It's disputed in the literature, I don't want to get into that dispute.

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Mark Kushner: But that electron has a reduction potential of almost minus 3 volts. It can kind of reduce anything. So you take this powerful electron plasma species, put it into the liquid, and now it becomes a very powerful liquid species.

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Mark Kushner: Several years ago, we asked… when we were still uncertain if these existed, we knew solvated electrons existed, we weren't sure if a plasma could produce them. We came up with a way to measure them. I said it forms essentially a 1S orbital.

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Mark Kushner: That means if you were to shine a light at it, it can absorb that light and get promoted into a 2P orbital, effectively. And that light happens to be that energy level

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Mark Kushner: change happens to be coincident with the red light. So if I shine red light at solvated electrons, like I'm showing here, they will absorb that light, and so any signal I measure on the back end will be attenuated. It will be reduced by the presence of solvated electrons. We do this total internal reflection.

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Mark Kushner: absorption spectroscopy to do this, we pulse the plasma, we lock in on that frequency so that we can measure it and hopefully reduce the noise, and that's what it looks like. It's not a really challenging experiment to set up, it's a really challenging experiment to get to work. But we were able to show that if you, change the wavelength of the laser just by using different diodes.

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Mark Kushner: You can reproduce what is the known absorption spectrum

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Mark Kushner: of the solvated electron. This gave us

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Mark Kushner: confidence that, yeah, we are, in fact, producing salvated electrons. Since that time, we've gone on to explore, different ways to interpret these, largely looking at their competition between their reactions that they drive, so they react with water itself to electrolyze it.

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Mark Kushner: and their diffusion into the liquid, and we can measure, for example, our total internal inflection, our TRS intensity as a function of our current density, and we can see interesting scaling behaviors, and using basic reaction diffusion models, actually come up with the theories that predict, in this case, a linear or a one-third scaling behavior.

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Mark Kushner: Now.

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Mark Kushner: There are a lot of questions you can ask in this configuration with the plasmic cathode. Once that electron comes in, what does it do? It could react with the solute.

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Mark Kushner: or a scavenger, it could react with water, but there's lots of other things that could be happening at the plasma-liquid interface, at that vapor layer, that creates all kinds of other reactive species, and they can dissolve in, and they can actually induce a lot of chemistry as well. And so.

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Mark Kushner: you start asking yourself, what chemistry does this electron drive? What's the competing chemistry? How do I be selective? All these really rich questions. And I'm not going to talk about them today. I am instead going to talk about the other configuration.

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Mark Kushner: So if you're interested, we wrote a review paper that's really a review of our own work, if I'm being honest, because we've published a lot of papers in this, on that behavior of the solvated electron.

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Mark Kushner: So I'm gonna focus…

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Mark Kushner: On the other configuration, where the electrons are supposed to be coming out, and we're supposed to be getting the other plasma species.

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Mark Kushner: directed into the solution. So, your positive ions from your plasma, so the plasma anode. And…

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Mark Kushner: We don't fully understand this configuration, but in principle, you have a plasma ion that's gonna come in, it's gonna have some energy, it'll lose a lot of it, because it'll, through collisions and the collision will sheath.

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Mark Kushner: It'll dissociate maybe that water to form an OH radical. It could ionize the water, itself, so it just hits the water high enough. We know this with high-energy ions, it'll do that, and it pops off an electron through an ionization process.

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Mark Kushner: It could actually just do a charge transfer reaction, and ultimately you'd end up just with more OH. And so I told you solvated electrons

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Mark Kushner: really powerful reductants, these OH are really powerful oxidants. And so, now you have these oxidizers that, are… could be driving chemistry. But from a plasma physics perspective.

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Mark Kushner: We also know that electrons need to somehow come out to continue the circuit, right? Otherwise.

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Mark Kushner: all those electrons just gather at the interface, and we lose our current. And so, there's sort of a question of how do we get electrons out? Are these solvator or aqueous electrons, getting out?

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Mark Kushner: And so, I'm going to talk about this configuration, and an application of it, and we can go back to fundamental plasma physics, where we think of the two charge creation processes, what is sometimes called the alpha process and the gamma process, or the Townsend processes.

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Mark Kushner: Right? Where in the alpha process, we have a free electron, hits a gas molecule or atom, ionizes it, produces a second electron, and you get the electron avalanche. Exponential growth.

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Mark Kushner: But if there's nothing else happen, all those electrons will get to the anode, and your circuit will die. So you have to be injecting electrons, and typically what that means is that an ion will come to the surface.

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Mark Kushner: interact with the cathode in some cool way, and eject electron, what we call secondary emission. It's usually, like, an Auger omission, it could be a photo emission, which is not necessarily electron-driven, but we know we need to get an electron out.

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Mark Kushner: And we call the emission coefficient the ratio of the emitted electrons to the incident ions, P for positive ion in this case.

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Mark Kushner: So, what does that look like with water? I don't know, right? Water doesn't have the band structure of a metal. Doesn't even really have the band structure of most dielectrics, either. And so, that is a really kind of weird

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Mark Kushner: weird thing. How does that really work? And so, we decided to start Trying to figure it out.

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Mark Kushner: And one way you can estimate the secondary emission coefficient is by measuring Passion's curve. Passion's curve is a curve that predicts breakdown, has a function of your pressure and your distance between electrodes.

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Mark Kushner: Where A and B are some function of the gas, and it says your breakdown voltage is going to be related to PD by this highly nonlinear expression, and dependent on your secondary emission

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Mark Kushner: of electrons, so this gamma. And this is well known, the measurements, and the theory go back well over a century now.

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Mark Kushner: And…

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Mark Kushner: If you have a needle pointed at an electrode, rather than two flat electrodes, you can modify this expression a little bit to account for that nonlinear configuration that you might have. And so, we set up an experiment and measured Passion's curve, or measured breakdown.

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Mark Kushner: and actually tried to fit, Passion's curve. So we have

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Mark Kushner: an electrode suspended over the water, we have water, and we just change the gap height, which changes PD, and we measure how much voltage we need to apply to turn on the plasma. Then we can curve fit

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Mark Kushner: our modified passion curve to that, and extract an equivalent gamma. What we see is that gamma for these systems is like 10 to the minus 5 to 10 to the minus 6. Now, for a typical metal, it's going to be

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Mark Kushner: 10 to the minus 1, 10 to the minus 2, maybe 10 to the minus 3. We're several orders of magnitude lower. What this suggests is that it's extremely inefficient.

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Mark Kushner: Right? One in a million ions somehow kick an electron out of the liquid.

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Mark Kushner: So we wanted to get a sense of how is that working, and I said already, well, we think we created these solvate electrons, maybe those are what are coming out.

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Mark Kushner: And so, how could we potentially get to that? And one way we did is, instead of using one cell, we split the cells. So we now have two cells, and that allows us to isolate this, and we can measure the system voltage

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Mark Kushner: across the entire cell. And if we…

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Mark Kushner: use a very high conductive electrolyte, there's no voltage drop across the liquid.

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Mark Kushner: So then the voltage drop is really just the drop across the plasma, which is really just the voltage drop across the sheet. The sheet is where secondary emission is happening, so if we somehow modify secondary emission.

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Mark Kushner: We should see the system voltage, i.e. the plasma voltage, change.

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Mark Kushner: Right? That makes sense. And so, our thought was, let's get rid of all the electrons.

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Mark Kushner: So we put in what are called scavengers.

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Mark Kushner: And these react with electrons really quickly, nitrite and nitrate. And in fact, there's a scenario called the pre-solvated electron, where the electron is in the solution, hasn't quite thermalized to equilibrium yet.

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Mark Kushner: So it's not quite in that well called pre-solvated, and it's known from the radiation community that these also scavenge pre-solvated electrons.

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Mark Kushner: And we did that, and literally nothing changed about our system voltage.

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Mark Kushner: So that suggested to us that whatever the electrons that are being emitted

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Mark Kushner: are coming from, they're not solvated. They haven't created this nice 1S orbital shell, for them. So we came up with this hypothetical diagram.

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Mark Kushner: like, tried to explain it. I'm not sure that it does. We actually… we're having our own doubts about it. But basically, we'll have argon, in this case, come in, hit the water. That argon is going to ionize the water, and before the electrons can do anything, they're in the conduction man.

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Mark Kushner: And they just… one out of a million of those just decide, hey, we're gonna leave today. And they come out into the gas. Then the rest will go through a localization and a solvation process to an equilibrium state.

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Mark Kushner: But then, also, the argon could be doing all kinds of other things, so there's your secondary mission, as well, and driving all this interesting OH-based chemistry.

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Mark Kushner: as well.

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Mark Kushner: Okay, so that was our attempt at understanding solvated electrons and their role, but it was all…

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Mark Kushner: By measuring other things.

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Mark Kushner: I told you we could measure them in the plasma cathode configuration. Our next question was, can we actually measure them directly? Do we even know solvated electrons exist? Is this a thing that happens? Right? The argon energies could be as low as 1EV. That's not enough to ionize water. So we don't even know if this actually happens. So… so?

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Mark Kushner: Let's go try N.

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Mark Kushner: Measure solvated electrons.

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Mark Kushner: So we inverted our system. It was harder than you think to invert our system, to invert the electronics and the switching circuit.

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Mark Kushner: But we're able to do it. Turns out our plasma was more well-behaved. And what I'm showing you is what a typical TRS measurement looks like. So, we first, we shine the laser, the plasma is not on, that gives us a baseline signal.

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Mark Kushner: And then we turn on the plasma, and we get this signal, which is the inverse, right? Like, we see a reduction in signal, and we run that for, you know, about 100 seconds.

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Mark Kushner: And then we turn off the plasma… I mean, turn off the laser, and we see any light that the plasma is generating on its own, so we can subtract plasma plus laser from plasma alone, and that gives us our total signal. What you see is a bunch of raw data. It's not terribly clean. It's pretty noisy data.

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Mark Kushner: But you look at the average of the data, and you definitely got enough signal relative to the noise. So we were… we were pretty confident that this was working. We did a little bit of measurements to try to get to the,

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Mark Kushner: the spectrum that's not a great fit, at least shows the right trend. It tails off as you go towards the blue, but what we ultimately use to convince ourselves, yeah, we do have solvated electrons, we can measure them.

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Mark Kushner: is we put in that scavenger. So we put in just a bunch of the scavenger. Well, so this is our raw signal, and what we saw is that it killed the scavenging electron signal. That's basically your noise floor. So that was really confident. But…

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Mark Kushner: I said that we also have all this OH.

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Mark Kushner: That OH, that hydroxyl radical, reacts with solvator electrons really quickly in and of itself.

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Mark Kushner: And you get OH-. So our signal may be artificially suppressed, because we're losing electrons to the OH that's naturally present. So then we throw in methanol to scavenge the OH, and sure enough, our signal comes back.

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Mark Kushner: to an even higher level, again, telling us that we probably are, in fact, measuring solvator electrons. They're not participating in secondary mission, but it's good to know they're there, right? We've assumed they're there, but now it's good to know they're there.

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Mark Kushner: One question we asked ourselves is, okay.

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Mark Kushner: how many… how efficient is that production of solvated electrons? How many ions produce a solvated electron? And one way to do it is to use an indicator reaction. In this case, use chloroacetate, where when it reacts to the solvate electron, you get a chloride ion.

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Mark Kushner: That pops off, and you can just measure that with an ion-selective electrode. And that was actually done in 1973 by Goodman and Hickley.

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Mark Kushner: Now, if you look at some potential reaction paths for solvate electrons, you can come up with an expression that looks at the efficiency, eta's efficiency.

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Mark Kushner: Faraday efficiency of measuring chloride has a function of the different reaction rates of all these competing reactions.

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Mark Kushner: Well, when this inverse goes to zero.

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Mark Kushner: Then, your chloride efficiency is the efficiency of creating a solvated electron.

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Mark Kushner: So you measure…

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Mark Kushner: for different concentrations of chloroacetate, when the 1 over the chloroacetate goes to 0, you extrapolate, that intercept tells you now your efficiency of solvated electrons. And so, Hickling and Goodman did this.

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Mark Kushner: In a hydrogen atmosphere, so different than what we did, but they said, we get about 3 electrons per ion and about 2 hydroxyl radicals per ion, okay? And they were like, look, we get beyond Faradake efficiency. We get more electrons per charge injected.

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Mark Kushner: Okay.

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Mark Kushner: So, so we were like, oh, we wonder if that's still true for our system.

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Mark Kushner: So we did the same measurement. We had to do a lot to make sure there wasn't OH in the system, so we had to add an OH scavenger, and what we measured in our system

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Mark Kushner: is actually roughly closer to 1 electron per incident ion. And…

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Mark Kushner: From there, we got to about 2 OH prime, so about the same amount of hydroxyl, but fewer solvate electrons. Still, this is remarkably efficient.

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Mark Kushner: This says that every single ion that comes in actually produces an electron.

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Mark Kushner: I'm not even sure if I believe it. I know I measured it, but it still kind of stuns us, especially when we're saying only, 1 million actually come out in any sort of way, shape, or form. But it suggests that, yeah, we might actually, in fact, have solved electrons.

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Mark Kushner: So, this number is important, this 2OH, because I'm going to come back to it.

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Mark Kushner: So, we were playing around with this configuration, and I needed an application. I needed something to do besides the fundamental.

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Mark Kushner: And this young woman named Savannah Benjamin came to me. I knew her advisor, and she had a question about sprays. I told you I did other kind of fluid dynamics. I had done some spray work, and she had a question about sprays, and what she was doing was she was trying to come up with a way to extract uranium.

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Mark Kushner: So, uranium broadly, appears in water in different ways. In mining, right now, almost all mining's done in Russia, or Kazakhstan, or places in the middle…

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Mark Kushner: of Asia, it's through in-situ leach mining, where you basically dump in water, and dissolve out urinal, which is this ion, UO22+,

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Mark Kushner: You do it as a sulfate to our carbonate.

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Mark Kushner: And then you take it and you extract that urinal out as uranium. That can power nuclear submarines or power plants, or something like that. 56% of the world's uranium is mined that way. 20 years ago, it was less than 10%. So when we talk about resiliency of our energy programs, our power, our nuclear power is now depending on

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Mark Kushner: Tons of mining occurring in countries in…

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Mark Kushner: portion of the world that is probably… there are some security concerns, let's put it that way. But there's also environmental contamination, so wine mining weapons programs from water, extraction from seawater. And so the way they extract the urinal is they actually react it with hydrogen peroxide, H2O2.

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Mark Kushner: And they get this… this… this thing com… that comes out called StudTite. It's a… it's literally a yellow powder. So she's asking me, because she had been using ozone, and she thinks if she does it in droplets, it'll be more efficient. And it was a fun, sort of.

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Mark Kushner: unwieldy conversation when I said, exactly what are you trying to do? She's like, well, I'm trying to have

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Mark Kushner: you know, more hydrogen peroxide. And I said, well.

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Mark Kushner: we create just a ton of hydrogen peroxide, because you create a bunch of OH radicals in our system. I bet you you could just try this with one of our systems. I have this student, his name's Daniel Martin. She's like, oh, I'm really good friends with Daniel Martin. He's actually waiting in the hall for me right now, because we're going to, like, go get coffee. And I said, well, go talk to him.

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Mark Kushner: And let's see if this works. And so, Daniel packed up our equipment, brought it over to their lab.

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Mark Kushner: I wasn't allowed… allowing uranium in my lab. And sure enough.

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Mark Kushner: We just set up a simple reactor. Pre-treatment, the urineal looks this yellow. Post-treatment, you get this cloudy, turbid, powdery form that occurs. We do have a patent on this,

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Mark Kushner: process that has a funny story itself, but then we're able to just put aesthetic reference into an IR spectrometer, look at our

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Mark Kushner: generated after we dry this out and extract it, and it's virtually identical. We are producing studite. Okay, here's the sidebar, funny story in the patent pending. We file a patent on this, we're really excited. We get this crazy email about a year ago, a year and a half ago, saying, this patent application has been flagged by the U.S. Department of Defense.

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Mark Kushner: You no longer own this technology, stop working on it.

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Mark Kushner: And, and so we tried, through a variety of back channels to get an explanation. No one would give us an explanation. And then 6 months later, we got another email saying, we are now releasing this technology back to you.

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Mark Kushner: No idea what happened. So we paused this project for a while, let's put it that way.

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Mark Kushner: We did some interesting studies where you could, we would measure the amount of uranium removed,

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Mark Kushner: Over time. So, with greater exposure, we could remove almost 100% of that, uranium.

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Mark Kushner: And depending on the initial concentration, that removal efficiency could be even above 100% sometimes, which is… we are defined basing purely on the current, so that means that you're getting greater than Faraday efficiency.

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Mark Kushner: So, my student came to me, and he said, I wonder if there's, like.

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Mark Kushner: if this is transport limited or kinetically limited, and what sort of predicts this behavior based on the initial concentration of urinal in our system. So we came up with, essentially, a kinetically limited model, similar to the one I just showed you, that depends on the amount of OH we have in our system.

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Mark Kushner: and the amount of our initial concentration, and he said, well, we know that we get about 2OH per ion.

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Mark Kushner: So, let's assume we get 2 OH per ion. I know that value now.

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Mark Kushner: And…

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Mark Kushner: So, there's a lot more data, but I'm showing you this clean version. I encourage you to read the SI, because this is all explained in detail, but we actually get a pretty good fit, even with the additional data, that suggests we're probably kinetically limited and not transport limited in this particular instance.

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Mark Kushner: So…

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Mark Kushner: That's the end… first half of my story, okay? Plasma electrochemistry, I showed you some interesting plasma physics with the new configuration, how we believe we're getting solvated electrons, and then,

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Mark Kushner: a potential application relevant to goods production in the United States. So my next one…

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Mark Kushner: It's going to be plasma-driven materials processing.

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Mark Kushner: Okay, this started… oh, and this project is going to look at, plasma surrounding aerosol droplets, so an aerosol-plasma interaction. And this project started in a completely different way.

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Mark Kushner: And so, my collaborators came to me, or they weren't collaborators at the time, my friends came to me, colleagues, and they wanted to look at making thermoelectrics. And the way you typically make a thermoelectric material, which converts heat to electricity, or vice versa.

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Mark Kushner: is you have some bulk material, you make an ingot, you metalize it, you cut it, you create a model assembly, and you create a device. You can buy those devices

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Mark Kushner: Online for a couple of bucks, okay? Thermoelectric generators and refrigerators. But, they're super rigid, there's a lot of wasted material, it's not a very efficient process.

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Mark Kushner: my colleague Yan Leon Zhang actually has a printing process that he can print aerosol inks.

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Mark Kushner: with nanomaterials in them, so you put thermoelectric nanomaterials in it, print… make it into an ink, you print it, you can do conformal printing, there's no waste, you can print all kinds of devices. He's put it into… he's printed on components of cars, nuclear reactors, all kinds of cool stuff.

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Mark Kushner: So we wanted to look at this whole process, see if we can optimize it. There's a really important post-processing step, and it is after you print the ink.

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Mark Kushner: you actually need to centrate. You need to take those nanoparticles in the ink, remove some of that ink, the surfactants, the organics, so those nanoparticles now form a cohesive network and are conductive, okay, to form your thermoelectric material. And the typical way you would do that

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Mark Kushner: Is you take your ink, you would print it on whatever your surface is, and then you'd put it into an oven for, like, 400… 400 degrees for, like, 3 hours.

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Mark Kushner: Right? And I purposely show an orange here, because if you put an orange in a 400 degrees C oven for a few hours, it's not going to look like that anymore, right? So it really limits the materials you could do. You have to do high temperature materials.

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Mark Kushner: Plasmas are a low temperature, you can touch them. Maybe we could do this in a low temperature way. And so that was the question we asked. It turns out other people had kind of looked at it at moderate temperatures, this is not moderate, 700, but to about 70 degrees C. We thought we could do even better, okay?

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Mark Kushner: And so we created a plasma jet sintering. This is a DBD plasma jet internal electrode, and then a surrounding external electrode, and it just impinges on a surface. Typically, we use argon, we can use helium, we can use nitrogen. You don't want oxygen because you don't want to oxidize it.

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Mark Kushner: Okay? We print our device, we mount it, and we hit it with our plasma jet, and we can measure the surface temperature from the back. We make these super thin using an IR camera.

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Mark Kushner: Okay?

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Mark Kushner: And what we did was we pulsed the plasma, not in sort of a nanosecond, but literally on a hertz, almost, scale, right? Or 0.1 Hz. So you turn it on, because it'll kind of heat the surface, and then you turn it off, the surface cools, turn it on, off, on, off, every 30 seconds.

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Mark Kushner: And the surface doesn't get above 28 degrees C, right? So you can still touch that. It's much less than 70 degrees C. Okay? And sure enough.

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Mark Kushner: we got sintering. So what I'm showing here is the sheet resistance, or in the open symbols, the conductivity, and over time, it gets more conductive or less resistant. And there seems to be

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Mark Kushner: two phases that are going on. One phase is in the first 20 minutes, you're removing parts of the ink itself, right? So you're getting rid of surfactants, organic materials, and all those things.

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Mark Kushner: In the second phase, you're actually promoting surface diffusion that densifies the material. So, this is what it looks like uncentered from an SEM, and then as we center it.

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Mark Kushner: you see densification. Now, this was all done with silver, not thermoelectric materials, but the basic principle was proved, okay?

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Mark Kushner: We also proved we could actually do it at low temperatures. So this is sintering on a tomato.

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Mark Kushner: We centered on a piece of plastic that was sitting on someone's arm, and against my permission, they actually centered on their finger, right? So they just printed a line in their finger and centered it, okay? So we can center really low temperature.

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Mark Kushner: I'm not going to go into it, but for those of you who have been doing plasma electro… plasma chemistry, in general, we did follow this up with a fun study to show that it actually follows

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Mark Kushner: An Arrhenius but non-arerhenius-like relationship with the specific energy input, or the amount of energy per molecule that the atom… that plasma puts into the gas flow.

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Mark Kushner: And so you get this non-rhenous relationship. You also see that in areas of plasma catalysis and polymerization as well.

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Mark Kushner: Now…

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Mark Kushner: What we really wanted to know is, do we need to do this after the fact? Could we do this while we were printing? It's a long process, even after the fact, and it turns out that there was a paper, a few years ago where they had this sort of basic idea, and they started a company called Space Foundry. So what we did was we actually went and bought a Space Foundry printer.

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Mark Kushner: Does anyone here work for Space Foundry?

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Mark Kushner: Anyone, like, have connections to Space Foundry?

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Mark Kushner: All right, the printer doesn't work. Our collaborators at Idaho National Lab bought it, my graduate student spent, like, 6 months there and came back and was like, we tried everything. The inks don't work, the plasma's unstable. It's just the way they were doing it didn't make any sense. So he said, maybe we can do it better.

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Mark Kushner: Our first way was to just say, hey, we have our plasma jet, we have our inkjet, let's just

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Mark Kushner: do them right at the same time. So what you see here, that's the tip of the ink, it's depositing silver in a glass slide. You see our plasma coming in. What's really cool is there's a fundamental phenomenon going on here, is our plasma isn't really right on top of the ink.

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Mark Kushner: itself, it's kind of offset, but because it's a dielectric, it forms a surface ionization wave and spreads, and sure enough, we could get sintering that way, okay?

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Mark Kushner: That's the first generation, but we wanted to do… first of all, it's complex, we're limited by, some of the materials we had in the lab, so we came up with the second generation, which we call Cojet printing. This is also patent pending.

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Mark Kushner: Where our aerosol jet, we have an aerosolizer that creates a gas flow, and we have the sheath gas that focuses it. So you actually have two gas flows.

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Mark Kushner: Then we add a third gas flow for a plasma working gas, embedded electrodes inside, they form a plasma not in contact with the aerosol, but it all propagates out, and now you're actually interacting with the aerosol. You actually start the evaporation process, you start everything.

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Mark Kushner: While in contact, and then it gets deposited, and you're also processing during the deposition itself.

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Mark Kushner: Does that work?

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Mark Kushner: Well, it would be really weird if I showed you it and it doesn't work. The answer is yes. This is what it looks like live.

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Mark Kushner: So again, you see an ink line that we're just depositing more and more ink on right now, with the plasma active.

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Mark Kushner: And sure enough, you get improvements. We did a wide variety of studies.

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Mark Kushner: That showed that we could get sintering, so we got to high conductivity, okay? Has a function of our pulse voltage for our plasma, has a function for our carrier gas flow rate, so the gas that carries the aerosols. The plasma flow rate could affect the width and the resolution. These aren't great, we've gotten down to 50 microns, but the point is that it worked.

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Mark Kushner: One thing that we observed, though, is that if we put more energy in the plasma, it worked better, but at some point, the plasma would start to filament.

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Mark Kushner: And if we decrease the gas flow.

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Mark Kushner: At some point, the printed material would start to crack.

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Mark Kushner: And if any of you have worked with plasmas, whenever they see a crack, or an edge, or anything sharp, they're like, that's where I want to go. So then the plasma would be attracted to the crack, form a filament, and it would kind of destroy things. So he said, hey, maybe there's a smarter way we can do this.

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Mark Kushner: So that… there's an example of the plasma jet, and then we see the plasma being attracted to cracks it's not even in contact with. So you're getting non-adjacent plasma formation.

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Mark Kushner: And we decided to add a camera and do a little machine learning

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Mark Kushner: algorithm, where what we would do is it would look at an image and assess, and I'm not a machine learning expert, so don't ask me.

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Mark Kushner: any questions, not even easy ones. If it started to see a plasma filament form, and if it did, it would reduce the voltage, okay? If it didn't, it would say, maybe I can increase the voltage a little bit and get better efficiency.

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Mark Kushner: Okay?

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Mark Kushner: If it started to see a crack form, it would actually turn off the plasma and just deposit ink. So you'd have, like, a plasma layer, a non-plasma layer, a plasma layer, a plasma layer, plasma layer, a non-plasma layer. We call that a repair.

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Mark Kushner: Okay? And what we saw is that without having this direct compensation while the printing, we got a lot of cracks and filaments, and only a small incidence count that actually were good, and then we basically got completely away

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Mark Kushner: from any filament formation and almost all crack formation, and we didn't sacrifice total performance in any way. We still got essentially the same

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Mark Kushner: outcome, the same connectivity. So, we've done this for silver now, we've expanded it to other materials, golds, and things like that.

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Mark Kushner: I alluded to the fact that this is… can do really low temperature, so… here we are. This is… I don't know why it's a boat, but we 3D printed a boat and printed right on that. This is a rubber, this is gelatin. Actually, it's a gummy bear, right? And then this is my favorite, a leaf.

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Mark Kushner: And in fact, we… we… Printed it on a live plant.

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Mark Kushner: And so what you see here is this printing, essentially interdigited electrode. It looks a little bit different, but an interdigit electrode on the leaf, and we can use it as an inductance center. And so we run a current through it, and what it can do is tell us some state of the plant itself.

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Mark Kushner: And so…

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Mark Kushner: That's what it looks like, this sort of interdigited electrode, and the current through the leaf is what your signal is, and what we found is that when the leaf dries out, the impedance goes down. If we turn on lighting.

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Mark Kushner: to produce photosynthesis, we see it come up. If it's fully hydrated, so if it's not…

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Mark Kushner: dry, you actually get this really nice signal as well. So now we have an in situ hydration sensor that we printed. It took only a few minutes. We could… we, used an argon helium instead of an argon-argon plasma.

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Mark Kushner: But that's… that's besides the point. The cool thing was that we're able to do this, and you can imagine…

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Mark Kushner: well, maybe we don't need a bunch of plant sensors, but other sensors he can have. So we actually have paper being presented next month on an actual, sensor for human, monitoring as well.

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Mark Kushner: And so, that is another story where we have taken a plasma-liquid interaction, this time an aerosol, and showed some way it could affect goods production. In this case, low-temperature sensors and materials. So, with that, I just want to wrap up my time here.

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Mark Kushner: I want to make sure I thank a lot of people who, worked on all this. On the plasma electrochemistry is Paul Rumbach and Dave Bartels, Juan Sankrin, who some of you probably know from Illinois, Daniel Elg, Hernan Delgado, Daniel Martin, and Hong Nun, and then on the plasma-driven material processing, the main collaborators are Yan Leon Zhang, Nasla Turan did all the cinturing work with Mortazis A.D. Yaivish.

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Mark Kushner: And then Jinu Yong and Yipudu did all the work on the printer, and of course, I thank my sponsors for supporting this work. With that, I'm happy to take questions.

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Mark Kushner: Thank you very much. Are there questions?

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Mark Kushner: Good to see you, yeah, thank you. Good to see you. Fantastic talk. Yeah, so in a patron's curve, measurement,

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Mark Kushner: what we will get if you sweep the polarity? That's a great question.

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Mark Kushner: So when you switch the polarity, right, the emitting electrode is no longer the liquid, it's the solid electrode at top. We know just from experience, although we haven't actually done a direct comparison, it's easier to form a plasma in that configuration.

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Mark Kushner: So my guess is you'd get something that looks more similar to a typical two-metal electrode type passions curve, but we would have to do the measurement.

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Mark Kushner: Because, at the…

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Mark Kushner: Because if you have a needle that has come close to a flat surface, if it's sufficiently close, it will also be able to induce a mild fuel enhancement. Yeah, yeah, yeah, yeah, certainly. Yeah, but we're a millimeter away, so a millimeter to two millimeters, so we're not close enough that you actually get anomalous behavior due to the gap spacing.

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Mark Kushner: Yes.

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Mark Kushner: Thanks for the, presentation. I have a couple of questions. First one is, was there any radiation safety concern during the uranium attraction experiment? So,

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Mark Kushner: Yes, we are fortunate at the University of Notre Dame to have a radiation lab that's sponsored by the Department of Energy. We have a variety of gamma and beta sources, and so they have the correct facilities to deal with any

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Mark Kushner: uranium and other actinide radiating materials, and so that's where we did all that work. I just made sure that we did it… did all the safety training, and then allowed my collaborators who work with the materials to be the ones who are

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Mark Kushner: really doing the work. Yeah. Thank you. My second question is.

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Mark Kushner: So for the plasma jet printing, you said at the sheath, sheath gas did improve the resolution. I wanted to know, is there, like, a ratio of a mixture makes a difference?

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Mark Kushner: Mixture of what? The two gases? Yes. Yeah, absolutely. The gas composition and the relative flow rates all influence

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Mark Kushner: both your operating regime, so where it actually successfully prints, and also your performance in terms of what the print looks like in terms of resolution, conductivity or efficacy, if you want to think about it that way. And so that took… we took a very Edisonian approach.

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Mark Kushner: Where we were… we tried…

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Mark Kushner: argon and nitrogen, nitrogen and argon, argon and helium, argon and argon, and all these different configurations. We tried different flow rates, and it was a lot of just trial and error to get to, what we show in the paper, the conditions that really were the most repeatable.

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Mark Kushner: Yeah. Thank you.

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Mark Kushner: Scott.

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Mark Kushner: So, in the applications where you're, like, solubleating electrons, what's the main…

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Mark Kushner: figure of marriage or anything you want to improve. Is it, like, the current of electrons you can test in the circuit? Yeah. So, that's a great question. What do we want to prove… improve? And I think there's different ways of looking at it, right?

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Mark Kushner: I'll tell you what we do, and then I'll tell you other perspectives. What we primarily focus on is Faraday efficiency, which is the charge efficiency. So for every

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Mark Kushner: charge that were created in the circuit, does it actually produce a chemical outcome that we desire, or not produce a chemical outcome we desire? And in conventional electrochemistry.

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Mark Kushner: they often use the Faraday efficiency as a good measure, because there's side reactions, typically water electrolysis, or hydrogen-oxygen evolution, that inhibit performance. And so, when,

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Mark Kushner: people have been looking very closely, for example, upgrading CO2 electrochemically to things like methanol or oxalate. There was a techno-economic study done maybe a decade ago that showed that you need to be above the

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Mark Kushner: something like 70% Faraday efficiency, and at the time, no one really had a system that was that high, but it needed to be there to have any economic viability. And so, we've always used that as our example. Now, the thing is, that's not energy efficiency.

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Mark Kushner: That's not the efficiency in terms of the amount of energy you get in versus either the energy content that you get out of the chemicals you produce, or the energy of the transition, or whatever that. And our energy efficiency is actually quite low.

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Mark Kushner: At one point in time, we estimated it, it was, like, less than a percent. And… and so from a…

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Mark Kushner: if you're looking to have a more energy-efficient process, that's not a great figure of merit. The other one that we look at.

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Mark Kushner: other than that is just selectivity. So if you have potential multiple different products, the question you ask is, how do I make sure I have the majority of one product versus other products? And it can be interpreted as an efficiency, but…

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Mark Kushner: selectivity is a catalysis term that we've also kind of adopted. Does that make sense? Sure.

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Mark Kushner: I think there's a question over here, yeah. Great talk. So, I was wondering, like, when you calculate how many solid electrons and hydroxyl water costs you produce per ion input, I kind of assume that you have, like, first-order reaction mechanism? Yeah.

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Mark Kushner: And how confident you are about that assumption, and if that affects, like, the calculation at all? I mean, we're confident it's first order.

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Mark Kushner: Yeah, yeah,

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Mark Kushner: what we're not confident in is whether there's other things happening. Okay. So, and I'll give you something very specific on that… that front, and I'm not going to take it out of presenting mode, because I think it might mess up the sharing, but…

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Mark Kushner: One of the things that I said when we were trying to measure that solvated electron in our system was that you also produce a lot of hydroxyl radicals, right?

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Mark Kushner: And, and those hydroxyl radicals

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Mark Kushner: react directly with the solvated electron, so if you can remove the hydroxyl radicals, you should get more solvated electrons, right? Well, the thing is, you put methanol in to react with the hydroxyl radicals, that will form some other organic

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Mark Kushner: That could also react With solvated electrons, or react with something else.

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Mark Kushner: And… and produce something else that reacts to solvated electrons. So the reaction chemistry gets really hard. So it's less about confidence, about whether the reactions we've identified are first order. They're well known.

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Mark Kushner: The question is, have we identified all the reactions, and the ones we haven't… that we identified that we don't know the reaction rate constants? Because that is what really dictates

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Mark Kushner: Where the reaction propagates.

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Mark Kushner: Thank you.

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Mark Kushner: Dara, I think Daniel Peters has a question online? I think we have.

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Daniel Peters: Yes, am I audible?

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Mark Kushner: Daniel, why don't you unmute yourself and ask your question, or put it in the chat?

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Daniel Peters: Am I audible now?

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Daniel Peters: Can anyone hear me?

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Mark Kushner: Can we make it a little louder?

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Mark Kushner: Yes, I…

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Daniel Peters: Great. Question on slide 8.

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Daniel Peters: On slide 18.

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Daniel Peters: there's the… the… solvoid electrons? How long do they stay in solution after the plasma is turned off?

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Mark Kushner: Yeah, so that's a great question. In this, in this configuration.

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Mark Kushner: If you have nothing else in your water, if it's completely neat water, maybe some salt that we know doesn't react with sulfated electrons.

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Mark Kushner: What dictates the lifetime of the solvator electron is a second-order recombination reaction with water itself.

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Mark Kushner: Let's see, I'm trying to remember the order of my slides… It's…

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Mark Kushner: this reaction right here, okay. So two electrons and two waters basically come together. It's a real diffusion-limited process, to form two OH- and then hydrogen gas. So it's basically a water electrolysis.

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Mark Kushner: Turns out that this reaction, which is second order, so it goes as the concentration squared, has a really, really high rate constant. One of the highest rate constants out there. And so, at the typical currents that we work, the lifetime is way less than a microsecond.

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Mark Kushner: Now, if you go to lower currents.

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Mark Kushner: you have less concentration of electrons, so then that quadratic term, that concentration squared term in the reaction rate, gets smaller, so your lifetime might be a little bit longer.

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Mark Kushner: We typically don't think of it in terms of duration, we think of it in terms of penetration, which is the electrons

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Mark Kushner: Once they enter, can start to fusing into the solution.

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Mark Kushner: And so, we can predict how far they diffuse based on this reaction right here, and it's a strong function of the current density, but you get upwards between, in water, 10 to 100 nanometers of penetration, depending on conditions.

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Daniel Peters: Okay, thank you.

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Mark Kushner: Yep.

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Mark Kushner: Any last questions? Well, two last questions there, Rebecca. So, kind of a hybrid of the questions, on uranium and measure of merit.

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Mark Kushner: what would be the advantage of processing uranium this way, as opposed to trucking in a bunch of hydrogen products? Yeah, it's exactly what you just said.

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Mark Kushner: you avoid trucking and a lot of hydrogen peroxide. And so, the idea is that you're not… you have no other additives, you can do it in air, we've done it in air, argon, pure oxygen, all with different efficacies, but they all work.

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Mark Kushner: And you avoid, basically, the off-site production of hydrogen peroxide, which I actually learned today from one of your colleagues, Brian, involves a lot of, dangerous components itself.

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Mark Kushner: but then the transport, because hydrogen peroxide is a contaminant or a pollutant itself. So you kind of avoid that. Now, we've done some back-of-the-envelope calculations to figure out whether this has got really the techno-economic

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Mark Kushner: potential that we think. The first calculation suggested it not, but I think they made some bad assumptions, so I went in and redid them. And I think you're within

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Mark Kushner: an order of magnitude right now, with zero optimization. If we just found components off the shelf that people have done, you could probably get within an order of magnitude of being cost-effective. Obviously, with optimization, you might get there faster.

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Mark Kushner: I'm curious about the machine learning aspect of printing. I know! I know, but I know you can answer my question. Okay.

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Mark Kushner: The first one is that I think I understand that you attach a camera to the combination of nozzles, and you're using purely visual inspection to identify both filaments and cracks in the film as it's deposited? Yes, that is correct. Correct, thank you. The next follow-on to that is, did you consider adding other diagnostic tools to

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Mark Kushner: that ensemble, in order to understand, for example, plasma chemistry, or temperature, or other factors that may play a role in the process? We did. We considered a few things.

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Mark Kushner: And we… We kind of…

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Mark Kushner: dismiss them in different ways. So, we thought OES for a while. To get the signals we want, you need to acquire over…

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Mark Kushner: a sufficiently long time that we didn't think, we could actually affect the printing process. So there was a time, mismatch there, whereas a camera's at 30 frames per second, right? Like, you get a lot of information quickly.

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Mark Kushner: The thing we really wanted to do was an in-situ connectivity measurement, because that would tell us… so we tried different ways of setting it up, and basically, any time we tried to add those additional electric fields.

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Mark Kushner: it messed up the plasma, and so the plasma then would be attracted to it. And it took us a while to figure that out. It even took us a while just to figure out how to properly isolate the substrate we were printing on from the surroundings to avoid issues with unintentional grounding, and therefore unintentional

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Mark Kushner: conduction pathways, so…

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Mark Kushner: Thank you. One last one, which may be in the machine learning side. Okay. Materials and plasma side, but how many test cases did you have to feed the model before you saw the improvement?

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Mark Kushner: So we train the model on, on well over a thousand images. Now,

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Mark Kushner: in terms of the number of cases, I would actually have to go ask my postdoc to figure out the number of ones. Yeah, and we have another machine learning project right now that, we've had to train it on around 9,000 images to get what we want, but on 9,000 of training images, it's able to analyze another 4,000

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Mark Kushner: pretty well. Like, we're pretty confident in those results. The problem is that we had to obtain 13,000 images to do it, which, luckily there's an undergrad that

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Mark Kushner: Needed some summer pay. With that, David, thank you very much for one of the… Thank you.