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State of aggregation: Plasma
Dr. Hendrik Kersten / Physical and Theoretical Chemistry
Photo: Private

The order of matter in shape and volume stability

Around 100 years ago, the fourth aggregate state was discovered: plasma
A 100 years ago interview with natural scientist Dr. Hendrik Kersten

What is an aggregate state?

Kersten: We will build a snowman together to answer this question. But first we will have a short crash course on matter. Now onto the crash course:
The individual atomic and molecular building blocks of matter are in constant motion. The warmer they are, the more kinetic energy they have. On the other hand, there is a certain binding strength of these individual particles among each other. Depending on the type of particle, they sometimes stick together more and sometimes less well. Precisely this relationship between kinetic energy and binding strength ultimately determines the form in which these many individual particles can appear as a coherent "something". Due to this ratio, many, many individual particles form a unit perceptible for us with its characteristic properties. These properties only occur through the collective behaviour. Every particle on its own does not possess these properties.  

1st state: We need lots and lots of water molecules to build a snowman. Their ratio of kinetic energy and bond strength must be well matched. Let us start with a small snowball (about 1,000,000,000,000,000,000 - one quadrillion particles) and roll it over the snow-covered ground. For it to become a proper sphere for the base and to maintain the shape we have given it, even in the face of gravity, the matter must 1. be as cold as possible (i.e., have little kinetic energy) and 2. have good bonding strength among the water molecules. The finished snowman, with a height of 1.5 m, a burly belly and a roughly estimated weight of about 40 kg, then consists of proud 1,300,000,000,000,000,000,000 (1.3 quadrillions) water molecules. It is interesting to note that the microscopic spatial arrangement of a water molecule with respect to its neighbors, is, in terms of distance and orientation, (approximately) the same throughout the snowman and very characteristic of this relationship of kinetic energy to bond strength. If the weather conditions (temperature, humidity and pressure) remain reasonably constant, these many, many water molecules also retain their unity in the form of the microscopic arrangement. Thus, they ultimately maintain the 1st state of our snowman.

2nd condition: The air molecules move faster and hit the surface of the snowman with more energy, when it gets warmer outside. As a result, the water molecules of the snowman begin to move faster. At some point, they wobble back and forth so much so that the kinetic energy becomes greater than the bond strength that had provided this particular spatial arrangement of the molecules in the 1st state. This does not mean that the water molecules suddenly no longer adhere to each other. Only the unique, spatial arrangement of the particles among each other, which led to the stability of the total work of art "snowman", can now no longer exist due to the new ratio of kinetic energy to bond strength. The 1.3 quadrillions of water molecules now form a new microscopic arrangement, with a different distance and significantly more flexible orientation to its respective neighbors. As a whole, these molecules form a new, 2nd state, with different properties. Together, they are now no longer dimensionally stable, but form a more fluid entity.

3rd state: At this point, I have underestimated a little bit. It is not quite true that all 1.3 quadrillion water molecules really change into this new state. Not all particles move at the same speed at a given temperature. A few are slower and a few are faster than average. Fortunately, the size of the fraction of molecules with a given speed at a given temperature follows a well known law, so we have that fully under control. Some are even so fast that the bond strength to its neighbors is no longer sufficient to keep that molecule in the overall bond. It simply flies up and away as a single particle. If this pile of water molecules gets even warmer, more and more particles will be able to overcome the binding strength of their neighbors and fly away as single particles until the entire 1.3 quadrillion particles are only present as single particles in space. The snowman has then dissolved into "air", so to speak. The particles still exist, of course, they are just no longer directly connected. Also this new and common 3rd state of these 1.3 quadrillion particles has certain properties. Since the distance of the water molecules among eachother has become substantially larger it clearly takes more space. Another reason for that is that is has no own form anymore, but simply fills the form evenly which one allocates to it.

As we have seen by now, the snowman can exist in three states, which are convertible into each other. They are also reversible by changing the temperature and thus the ratio of kinetic energy to bond strength. These characteristic properties of the three described states of the snowman have also been observed in many other materials by simply changing the temperature. Humans like to think in categories with criteria of properties in order to be able to classify things in the world around them accordingly. In order to structure the complexity of the world so that we have a hint of a chance to understand something. That is what happened here. At first, we take an etymological view on the term: "aggregate" in Latin denotes something like "accumulation". Correspondingly, the "state of aggregation" is a "characteristic state of matter that denotes the specific properties that result from the accumulation of many particles". The criteria for these three classical states are banal and easy to determine for everyone: We simply classify matter according to whether it shows shape and volume stability. That is it. The first state of the snowman was both shape and volume stable and is commonly known as the solid state of matter. The second state is the liquid. This retains its volume, but because of its flowing properties, it has no shape retention. The third state is the gaseous. Here, the particles do not retain their volume as a whole. They evenly distribute themselves in any space that is made available to them. The particules can neither form a collective shape that could be perceived as an independent object.  

About 100 years ago, the fourth physical aggregate state was discovered. What is this about?

Kersten: The question is whether it was discovered or simply defined. This state of matter is pretty cool. Under the conditions here on earth, it does not occur naturally, but somehow, has to be "forced" every time. It is funny that in the rest of the universe about 99% of the matter exists quite naturally exactly in this state of matter. The best example is our sun. The characteristic of this state of matter is the stable coexistence of positive and negative charge in the gas phase. This means that some electrons could somehow detach from the atoms and are now flying freely and peacefully through the space next to the positive residual atoms. So how can it happen that electrons detach from the atom? Justifiably, the answer to this places this state of matter exactly in the order as the fourth state of matter. Not between one and two, or two and three, i.e. as a supplement due to a chronological discovery faux pas, but from purely physical logic. At the example of the snowman we just have seen that we could reach the aggregate state one, then two, then three by simple an increase of the ratio of kinetic energy to bond strength. The movement of the particles increased bit by bit, so that the binding strength gradually was not sufficient for the cohesion of the particles among each other anylonger. Now, we simply continue this consequently. From the physical point of view, the forces which hold electrons in the atom are of the same nature as the forces which held the snowman together...only a little stronger. We simply have to make the gas phase even warmer to get from the third aggregate state, the gas phase, to the fourth aggregate state. So warm that the kinetic energy becomes such intense that even the bonding strength of a few electrons in the atoms is no longer sufficient to keep them in the atoms. These few electrons now separate from the atoms as free, separate particles, like as the individual water molecules previously separated from each other during the transition from the liquid phase to the gas phase.

However, as just mentioned,quite a lot of heat, i.e. kinetic heat is necessary for this electron separation of the atoms. On earth, we do not have any naturally occurring sources for this, but we have to put in a lot of work to bring about this state through targeted energy supply. A good possibility for this is the application of electric fields, in which existing charge carriers get proper kinetic energy in the gas phase. But for the reasons mentioned above, this state of matter is not easily found on the street or in the forest, whereas the other three states are ubiquitous. However, we can constantly admire this state and enjoy its great properties, when we look up to the sun in the sky during the day, because the sun is hot enough. Hot enough for its radiation to completely convert even our 1.3 quadrillion water molecules piled up into a beloved snowman into the gas phase.

Now, we still have to cover the naming. Instead of simply numbering the states, they have been given great names, so the fourth aggregate state is called plasma. So we have: solid, liquid, gas and plasma. If you want to be very picky, you could make even smaller distinctions in these individual states, then we speak of so-called "phases". But we do not want to be picky now.  

In the media, both Fritz Winkler and Irving Langmuir are named as the discoverers. Which chemist was the real discoverer?

Kersten: Right, someone must have given the name to the child. The plasma community agrees that the term plasma (as just described for the forth aggregate state) was introduced by Irving Langmuir in his 1928 publication "Oscillations in ionized gases". I cannot answer exactly, but only assume how Mr. Winkler could appear in this context on some web pages. At the beginning to the middle of the last century Winkler was successfully engaged in the development of large-scale chemical plants at BASF. He developed a reactor that converts the solid coal into particles as tiny as possible and swirls them with the gas phase for the most efficient chemical conversion of coal with gases. They thus create a large surface area and the longest possible reaction times. This forms an aerosol, a mixture of solid phase and gas phase, which can possibly lead to a unit of matter with very characteristic properties due to the special bond strengths between the particles and the gas molecules and the special conditions in the reactor. This could actually be called an aggregate state according to the above crash course on matter, as the title of Paul Feiler's 1972 publication "The Fluidized Bed: A New Aggregate State" suggests. However, I dare to doubt that this is the same state according to the above definition (separation of positive/negative charge in the gas phase), for that I would have to have a closer look at the publication. I suspect that in the compilation of the web pages on which Mr. Winkler is mentioned as the originator of the plasma term, something was mixed up and the fourth aggregate state, according to the abovementioned definition,  was confused with the fluidized bed from the mentioned publication by Feiler. Unfortunately the references are very poor on these web pages.

What are the new, different physical properties of plasma?

Kersten: That is really exciting. Now, you could say, okay, instead of only neutral particles, there are also some charged particles in the gas phase, so what? What makes this fact so special? One must emphasize quite clearly, only because a few charged particles possibly exist in the air (and in fact they constantly do so in small concentrations, conditioned by the natural radioactivity and cosmic background radiation) we does not immediately has to be plasma. Just as in the classical three aggregate states the particles show a very characteristic collective behavior, e.g. 1.3 quadrillions of water molecules flow coherently over the ground. One only speaks of a plasma when the concentration of charged particles, their energy and their average distance are so perfectly matched that a collective behavior becomes noticeable. There are criteria for classification in the plasma, just as the collective stability of shape and volume were criteria of classification in the other three states, although these are not quite as easy to identify. One criterion, for example, is that the concentration of free electrons and positive charges in the piece of matter must reach a certain value and be nearly the same for both varieties. If this is fulfilled and now takes a look from the outside at this space filled with matter, it appears as "quasi" neutral. So although there are so many free charge carriers flying around, the forces of the charges cancel each other out when viewed over the entire space and the plasma appears to the outside as almost "not" charged. You might think now that this is stupid, what is the point if it is neutral to the outside again? But the cool thing is that this "structure" can now appear as one unit. Let us imagine that we would be able to deflect the light electrons in this gas to the upper right corner for a short time. The much, much heavier positive ions are too inert to follow them quickly. What have we done now? Suddenly, we have a large amount of charges separated spatially from each other. We have thus initiated incredibly strong restoring forces in the gas. The negative cloud wants to retrieve those with a proper swing, because positive and negative attract each other. In the end, the electron cloud starts to swing back and forth around the positive ions. It is very comparable to a pendulum that has been deflected and allowed to swing. It is also said that the plasma has a natural frequency, namely the number of times this electron cloud swings back and forth per second. Now, this is not only a theoretical consideration, but happens about 100 km above our heads in the ionosphere in reality. As the name suggests, there are a lot of free charge carriers in this layer around our planet. The hard radiation of the sun ionizes the molecules (mainly oxygen and nitrogen) that are located there. This layer is a type of protection layer for life on this earth, because if the hard radiation would not be intercepted, not only the snowman would have a problem. This ionosphere fulfills the plasma criteria relatively well. It shows the expected collective behavior, e.g. the plasma oscillations. This has strong implications for radio communications: electromagnetic radiation in the radio frequency range (MHz) is able to interact with the oscillating charge clouds of the ionosphere. This manifests itself in the fact that, e.g. the short wave can be reflected at the ionosphere back to earth particularly well, and can thus bridge long distances as a space wave.

I would like to emphasize one characteristic of most plasmas at this point. The glow. When you type "plasma" in the Google image search you will see a bunch of colorful and glowing photos. It always pleases me to see them even though I have seen a lot of plasmas and sometimes these glowing phenomena are actually secondary to the actual application.  

For which areas is plasma research important?

Kersten: Oh, for many. There are a lot of very reactive species in the plasmas, due to the overall high energy state of plasmas. That has great technical significance. However, it is not easy to say to the plasma "please make me such and such reactive species." One likes to use plasmas to modify surfaces in a very specific way especially in semiconductor technology. Either to deposit thin layers or to ablate thin layers. Contradictory? You see, a plasma can do both, and it is an important area of research to manipulate this state of matter so that it does either one or the other, or something completely different. There are also many current research areas that want to take advantage of the unique reactive species from plasmas to develop new synthetic pathways and thus, for example, new materials. Recently, I had the privilege of being a reviewer for a BMBF process involving research on the large-scale conversion of CO2 into fuels using plasma reactors. The idea of recycling climate-damaging gases into reusable fuels. Another example goes in the direction of energy research. An old dream to match the sun and obtain usable energy through nuclear fusion. Fusing hydrogen nuclei into helium is, unlike coal and nuclear fission, a relatively clean process. Hydrogen in, helium out. However, to get atomic nuclei close enough to actually fuse, you really have to collide them with a lot of kinetic energy. And where could you get that much kinetic energy? Exactly, from plasmas, so here, too, there is still a great need for research.
As I said, this is a highly topical, exciting state of aggregation in which there is still much to discover.

We are surrounded by plasma states even in our everyday world. What are they, for example?

Kersten: I had just omitted a natural, on the earth observable plasma phenomenon, and these are thunderbolts. One finds further, but then artificially produced plasmas, e.g. in fluorescent tubes and other plasma lamps in any forms and colors, plasma screens (although these were replaced in the meantime in their performance by LED screens), with welding, plasma cutting, or there are also these funny plasma lighters for lighting candles, and many, many more.
One of the largest research projects worldwidde, the new international accelerator center FAIR is currently being built in Darmstadt. With FAIR, matter will be created and researched in the laboratory in a way that otherwise only occurs in the universe. Scientists from all over the world expect to gain new insights into the structure of matter and the evolution of the universe, from the Big Bang to the present day. At the same time, the experiments made possible by FAIR open up the prospect of exploring the physical principles of so-called inertial fusion. Some scientists see this as the future of energy supply for mankind. In this process, small hydrogen capsules are to be compressed by bombardment with heavy ion beams in such a way that the fusion process to helium is set in motion, releasing usable energy. Is this a serious alternative in energy supply?

Kersten: It is a fact that nuclear fusion releases a lot of energy even with a small amount of starting material. Remember the sun and the explanations just given about plasma research. Sad fact is also, that this was already shown on earth with the hydrogen bomb very impressively. The difficulty lies in the controlled dosage to use this energy. So it is not a physical impossibility, but "only" a technical hurdle, which is still a hard nut to crack. This possibility of a usable energy source is to be taken seriously in my eyes, but maybe not tomorrow yet, and also not next week. Now, I am not a real "expert" on nuclear fusion details, this is clearly beyond our laboratory capacities. We have already ignited and examined plasmas with our lasers in the laboratory. But, for the necessary energy input for the initiation of nuclear fusion this was by far not enough... otherwise one would have heard about it already in the news.

Uwe Blass (Interview on January 21, 2021)

Dr. Hendrik Kersten studied Food Chemistry (not to the end) and Chemistry at the University of Bonn and at the University of Wuppertal and received his PhD in 2011 in the field of Mass Spectrometric Analytical Methods for Atmospheric Chemical Processes (Wuppertal and Kelowna/Canada). Since 2011, he has been working in Physical and Theoretical Chemistry in the Department of Chemistry and Biology in the Faculty of Mathematics and Natural Sciences. His research areas include many topics related to Mass Spectrometry and the Characterization and Use of Plasmas (habilitation thesis on this is in progress).