Plasma balls: creating the 4th state of matter with microwaves
Submitted by rau on 13 August 2009
Scientists from Tel Aviv University in Israel have now deliberately created fireballs in a microwave cavity at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France: they wanted to find out what was in them. Their results show that highly ionised nanoparticles (a dusty plasma) can be made in your school’s microwave oven.
Although Jerby and Dikhtyar were not fundamentally interested in fireball production to begin with (just desperate to get rid of it), as time passed, their interest increased. The next few years were devoted to systematic work so that they could intentionally generate fireballs from molten spots of glass. Now, in addition to using their drill to make holes, they can use a modified version of it to make fireballs.
There are some videos of Professor Jerby’s fireballs on his websitew3, but plenty of amateur investigators around the world have performed the experiments, too. No one should even think of repeating the experiment done by Bill Beaty, an engineer at the University of Washington in Seattle, USA (the risk of hot flying glass or a broken microwave seems much too great to me) but his videow4 is too entertaining to miss.
Obviously Jerby, Dikhtyar and their colleagues wanted to understand their fireballs. It appeared as if they were drawing material out of the molten glass (see image below and the videos on Professor Jerby’s websitew3), but if there were glowing particles suspended in the air, they had to be really small. If there were particles even as large as a couple of microns the fireballs wouldn’t just disappear (as they do) when the microwaves are turned off – the particles would scatter visible light in the same way as water droplets in a cloud (which have an average size of about 10 μm) and you would see a cloudy haze of glass droplets.
Electron microscopy is usually the first technique that scientists use to characterise sub-micron structures – such as the hypothesised particles in the plasma balls – but you cannot create a sample of a plasma ball to be put in an electron microscope’s vacuum tube. However, the technique of small-angle scattering (see box) provides a way to tell whether there are particles in a plasma ball, and – if so – to characterise any particles found.
At ESRF, the researchers created fireballs in their cavity using the modified drill. X-rays (wavelength 0.1 nm) were fired from the synchrotron down an evacuated tube, through a cover (transparent to X-rays) and into the microwave cavity filled with air at atmospheric pressure. The X-rays shot through the fireballs (which stayed immobile for about 1 s) and exited the cavity into a second evacuated tube which led to an X-ray detector 5 m away (see diagram). The small-angle X-ray scattering patterns produced were recorded every 0.1 to 0.3 s.
These patterns proved that the fireballs were indeed full of particles with an average radius of about 25 nm – i.e. they are nanoparticles. The data also showed that the particles varied widely in size (as is typical of aerosols) and that there were about 109 particles per cubic centimetre. This makes the volume fraction of solid material (the ratio of volume of solid to total volume of space) in the fireball around 10-7 or 10-8. There was really only a very, very, small amount of matter in the cloud. The analysis also suggested that the particles had quite a rough surface: the scientists found the surface to have a fractal dimension of 2.6 (2.0 corresponds to a smooth 2D surface, 3.0 to 3D).
The SAS technique can tell you the average size of a particle (in the range of about 1 nm to a few hundred nanometres), the size distribution of particles, the shape of the particles, and something about their internal structure, surface roughness or the interparticle separation, but not all this information can be extracted at the same time from any given data. The scattering pattern is relatively lacking in sharp features (usually there is just some overall decline in scattered intensity as scattering angle increases, occasionally there may be one broad peak) and does not uniquely define the scatterers: a very similar small-angle scattering pattern may, for example, be produced by a polydisperse (of varying size and shape) population of spherical scatterers or a fairly monodisperse (of same size and shape) population of cylindrical scatterers. Consequently, data analysis proceeds by a ‘guess, check and revise’ method, where a plausible model is used to calculate a predicted scattering pattern, which is then compared with the actual data, and the model is revised accordingly with all steps being repeated iteratively.
But why do the particles glow? Why do the researchers say they form a plasma ball? While the particles are being microwaved they absorb microwave energy and heat up to about 730 °C (1000 K). This energy is re-radiated in the form of intense visible light. At 730 °C the particles will also emit electrons due to thermoionic emission, thus making the fireball a dusty plasma (a cloud of solid particles that have lost electrons and are thus highly ionised).
Using X-rays at ESRF, the scientists also investigated what happens to the fireballs when the microwaves are turned off. Visually the fireball vanishes after about 30 ms, but the X-ray data continued to detect particles for about 4 s. The particles were there, but invisible to our eyes because they were so small. These X-ray data showed that the particles (which were charged and stable while being microwaved) initially simply diffuse away as the fireball cools and then, as cooling continues, tend to aggregate and form large clusters (Mitchell et al, 2008).
Professor Jerby has since returned to ESRF with a collection of different materials to microwave. He says, “We examined the structures of plasma balls made from a variety of materials, including copper, salts, water and carbon. It seems that we are able to generate plasma balls from almost any material now....” This means that he now has a method of directly creating nanoparticles of many different substances. This is very interesting, because nanoparticles are increasingly important in a wide variety of applications, and producing them is not always easy. Nanoparticles are being used in medicine (e.g. drug delivery), in catalysis (for cleaning up pollutants), and even in treatments for smelly socks (which rely on nanoparticles of silver to kill bacteria; see Benn & Westerhoff, 2008). For a good overview of nanotechnology, see Pickrell (2006), and of how to use nanotechnology in the classroom, see Mallmann (2009).
This is all a long way from drilling holes in ceramics though, and when asked what he was going to do next, Professor Jerby replied: “I hope to generate energy from common materials in an efficient and practically feasible manner.” In the mean time, remember that any attempt to dry your nanotech socks using a microwave oven could lead to fireworks!
The author would like to thank Professor Jerby, Tel Aviv University, Israel; Dr Narayanan from ESRF; and Dr Schrempp, from Los Osos High School, Rancho Cucamongo, California, USA, for help with this article.
Abrahamson J, Dinniss J (2000) Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil. Nature 403 (6769): 519-521. doi: 10.1038/35000525. Download the article free of charge here, or subscribe to Nature today: www.nature.com/subscribe
Mallmann M (2009) Nanotechnology in school. Science in School 10: 70-75. www.scienceinschool.org/2008/issue10/nanotechnology
Pickrell J (2006). Instant expert: nanotechnology. New Scientist. www.newscientist.com/article/dn9939-instant-expert-nanotechnology.html
Stanley H (2009) Microwave experiments at school. Science in School 12: 30-33. www.scienceinschool.org/2009/issue12/microwaves
w1 – YouTube: www.youtube.com
w2 – For more information about the microwave drill, see Professor Jerby’s web page: www.eng.tau.ac.il/~jerby/microwave_drill/index.html
w3 – Professor Jerby’s web page includes several videos of fireballs and their generation: www.eng.tau.ac.il/~jerby/Fireballs.html
w4 – Bill Beaty’s microwave experiment to melt a beer bottle: www.metacafe.com/watch/1004040/melt_a_frickn_beer_bottle
w5 – Jean-Louis Naudin describes how to create ball lightning in a microwave, using a piece of aluminium: http://jlnlabs.online.fr/plasma/4wres/index.htm
w6 – More information about ball lightning can be downloaded here.
Halina Stanley is a physicist by training. She spent ten years as a research scientist in industry and academia using neutron and X-ray scattering techniques to characterise materials before joining the American School of Grenoble, France, where she teaches physics, chemistry and mathematics to secondary-school students.