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Home » Page 1054

Liquid flow in capillaries contradicts conventional wisdom

Fill a narrow glass capillary tube with water, then turn it horizontal. Conventional wisdom says that the water will remain inside the tube. But a group of scientists in Spain and the UK has found that when the internal cross-section of the tube has a certain shape – triangular or a squashed ellipse – the tube will empty, however narrow it is. This phenomenon could represent a new way to control flow in microfluidics. It might even already be exploited in nature, for example in plants or blood vessels.

Innocent question

The authors of a new paper first began thinking about the behaviour of liquid in a horizontal capillary tube about eight years ago, when mathematician Carlos Rascón of the University Carlos III de Madrid, Spain, and theoretical physicist Andrew Parry of Imperial College London, UK visited Dirk Aarts at the University of Oxford, UK.

Aarts was experimenting on the behaviour of colloid–polymer mixtures in slit-like capillaries, and he commented that at the end of the day’s work they’d turn the capillaries horizontal and see the meniscus slump. He asked his visitors the innocent question “Do you know what shape it is?” And the trio realized that none of them knew how to work that out.

Keeping it simple

The equilibrium shape of the meniscus comes from minimizing the total free energy of the liquid confined by the solid walls of the tube. It can have a range of shapes, from a simple sloped meniscus, to an s-shaped cross-section in which a “tongue” of liquid extends along the base of a capillary.

The 3D shape is not easy to calculate and so, in 2012, the researchers’ first step was to calculate a reduced-dimension solution. They showed that for a simple 2D slit (with a 1D meniscus) there is a critical slit width at which the meniscus length becomes infinite, meaning that the slit-like pore empties.

Solving the problem in 3D for an arbitrary capillary shape is much harder. Rascón and colleagues simplified the calculation by considering the energy needed to form a meniscus with a constant cross-sectional shape all along the tube: that is, one where the liquid “flattens out” and the tube empties. When this energy falls to zero, such a tube-emptying meniscus becomes stable. Using this method, the researchers could find the “emptying conditions” for a tube, which depend on parameters such as the contact angle of the bulk liquid with the wall (a measure of how strongly the wall attracts or repels the liquid), and the shape and width of the tube.

Surprise result

To the researchers’ surprise, they found that changing the liquid type and what the vessel is made of – which affects the contact angle – doesn’t have a big effect on whether a tube empties or not. Only for a narrow range of tube sizes does it matter at all; outside of this range, the tube will be filled or empty for any liquid.

The shape, on the other hand, matters a lot. For sufficiently flattened ellipse-shaped cross-sections, the tube will always empty, both for very small and very large contact angles, regardless of the tube width. In the former (hydrophilic) case, the strong liquid–wall interactions “pull” the liquid along the tube and out of the ends, while in the latter (hydrophobic) case, the very weak interactions mean that the liquid just “slips out”. This behaviour contradicts the conventional wisdom that very narrow horizontal capillaries will always hold onto their contents. “We could not believe what we were seeing in the maths,” says Parry.

“We all thought we understood how liquid can clog a very narrow tube, being incapable of flowing out no matter how we orient the capillary,” says Joseph Indekeu, a specialist on wetting at the Catholic University of Leuven in Belgium. But these results suggest that very narrow tubes may always empty for particular shapes of cross-section.

Capillaries with an equilateral triangular cross-section may also empty spontaneously, the researchers find, but the precise conditions for it depend on the orientation. Such behaviour has been discussed previously by applied-mathematician Robert Finn of Stanford University in California, US, who in 2011 showed that a tube with an “ice-cream-cone” cross-section – round at the top, sharply V-shaped at the bottom – will empty even in the absence of gravity, thanks to capillary forces alone. He and others have a patent on an “astronaut’s drinking cup” based on this principle. There is, says Finn, a “moderately substantive” body of work on the question of capillary emptying in the fluid-dynamics literature, partly to understand the question of how liquid “plugs” might block flow down narrow tubes.

Shape-based switches

The researchers say that such findings might be put to use in microfluidic technology. A change in shape of the tube (such as flattening of a circular cross-section) might induce flow – a kind of “pumping” – without requiring any change in fluid pressure. “I like very much the possibility of ‘lock and go’ switches, akin to traffic lights, for directing liquids in networks of capillaries, operated by simply rotating non-cylindrical tubes around their axes,” says Indekeu.

It’s possible too, says Parry, that examples of flow controlled by geometry might appear in nature, for example through changes in the shape of blood vessels or the v-like grooves of leaves and folded membranes.

The research is published in PNAS.

Flash Physics: CubeSats could soon self-propel, ALMA unveils the birth of stellar siblings, patent award for US plasma lab

CubeSats could soon have on-board propulsion

CubeSats – small, low-cost satellites – could soon become self-propelled, thanks to a rocket-motor concept developed by researchers at Los Alamos National Laboratory in the US. While CubeSats are a cheap and easy way for relatively small research groups to launch satellites and access space, they traditionally do not have any on-board propulsion system – the nanosatellites are usually launched via a larger satellite and simply released into a specific orbit. “The National Academy of Sciences recently convened a meeting to look at science missions in CubeSats,” says Bryce Tappan, lead researcher of the CubeSat Propulsion Concept team, “and identified propulsion as one of the primary categories of technology that needs to be developed.” Recently, the Los Alamos researchers successfully tested a six-motor CubeSat-compatible propulsion array and according to Tappan, they are very close to being able to take the next step and show that the propulsion system works on a satellite in space. One of the main problems with CubeSat propulsion is that of safety – the fuels used in any such system are intrinsically hazardous ones such as hydrazine, and as multiple CubeSats are deployed by piggybacking on a larger mission, even a small margin of risk can be disastrous. To avoid these issues, Tappan’s team is developing a solid-based chemical-fuel technology – called a “segregated fuel oxidizer” system – that is completely non-detonable and where the solid fuel and solid oxidizer are kept completely separate inside the rocket assembly. The ability to self-propel would expand the capabilities of CubeSats, allowing them to enter higher orbits and achieve multiple orbital-planes in a single mission, and could also be used to make them “de-orbit” when their mission ends, reducing space junk.

ALMA unveils the birth of stellar siblings

Astronomers have spotted a relatively rare triple-star system surrounded by a disc with a spiral structure, using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The discovery lends support to a proposed process known as “disc fragmentation”, which allows for the formation of young binary and multiple star systems – such a triple-star system forming in a disc has never been observed until now. “What is important is that we discovered that companion stars can form in disc material surrounding a dominant star,” says team-leader John Tobin at the University of Oklahoma in the US. “We had observed this system in the past with ALMA’s predecessors, but this is the first time we have been able to clearly analyse the disc and the newborn stars within it,” he explains, adding that “triple systems like this one are rare, and this is the only one with a configuration like this, but we are actively searching for more.” The work may help to explain how binary star systems form – something that astronomers are still not sure about. The research is published in Nature.

Patent award for US plasma lab

The Princeton Plasma Physics Laboratory team that invented a new way to produce technetium 99m
Plasma pioneers: (Left to right) George Ascione, Charles Gentile and Adam Cohen from the Princeton Plasma Physics Laboratory have invented a new way to produce technetium 99m – a substance used in two-thirds of nuclear medical diagnostic procedures. (Courtesy: Elle Starkman/PPPL)

Researchers at the Princeton Plasma Physics Laboratory (PPPL) have won the 2016 Edison Patent Award for developing a new technique to create isotopes for medical imaging. The refrigerator-sized device can produce the radioactive element Technetium 99m (Tc-99m) – a substance with a half-life of six hours that is used in more than 60% of nuclear medical diagnostic procedures. Tc-99m results when Molybdenum 99 (Mo-99) decays – an isotope typically produced in a nuclear reactor. However, there has been a shortage of Tc-99m due to the closure or aging nuclear reactors worldwide. The technique developed by the PPPL researchers can produce Tc-99m from naturally occurring Molybdenum 100 (Mo-100). It involves firing neutrons at a metal plate to produce gamma rays that then strike a Mo-100 nucleus turning it into Mo-99. Due to the device’s size, the researchers say that that technique could allow many other countries to have access to Tc-99m imaging. “There was a lot of work that went into this and we’re just happy that we can potentially make a positive impact on helping people in the world who would not necessarily have access to this diagnostic technology,” says PPPL researcher Charles Gentile, who worked on the device. The prize will be awarded at a ceremony at the Liberty Science Center in Hoboken, New Jersey, on 3 November.

 

  • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on the strange behaviour of liquids flowing in capillaries.

A toe-tally terrific trio

Photo of three Chatty Feet socks
Toe to toe: the three ChattyFeet physicist designs.

By Matin Durrani

It’s not even Halloween yet and Physics World HQ has already received its first gift ideas for the Christmas season. Now most of us might roll our eyes if we were given a pair of socks for Christmas, but the footwear sent to us by UK firm ChattyFeet – slogan “Let the socks do the talkin'” – are sure to bring a smile to any physicist’s face.

The company has three different physics-related sock designs on offer, each depicting a cartoon image of a famous physicist and branded with a toe-totally amusing name. First up is a fetching blue number dubbed “Stephen Toeking” with the washing instruction: “Choose a slow spinning cycle to avoid a black hole.”

(more…)

The struggle for convergence

“Young man, if I could remember the names of these particles, I would have been a botanist.” Enrico Fermi’s reply to the unfortunate student who asked him the name of a certain subatomic particle came in the mid-1950s, when only a dozen such particles were known to exist. Within a decade, the number had swelled to nearly 100 as ever more powerful accelerators and detectors came online. But as Peter Watson explains in his latest book Convergence, although this proliferation in the number of particles “seemed counterintuitive at first, [it] would also help unify our understanding of certain aspects of the universe that had been beyond comprehension beforehand”. Indeed, it was not long before Murray Gell-Mann managed to turn what had become a nightmarish particle jungle into an ordered particle zoo, by devising a classification scheme called the Eightfold Way that grouped certain particles (the hadrons) into families of eight. In the process, this reorganization allowed the existence of undiscovered particles to be predicted.

Watson, a former journalist and a prolific (but always thought-provoking) historian of ideas, calls Convergence “a history of modern science but with a distinctive twist”. This twist, he claims, “has been there for all to see but so far it has not been set out as clearly as it deserves”. His main argument is that the various disciplines – despite their very different beginnings and apparent areas of interest – have in fact been gradually coming together over the past 150 years. Like Gell-Mann’s particle zoo in the early 1960s, these disciplines are “Converging and coalescing to identify one extraordinary master narrative, one overwhelming interlocking coherent story: the history of the universe.”

The two great unifying topics of the 19th century, Watson argues persuasively, were the conservation of energy and Darwin’s theory of evolution by natural selection. Each was the “fruit” of the coming together of the sciences: of heat, optics, electricity, magnetism and blood chemistry in the case of energy conservation; of geology, palaeontology, anthropology, geography and biology in the case of evolution.

Given this impulse to unite and simplify, Watson points out that physics has advanced when seemingly diverse phenomena have turned out to be different aspects of the same thing. Newton’s great discovery was that the same force that pulled the apple to the ground also held the Moon in its orbit around the Earth, and the Earth in orbit around the Sun. Magnetism, electricity and light were thought to be completely disparate phenomena until James Clerk Maxwell and Michael Faraday found that all were just different manifestations of electromagnetism. Albert Einstein’s theory of relativity grew out of his efforts to reconcile electromagnetism with classical mechanics.

It is hard to argue with Watson’s choice of Einstein’s unification of mass and energy and space and time as “the first great convergence event after the 1850s”. Max Born described Einstein’s masterwork as “the greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition and mathematical skill”. However, I do wonder how many would have chosen, as Watson has, Linus Pauling and his work on the nature of the chemical bond as one of their top three unifiers of the 20th century.

Watson also makes much of the Nobel laureate Steven Weinberg’s claim that convergence is “the deepest thing about the universe”. However, I find it remarkable that the “convergence of the sciences” (or, as Watson puts it “their synthesis, symphysis and coherence”) should be accorded such an accolade in the face of what those sciences have revealed to us about the nature of the universe and our place within it. It is widely accepted that when the universe was born in the Big Bang there was only a single force that soon shattered into four. Far from converging, these four forces have played largely separate roles ever since. The strong force holds the quarks together in the atomic nucleus. The weak force transmutes matter and makes the different elements. The electromagnetic force binds atoms and controls their chemical reactions. And then there is gravity, which has so far defeated all attempts to make it converge with the other three.

Weinberg was one of those responsible, in the late 1960s, for unifying electromagnetism and the weak force into the electroweak. Even so, I am surprised by his claim about convergence, which allows Watson to assert that “the most exciting intellectual breakthrough of all time” is “the way one science supports and interconnects with another, the beginning of a form of understanding like no other in history”.

Wherever experimental evidence can be coaxed out of nature, it suffices to corroborate or refute a theory and serves as the sole arbiter of validity. But where evidence is sparse or absent, other criteria, including aesthetic ones, have been allowed to come into play – both in formulating a theory and evaluating it. Watson believes that because of this, in some ways “physics has become mathematics”, arguing that we are currently “living in an in-between time, and have no way of knowing whether many of the ideas current in physics will endure and be supported by experiment”.

This, Watson explains, deeply worries the likes of cosmologists Joseph Silk and George Ellis. At the end of 2014, Silk and Ellis argued in a Nature comment piece that some scientists appear to have “explicitly set aside” the need for experimental confirmation of our most ambitious theories, “so long as those theories are sufficiently elegant and explanatory”. They further complain that we are at the end of an era, “breaking with centuries of philosophical tradition” of defining scientific knowledge as empirical.

As Silk and Ellis point out, this situation has come about because particle physicists have struggled to go beyond the Standard Model. Their most prominent attempt has been the theory of supersymmetry, but the problem is that no supersymmetric particles have been found, and Silk and Ellis fear that its advocates will simply “retune” their models “to predict particles at masses beyond the reach of the LHC’s power of detection”.

The result, Watson writes, is “a discernible sense of crisis”. The problem with a process of convergence, as he acknowledges, is that it presupposes a final end point. So while it may be true that “convergence is happening all over the sciences”, he admits, “The problem in physics is that it may be just too expensive to build the equipment that might bring that final convergence about.”

  • 2016 Simon & Schuster £25.00hb 544pp

Web life: Precarious Physicist

So what is this site about?

If teaching physics to undergraduate students strikes you as a secure, well-respected and at least somewhat highly paid job, the Precarious Physicist blog will challenge your assumptions. Its author, Andrew Robinson, is one of a large and growing number of university lecturers who work on short-term contracts with relatively poor pay, high teaching loads and little prospect of permanent employment. Or, as Robinson puts it: “Hello. My name is Andrew. I am 54 years old, have a PhD and I have a crap job in academia.”

That’s…blunt.

Indeed. But it’s also hard to disagree. As Robinson explains, his job as a contract instructor in physics at Carleton University in Ottawa, Canada, is “completely casualized labour. I have to reapply for my own job every four months; I have very poor benefits compared to tenured staff; I have no promotion or career development prospects at all”. By his calculation, Robinson also teaches “twice as many courses as tenured staff for around a third of their salary”, and although he has won awards for his teaching, he feels that his opinions on pedagogy “do not matter” to the university.

If it’s that terrible, why doesn’t he quit?

In part, it’s the students, who Robinson describes on his blog as “wonderful…the only reason I still do this job”. But there are personal factors, too. Robinson is originally from the UK, but he moved to Canada after his Canadian wife got a tenure-track job in physics at the University of Saskatchewan. “We had the classic two-body problem,” Robinson told Physics World. “I got into teaching by accident.” Asked to cover his wife’s physics course when she went on maternity leave, he discovered that he liked teaching and was good at it. Later, Saskatchewan gave him annual contracts to teach large lecture courses for first-year students – a job he describes as “a good match”. After a few years, however, their second child’s health problems forced them to move to be near family in Ottawa. Once there, Robinson found that conditions for contract teaching staff were much less favourable than those he’d experienced previously, but “there aren’t really any other jobs for a 50-something PhD scientist in Ottawa”, he says. In Canada, he adds, “a PhD is regarded much more as training to be a professor than it is in the UK or Europe”.

What topics does it cover?

In addition to the “crap job in academia” post quoted above, Robinson has analysed how his stipend and benefits stack up against those of his tenured or tenure-track colleagues (badly); skewered an essay that advised faculty to step away from the “frantic pace” of modern academia (“I don’t have this luxury”); and discussed the financial disincentives of trying new things in his classroom (“a huge uncompensated task”). But he also regularly writes about physics teaching, and his posts on this topic are as kind and patient as his diatribes against his employer are pointed and sarcastic.

Why should I visit?

Numbers of “contingent” (that is, neither permanent nor potentially permanent) faculty have been rising for years in many parts of the world. According to the American Association of University Professors, more than 70% of university-level instructors in the US are now in non-tenure-track jobs. Looking at it from the university’s point of view, this trend makes perfect sense: contract or adjunct faculty are cheap, well qualified and often very good at what they do, so why would they hire anyone else? Economic arguments aside, though, Precarious Physicist makes a powerful case that the current system is both unfair and unsustainable, and Robinson is taking a risk by writing it. As he repeatedly points out, his employer could decide at any moment not to renew his contract. Under the circumstances, paying attention seems like the very least the rest of us can do.

What can we learn from ultrahigh energy cosmic rays?

Cosmic rays are streams of highly energetic particles arriving at the Earth from sources beyond the Milky Way galaxy. In this video, Gordon Thomson from the University of Utah in the US explains what this phenomenon can teach us about the nature of the cosmos. Specifically, Thomson introduces the Telescope Array experiment in Millard County, Utah, which observes the secondary radiation produced by cosmic rays interacting with particles in the Earth’s atmosphere. The experiment is being used to create a map of the sky to identify cosmic ray sources and investigate how they might be able to accelerate particles to such high energies.

This video is part of our 100 Second Science series, in which researchers give concise presentations covering the spectrum of physics.

Physicists break record for laser-electron interaction

Researchers have converted the energy of an electron beam into a pulse of coherent light with an efficiency of 30% – much higher than the 0.1% efficiency of most free-electron lasers. While their demonstration produces only infrared light, the method could lead to efficient, high-power lasers operating over a range of wavelengths, including X-ray. Such X-ray sources could be used to etch circuits on semiconductor chips faster and more efficiently.

Using facilities at the Brookhaven National Laboratory in the US, the researchers demonstrated the energy conversion by first accelerating a beam of electrons in a five-metre-long tunnel. They then directed those electrons for several more metres along a helical path using magnets, before illuminating those electrons with an infrared laser.

The laser caused the electrons to slow down while emitting coherent light. Because the emitted light was the same wavelength as the laser, the process – known as stimulated emission – effectively amplified the original infrared laser beam. The degree of amplification depended on the starting energy of the electron beam, with the team saying it has broken the record for an interaction “between a free-space propagating laser pulse and a relativistic electron beam”.

The group successfully converted more than 30% of the electrons’ energy into a coherent light pulse that was three picoseconds long and had a peak power of about 100 gigawatts. Pietro Musumeci, a physicist at the University of California, Los Angeles, in the US, who was involved in the work, says that researchers first attempted this technique about 30 years ago but were only successful at converting beam energy into longer, microwave wavelengths.

Exceeding expectations

Musumeci’s group inferred the energy of the light pulse by measuring how much energy the electron beam lost and by comparing the measurements to a theoretical simulation, although the team plans to directly measure the pulse’s energy in a future experiment. He adds that it is, in principle, possible to convert up to 90% of the electron-beam energy into coherent light by making the experimental set-up longer. The laser would then stimulate the electron beam to emit for a longer time, thus converting more energy into coherent light.

The eventual goal is to create consecutive pulses to generate an efficient, high-power laser. Laser light is useful for scientific and industrial applications because it can be sharply focused and can efficiently fit “a lot of light in a small volume”, as Wim Leemans, director of the Berkeley Lab Laser Accelerator, who was not involved in the research, points out. This tightly focused light from a pulsed beam can be used, for example, to capture “snapshots” of rapid quantum phenomena.

However, coherent light sources are commonly inefficient, Leemans says. For example, an X-ray free-electron laser only has an efficiency of around 0.1%, making a technique that can convert 30% of the electron-beam energy into photon energy significant. Leemans, who is working on a portable X-ray laser that fits on a table, says that smaller laser sources could adapt this technique to achieve higher efficiencies.

Musumeci says that this technique could eventually be used to make efficient, high-power, 13 nanometre X-ray sources used to etch tiny circuits on semiconductor chips. “All the new generation chips in your iPhone and your computer are made with 13 nanometre light,” he says. “So if you have an efficient light source there, you can print more and faster.”

To tune the wavelength of the generated light pulse, he says, the infrared laser in their set-up would need to be replaced with a laser at the new wavelength, and the spacing of the magnets in the cavity would need to be changed.

The research is described in Physical Review Letters.

Flash Physics: Designer magnets from 3D printers, creating silicon-germanium glass fibres, towards a space-based gravitational-wave observatory

Creating designer magnets using a 3D printer

A new method that uses 3D printing to fabricate permanent magnets with specific, pre-determined magnetic-field shapes has been created by researchers at the Technische Universität Wien (TU Wien) in Austria. Their new technique allows for the production of complex forms of magnets, with precisely customised fields – these are especially required to create devices such magnetic sensors. “We often require special magnetic fields, with field lines arranged in a very specific way – such as a magnetic field that is relatively constant in one direction, but which varies in strength in another direction,” explains Dieter Suess, head of the University’s Christian-Doppler Advanced Magnetic Sensing and Materials laboratory. For this to be possible, the magnets need to produced with a specific geometric form – something the TU Wien team do on a computer, adjusting its shape until all requirements for its magnetic field are met. The design is then implemented via a special 3D printer, created by the team, which can handle magnetic materials. The magnet printer uses specially produced filaments of magnetic micro granulate, which is held together by a polymer binding material. The resulting object is made up of roughly 90% magnetic material and 10% plastic. Finally, the object is exposed to a strong external magnetic field, converting it into a permanent magnet. The team says its new process is fast, cost-effective and offers new possibilities including using different materials within a single magnet to create a smooth transition between strong and weak magnetism. The research is published in Applied Physics Letters.

Silicon-germanium glass fibres become a reality

While glass fibres are ubiquitous in most modern technologies – from internet cables to keyhole surgery – scientists are keen to expand the usage of such fibres by adding semiconductor core materials to them. Now, an international team of researchers, led by Ursula Gibson at the Norwegian University of Science and Technology, has created glass fibres with single-crystal silicon-germanium cores. Creating such fibres is a challenge mainly because silicon and germanium have different melting points. The researchers combined both materials in a fibre by scattering germanium flecks through the silicon fibre and then moving the fibre through a laser beam to rapidly heat it, thereby melting the semiconductors in the core in a controlled fashion. “If we take a fibre and melt the core without moving it, we can accumulate small germanium-rich droplets into a melt zone, which is then the last thing to crystalize when we remove the laser slowly,” says Gibson, adding that the team can “make stripes, dots…you could use this to make a series of structures that would allow you to detect and manipulate light.” The work could help in developing high-speed semiconductor devices, as well as in expanding the current capabilities of endoscopes, according to the researchers. The research is published in Nature Communications.

ESA seeks proposals for space-based gravitational-wave observatory

Illustration of the gravitational waves from merging black holes
Illustration of gravitational waves produced by a newly formed black hole. (Courtesy: NASA)

The European Space Agency (ESA) has put out a call for European scientists to submit proposals for the first space mission to observe gravitational waves. A space-borne observatory that will specifically study gravitational waves – ripples in the fabric of spacetime created by accelerating massive objects – was chosen in 2013 as the third large mission (L3) in ESA’s Cosmic Vision plan. The decision was no doubt boosted by the first direct detection of gravitational waves made by the ground-based LIGO detectors in the US in February this year – a second detection was announced in June. In 2014, a “Gravitational Observatory Advisory Team”, composed of independent experts, was appointed. In the team’s final report earlier this year, they recommended that ESA pursues the mission, having verified the feasibility of a multi-satellite design with free-falling test masses linked over millions of kilometres by lasers. The decision also follows the successful performance of ESA’s LISA Pathfinder mission, which demonstrated some of the key technologies needed to detect gravitational waves from space, which was launched in December 2015. “Gravitational waves promise to open a new window for astronomy, revealing powerful phenomena across the universe that are not accessible via observations of cosmic light,” says Alvaro Giménez, ESA’s director of science. Letters of intent for ESA’s new gravitational-wave observatory must be submitted by 15 November, and the deadline for the full proposal is 16 January 2017. The selection is expected to take place in the first half of 2017, with a preliminary internal study phase planned for later in the year.

 

  • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a new method for generating laser light.

Recipe for success with topological materials

Fang and Weng were named in the Physics World Top 10 Breakthroughs of 2015 for their work on Weyl fermions, which are quasiparticles that they found lurking in their Weyl semimetal. Weyl fermions also have unique properties that could make them useful for creating high-speed electronic circuits among other applications.

Fang explains how his team of theorists uses mathematics and computer simulations to predict which materials are topological. He also explains how topological materials could be useful in creating quantum computers of the future. Weng then takes up the challenge of explaining just what a Weyl semimetal is and why it is home to Weyl fermions. He also explains how the topological material could be used to create another elusive quasiparticle – the Majorana fermion.

US unfazed by Russia ending reactor co-operation

A spokesperson for the US Department of Energy (DOE) has told Physics World that Russia’s recent decision to end an agreement between the Rosatom nuclear agency and the DOE into the feasibility of converting Russian research reactors to low-enriched uranium (LEU) will have “no practical impact” on reactor conversion.

The agreement between the two agencies, which was initially suspended in 2014, was terminated earlier this month, with the Russian government saying it had done so following the US’s decision to end civil nuclear-energy co-operation with Russia in 2014. The Russian government also blamed “other hostile steps and statements”.

The US has been seeking to eliminate highly enriched uranium (HEU) from civilian research reactors worldwide since 1978. But the DOE spokesperson says that the National Nuclear Security Administration (NNSA) does not expect any impact on work to remove highly enriched uranium from third countries, as it “is covered by a separate agreement that still has bilateral support”.

Global programme

Naturally occurring uranium contains less than 1% uranium-235 – the primary fissile isotope of uranium – by mass. But the proportion can be increased by enrichment, with HEU fuel containing at least 20% of uranium-235. Research reactors, however, often rely on “weapons-grade” uranium, which contains at least 90% uranium-235, to produce beams of neutrons that are intense enough for research purposes.

The US had predicted that the 74 reactors around the globe that still use HEU fuel would switch to LEU by 2018, although a recent report by the US National Academies of Sciences, Engineering and Medicine concluded that will now take until at least 2035.

Russia is home to 32 of the research reactors that still use HEU, with the US report citing non-technical factors as the main hindrance to their conversion. It claims that conversion “is not a high national priority for Russia” and that in recent years Rosatom and the US DOE “have severed nearly all ties”, with Russia no longer willing to accept US funding for conversions and the DOE curtailing interactions between scientists.

Before relations deteriorated, in 2014 Russia and the US successfully converted one reactor – the ARGUS reactor at the Kurchatov Institute in Moscow – from HEU to LEU fuel. The two nations also concluded that it was technically and economically feasible to convert five more.

Russia and the US have also been working together to remove HEU from other countries. At the end of September they announced that they had successfully repatriated 61 kg of Russian-origin HEU from the Maria Research Reactor in Poland, which was converted to LEU two years ago. Poland is now HEU-free.

Independent approach

Russia says that if it decides to convert further research reactors it will now “conduct this work independently”, although the government statement makes it clear this is not a priority. “We believe that the conversion of nuclear reactors from highly enriched to low-enriched uranium fuel in [Non-Proliferation Treaty] member states is not an end in itself,” it says. “In a number of cases, including medical-isotope production, HEU is most effective, and abandoning it does not make sense from a technological and economic standpoint.”

Russia has also suspended an agreement on nuclear- and energy-related research-and-development co-operation with the US, and an agreement that commits the two countries to eliminate parts of their weapons-grade plutonium stocks. In a statement, the Russian foreign ministry says it had suspended the plutonium pact in response to “unprecedented sanctions-related pressure”, hostile steps by the US since “the reunification of Crimea with Russia” and “an active build-up of NATO’s military infrastructure”.

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