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Amazing science demo five: Double Doppler with Train Set

This is the fifth and final instalment of our “five amazing physics demonstrations” presented by science-demo guru Neil Downie and his adept assistant Matthew Isbell.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how you would present each demonstration to an audience. Full details of how to access the digital edition are available at the bottom of this article.

In this final demo of the series, Downie uses a model train set equipped with an ultrasound emitter to demonstrate the Doppler effect. Employing a bat detector to convert the ultrasound signal into an audible sound, Downie illustrates how the pitch increases as the train steams towards him, and then decreases as it moves away. The train set brings a potentially dry subject to life and Downie uses the set-up to explain how bats utilize the Doppler effect when hunting for prey.

Double Doppler with Train Set

So what’s this all about? The Doppler effect is of fundamental importance in physics, not least because measuring the redshift of light from galaxies lets us estimate how far away they are. When it comes to sound, however, demonstrating the Doppler effect is not easy. You could get someone on a passing train to play a trumpet or try whirling an electronic beeper around on a string – but neither is particularly simple. What is more, the Doppler shift is small and can be masked by changes in sound volume. This project is a much neater way of showing the Doppler effect and has the added bonus of bringing ultrasonic waves to life.

What bits and pieces do I need? You will need an electronic bat detector – a hand-held device that converts ultrasound into audible sound. You’ll also need an ultrasonic transducer, such as the kind fitted to cars to help drivers park. Such devices, which emit ultrasound at about 40 kHz, are normally hooked up to a separate microprocessor circuit to create pulses. So to get a continuous source of waves, you’ll need to connect the emitter to an oscillator circuit. Finally, you need to attach the transducer to a moving vehicle – a toy train running on a circular track is ideal.

How do I get going? Lay the track somewhere without too many flat walls or in the corner of a room (this reduces reflections). Turn on the ultrasonic emitter and put it in the train. Then switch on your detector. If you’re standing still, you’ll hear the pitch of the signal rise and fall with a characteristic “heeeEEEeeaaAAAAaaaaw” as the train first approaches and then moves off. There’s a steady high note as the train approaches, and a steady low note as it recedes, with a modulating pitch in-between.

And what physics will I learn? If you’re standing still and the train’s approaching you at speed v, the frequency of the sound received, f_r, will differ from the emitted frequency, f_e, according to the classic textbook formula f_r = f_e[c/(c – v)], where c is the speed of sound. (The formula is f_r = f_e[c/(c + v)] if the train is moving away.) If the sound were at an audible frequency of about 2 kHz, then for a source approaching at 0.5 ms^–1, the detected frequency would rise by just 3 Hz, which would be hard to hear given that a semitone in music is 120 Hz. A 40 kHz ultrasonic system moving at the same speed, however, will have a Doppler shift of about 60 Hz, which is easy to notice if you adjust the bat detector so it emits, say, 1000 Hz waves when it receives 40 kHz ultrasound – it will be about a whole musical tone different. You could also use your equipment to detect other sources of ultrasound, from rubbing fingers and whistling fluorescent lamps to electronics and clanging metals. Or even bats.

If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine

Trail runs cold on alien hotspots, for now

A search of 100,000 galaxies for signs of highly advanced extraterrestrial life, carried out by researchers in the US, has found no evidence that they harbour advanced civilizations. Scouring through observations from NASA’s WISE orbiting observatory, the researchers looked for unusually high amounts of mid-infrared radiation, which could mark out a colonized galaxy. The research is a significant expansion of previous work, which only studied about 100 galaxies. Although the team has not found any obvious signs of advanced life just yet, it has come across some puzzling new sources of mid-infrared radiation that need further study.

In 1960 the eminent physicist Freeman Dyson suggested that highly advanced alien civilizations beyond the solar system could be detected by the inevitable waste heat that they would produce in the form of mid-infrared emissions. He reasoned that such civilizations would have developed technologies that extracted useful energy from starlight by converting it into mid-infrared radiation. In a paper published in the journal Science, Dyson proposed that “a search for point sources of infrared radiation be attempted, either independently or in conjunction with the search for artificial radio emissions”. But such searches were only made possible once space-based telescopes like WISE were developed.

Hot on ET’s heels

Together with colleagues in the US, Jason Wright of Penn State University began the so-called Glimpsing Heat from Alien Technologies Survey (G-HAT), which makes use of WISE. “Whether an advanced spacefaring civilization uses the large amounts of energy from its galaxy’s stars to power computers, spaceflight, communication or something we can’t yet imagine, fundamental thermodynamics tells us that this energy must be radiated away as heat in the mid-infrared wavelengths,” explains Wright, adding that this “same basic physics causes your computer to radiate heat while it is turned on”.

Team member Roger Griffith, also at Penn State University, combed the entire WISE catalogue of detections – nearly 100 million entries – for objects consistent with galaxies emitting too much mid-infrared radiation. He then individually examined and categorized around 100,000 of the most promising galaxy images. The researchers’ extensive search did not throw up any obvious candidates for an alien civilization reprocessing more than 85% of its starlight into the mid-infrared wavelength. But they did find about 50 galaxies that have unusually high levels of mid-infrared radiation. “Our follow-up studies of those galaxies may reveal if the origin of their radiation results from natural astronomical processes or if it could indicate the presence of a highly advanced civilization,” says Wright.

In any case, the very fact that the extensive search did not find any colonized galaxies is curious in itself. Many of these galaxies are billions of years old, giving plenty of time for technologically advanced civilizations to form and establish. “Either they don’t exist, or they don’t yet use enough energy for us to recognize them,” says Wright.

WISE curiosities

“Once we had identified the best candidates for alien-filled galaxies, we had to determine whether they were new discoveries that needed follow-up study or well-known objects that had a lot of mid-infrared emission for some natural reason,” says Matthew Povich, who is part of the G-HAT team and is based at the California State Polytechnic University in Pomona. All told, the researchers found about half a dozen objects that were both unexpected and potentially interesting to study in the future to determine exactly which natural astronomical phenomena may be causing the results.

Among the discoveries within our own Milky Way galaxy, the team spotted a bright nebula around the nearby star “48 Librae”, and a cluster of objects easily detected by WISE in a patch of sky that appears totally black when viewed only in the visible spectrum. “This cluster is probably a group of very young stars forming inside a previously undiscovered molecular cloud, and the 48 Librae nebula is apparently caused by a huge cloud of dust around the star, but both deserve much more careful study,” says Povich.

Wright believes that more careful future observations of light from galaxies with higher-than-expected mid-infrared emissions will “push our sensitivity to alien technology down to much lower levels, and to better distinguish heat resulting from natural astronomical sources from heat produced by advanced technologies. This pilot study is just the beginning”.

The research is published in the Astrophysical Journal Supplement Series.

Cyclotron radiation from a single electron is measured for the first time

Photograph of a Project 8 team member adjusting the experiment — Going in circles: a Project 8 team member adjusts the experiment. (Courtesy: Project 8)

The cyclotron radiation emitted by a single electron has been measured for the first time by a team of physicists in the US and Germany. The research provides a new and potentially more precise way to study beta decay, which involves the emission of an electron and a neutrino. In particular, it could provide physicists with a much better measurement of neutrino mass, which is crucial for understanding physics beyond the Standard Model.

The Standard Model of particle physics assumes that the mass of neutrinos is zero, but in 1998 the Super-Kamiokande detector in Japan showed conclusively that the particles undergo oscillations and therefore must have mass. Knowing the masses of the three known types of neutrino is crucial to understanding physics beyond the Standard Model, but actually measuring the masses is proving extremely difficult. “Currently, we know more about the mass of the Higgs boson, which was discovered two years ago, than we do about the mass of the neutrino, which was discovered 60 years ago,” says Patrick Huber of Virginia Tech in the US.

Studies of neutrino oscillations tell us only that the average neutrino mass must be at least 0.01 eV/c², so researchers are also trying to measure the mass using conservation of energy in beta decay. This is a nuclear process that involves the emission of an electron and a neutrino – strictly speaking, an electron antineutrino. Neutrinos are extremely difficult to detect, so physicists instead measure the energy of the electron and use this to calculate the mass of the neutrino.

Upper bound

The best measurements so far give an upper bound on the electron antineutrino mass of 2.05 eV/c². Scientists are assembling a new detector called KATRIN at Karlsruhe Institute of Technology in Germany. This should measure a neutrino mass as small as 0.2 eV/c² – which could still leave a 20-fold uncertainty in its value. But KATRIN is the size of a building, and further improvements in measurement accuracy by this method would require an even larger, more expensive spectrometer.

Now, physicists at Pacific Northwest National Laboratory, National Radio Astronomy Observatory, University of California Santa Barbara, University of Washington, the Massachusetts Institute of Technology and Karlsruhe University have set up the Project 8 collaboration, which is taking a different and possibly more elegant approach to measuring neutrino mass. When an electron passes through a magnetic field, its path curves into a circular orbit, and this causes the electron to emit cyclotron radiation at microwave frequencies. The nature of this radiation is dependent on the energy of the electron, and therefore measuring this effect could provide a much more simple and precise technique of measuring the energy than is currently used at KATRIN. The challenge, however, is how to detect the extremely weak femtowatt signal of cyclotron radiation from a single electron.

Now, the Project 8 team has taken an important step in that direction by being the first to detect this cyclotron radiation. Its prototype tabletop apparatus is located at the University of Washington in Seattle, and it uses a centimetre-sized gas cell that is filled with krypton-83 – a gas that undergoes beta decay. In an actual neutrino-mass experiment, the krypton would be replaced with tritium, but this introduces additional technical and safety considerations that will be considered in the future. The cell is placed inside a superconducting coil to generate a magnetic field. Electrons emitted by the beta decay travel in very long circular paths inside the tiny cell, emitting cyclotron microwave radiation, which is then detected by cooled, ultralow-noise detectors.

Small and simple

The researchers measured the energy of single emitted electrons with an accuracy of 30 eV. While this is far too low to obtain a reliable calculation of the neutrino mass, the team is now working to optimize the device to improve its resolution. “The apparatus that we built was very, very small”, says team member Benjamin Monreal of the University of California, Santa Barbara, “And that made the electronics very simple. We’re now preparing the readout designs, the antenna designs, the amplifier designs and the software to try to scale up.”

Huber, who was not involved in the research, is impressed, “They have successfully completed the first, very crucial step,” he says. “From here on, careful engineering and scaling of the device should get them to a point where they can compete with KATRIN.” However, he says, “there are probably more physics experiments that have failed because of ‘mere engineering challenges’ than for any other reason”.

The research is published in Physical Review Letters.

Secret of record-breaking superconductor explained

Conventional superconductivity can occur at much higher temperatures than previously expected, according to calculations made by an international team of physicists led by Matteo Calandra of the IMPMC Institute in Paris. The researchers have developed a theoretical model for the record high-temperature superconductivity reported last year in hydrogen sulphide, which the team says arises from relatively simple interactions similar to those underlying conventional low-temperature superconductors. This is different to other high-temperature materials in which the superconductivity is caused by complicated and poorly understood processes.

Low-temperature superconductors are usually well described by the BCS theory of superconductivity, whereby interactions with lattice vibrations called phonons cause electrons to pair-up to form “Cooper pairs” that can travel through the material without encountering any resistance. Such materials stop superconducting above a transition temperature (T_C) fairly close to absolute zero – the highest to date being just 39 K. High-temperature superconductors, in contrast, have transition temperatures up to 133 K.

Despite the vast amount of research done on high-temperature superconductors since the first such material was discovered in 1986, much of the physics underlying their superconductivity remains unknown. This mystery appeared to deepen late last year when Mikhail Eremets and colleagues at the Max Planck Institute for Chemistry in Mainz, Germany, found that when hydrogen sulphide is subjected to extremely high pressure (200 GPa) it has a T_C of 190 K. While the T_C of high-temperature superconductors can be increased by applying pressure – the current record is 164 K – hydrogen sulphide looks set to become the new record-holder if the measurement can be confirmed.

Conventional yet high temperature

The strange thing about hydrogen sulphide is that – unlike other high-temperature superconductors – it does not also exist in a magnetic state, and therefore more closely resembles a conventional superconductor. This observation led Calandra and colleagues in Canada, China, France, Spain and the UK to use BCS theory as the starting point for their calculations.

Key to understanding superconductivity in hydrogen sulphide are the interactions between electrons and the vibrating hydrogen atoms. Hydrogen has a very low mass and therefore tends to vibrate at relatively high frequencies. These high-frequency modes interact very strongly with electrons and so should result in a superconductor with a very high T_C. Indeed, when Calandra and colleagues used BCS theory to calculate the T_C of high-pressure hydrogen sulphide, they obtained a value of about 250 K – much higher than the observed 190 K.

The team believes that the actual T_C is somewhat lower, because basic BCS theory assumes that the atoms in the material vibrate as simple harmonic oscillators. However, light atoms such as hydrogen undergo more complicated anharmonic oscillations, and this can weaken significantly the interactions that create Cooper pairs. After taking anharmonic effects into consideration in their calculations, Calandra and colleagues calculate a much more realistic T_C of 194 K – in close agreement with Eremets’ measurement.

Upping the pressure

The calculations also suggest that the interplay between anharmonic effects and other properties of the material will result in the T_C remaining constant in the pressure range 200–250 GPa. While observing this effect in the lab would be a good test of the calculations, Calandra says he is unaware of any measurements above 200 GPa. Indeed, he points out that the 200 GPa experiment was extremely difficult to make, and that Eremets and colleagues are probably the only researchers capable of studying hydrogen sulphide at higher pressures.

“Eremet’s discovery and our theoretical work pave the way for the quest for high-T_C superconductivity in hydrides and hydrogen-based materials in general,” says Calandra. “In this class of materials it should be possible to find superconductors with a T_C of the same order (or maybe more) than hydrogen sulphide at high pressure,” he adds.

Elisabeth Nicol of the University of Guelph in Canada is enthusiastic about the results. “What is amazing is that this says we can actually have an electron–phonon superconductor that operates at 190 K,” she says. Nicol, who was not involved in the calculations, adds that “While technically the theory of superconductivity itself does not put a limit on T_C, consensus has been that electron–phonon superconductors have low T_C. Clearly, we are learning that there are still possibilities out there for conventional superconductivity.”

The work is published in Physical Review Letters.

Hubble at 25, Star Trek selfies on the ISS, Wu-Tang Clan physics and more

Hubble's official 25 anniversary image of the Westerlund 2 cluster — Shine on: Hubble’s official 25 anniversary image of the Westerlund 2 cluster. (Courtesy: NASA, ESA, (STScI/AURA), A Nota (ESA/STScI), Westerlund 2 Science Team)

25 years ago today, the ESA/NASA Hubble Space Telescope (HST) was launched aboard the Discovery space shuttle and since then, it has changed the face of observational astronomy as we know it; taking millions of people worldwide from their homes to the most distant and far-flung reaches of the universe and the imagination. The telescope has also been instrumental in some of the biggest, Nobel-prize-winning discoveries in physics in the past two decades, including that of the accelerating expansion of the universe. The stunning image above of the giant cluster of nearly 3000 stars dubbed “Westerlund 2” was especially released yesterday to celebrate Hubble’s 25th anniversary. The stellar nursery is difficult to observe because it is surrounded by dust, but Hubble’s Wide Field Camera 3 peered through the dusty veil in near-infrared light, giving astronomers a clear view of the cluster. Once you are done staring in awe at the image, watch the short video below, put together by NASA on the HST’s lifetime.

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IYL 2015 gets Swiss design makeover

Posters inspired by IYL 2015 — IYL 2015 is the theme of this student poster project.

One of the big aims of the International Year of Light (IYL 2015) is to take scientific ideas out of the lab to show the world just how inspiring and useful they can be. In the process, it can forge relationships between different communities, including scientists, engineers, artists, journalists, architects, politicians, aid workers…the list goes on.

Here in Bristol, where Physics World is produced, we’ve seen a fantastic local example of this by way of an art project at the University of the West of England (UWE). Second-year graphic-design students were set the brief of creating posters themed on IYL 2015. Last night we hosted an evening at IOP Publishing headquarters to showcase the students’ work and to let them find out more about science publishing.

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Tunable plasmon laser could sniff out cancer

Researchers in the US say they have succeeded in tuning the wavelength of light emitted by a tiny laser made from plasmonic nanocavity arrays. The plasmon laser, which consists of arrays of gold nanoparticles surrounded by dye molecules dispersed in a liquid, is robust and can operate at room temperature. An important use for the new device could be detecting cancer biomarkers at very low concentrations, say the scientists.

Normally, the diffraction limit means that light cannot be focused to a spot smaller than half its wavelength – about 250 nm for green light. This puts a limit on how small a laser or other optical device can be made. But in recent years, scientists have managed to compress light down to much smaller length scales by coupling it to surface plasmons – conduction electrons that oscillate collectively at the surface of a metal. A similar effect occurs in arrays of tiny metal nanoparticles, where the resulting excitations of light and electrons are called “lattice plasmons”.

Most plasmon-based lasers operate at fixed wavelengths, and it is very difficult to adjust the wavelength, especially in real time. This is because the wavelength of a plasmon laser is defined by its gain material, which is normally a solid such as an inorganic semiconducting nanowire or an organic dye in a solid matrix.

Liquid gain materials

Now, researchers led by Teri Odom of Northwestern University say that they may have found a way to make a tunable plasmon laser by filling a plasmonic nanocavity array with a gain material that is a liquid solvent. This allows them to change the emission wavelength of the laser by adjusting the refractive index of the solvent used.

“Using liquid gain materials has two main advantages,” Odom explains. “The first is that organic dye molecules can readily be dissolved in solvents with different refractive indices. So, the dielectric environment around the nanoparticles can be tuned, which also enables us to tune the lasing wavelength in real time. Second, the fact that the gain materials are in liquid form allows us to manipulate the gain fluid within a microfluidic channel, which means that we can dynamically tune the lasing emission by simply using liquids with different refractive indices.”

And that is not all: the researchers say that their tiny lasers are easy to fabricate and can emit light over the entire gain bandwidth of the dye employed. “This means that with the same nanocavity structure, that is the same nanoparticle arrays, we can tune the lasing wavelength over 50 nm (from 860 to 910 nm) by simply changing the solvent the dye is dissolved in,” says Odom.

Molecular sensing

Xiang Zhang of the University of California, Berkeley, who was not involved in the project, says that making a tunable plasmon laser using microfluidics could lead to interesting practical applications. “Such a configuration could be very useful in biomedical diagnostics and molecular sensing in liquid environments. We could mix a cancer biomarker in the liquid gain, for example, and detect it with very high sensitivity, provided that heating is not an issue near the plasmon particles.”

Odom says that the tiny light sources could be used in ultrasensitive sensors to detect weak physical and chemical processes on the nanoscale. They might also be integrated in lab-on-a-chip devices, she adds.

The tunable plasmonic laser is described in Nature Communications.

This article first appeared on nanotechweb.org

Albert and Erwin: decline and fall

More than a century has passed since quantum theory began to pose teasing questions about how we interpret our world. Books abound that offer alternative views of the problems the theory raises, and Einstein’s Dice and Schrödinger’s Cat is another. It begins with a few central themes and then follows the later interaction-at-a-distance of two prominent figures who had their heyday in the early 20th century and lived on to become eminences grises.

As the author, Paul Halpern, acknowledges, the pairing of Albert Einstein and Erwin Schrödinger is a somewhat unbalanced one, at least in the mind of the wider public – Einstein the universal icon with the big hair, Schrödinger playing a central role for physicists but relatively unknown to others. Even the dilemma posed by Schrödinger’s cat is not widely appreciated; Halpern notes that it derives from an earlier example put forward by Einstein, one that involved exploding gunpowder rather than poisoned pets.

Einstein was a great and a good man, but perhaps he has already received rather too much biographical attention, in comparison with his contemporaries. We need a better sense of the individual researcher as a quasiparticle, moving and acting in a sea of influences from the wider scientific community. Accordingly, Halpern’s book contains a large cast of other characters, who appear in the potted history of physics that forms the background to his tale. However, much of that which feels fresh in the book lies in the recounting of Schrödinger’s adventures in Ireland, a country to whose shores he was driven by events in his native Austria and his distaste for the high tables of Oxford (which may have been mutual).

Schrödinger was brought to Ireland in 1939 by the country’s leader Éamon de Valera, who had a quixotic enthusiasm for mathematics and theoretical physics. To the very end, when he was nearly blind, de Valera would still struggle through textbooks on the subject. In earlier times, when he dominated public life, he could often be seen striding into the seminar room of the Dublin Institute for Advanced Studies – a centre created in his own image as well as that of its Princeton counterpart – to hear the latest developments. De Valera was enamoured of relativity, and according to some, he once sent a young civil servant to find out whether it could be applied to economic policy. I met the official in question many years later. He hotly denied it. Apocryphal or not, the story conveys an impression of life on the isolated offshore island where Schrödinger was beached – not quite a Crusoe, more of a Gulliver.

It is a pity, therefore, that this author does not have a better feel for Dublin life, and that some of his details seem unreliable (for example, the Trinity College physicist G F Fitzgerald is here referred to as “Edward”). His style is also racy, sometimes uncomfortably so: terms such as narky, cushy, bragging and mouthing off do not always sit well in the story of these two dignified intellectuals. However, the aspect of the book that saddens me most (but is common to most such popular writing today) is the total absence of the elegant mathematical formulae that are the very essence of theoretical physics. Granted, lay readers cannot be given an instant tutorial on technicalities; nevertheless, one might yet hope that a few excerpts could be exhibited, with some hint of their nature and sophistication. After all, a third of a millennium has passed since Leibniz and Newton introduced the kind of mathematics that – somewhat miraculously – is still used for quantum mechanics. Moreover, the reader who cannot admire a few symbols is unlikely to understand much of the author’s talk about tensors, determinants and complex numbers.

Einstein and Schrödinger rubbed shoulders and eventually locked horns over their respective attempts to produce new unified field theories. Neither was successful. In 1947 Schrödinger jumped the gun by prematurely and publicly announcing a major breakthrough in that quest. He did so in the sedate surroundings of the Royal Irish Academy, where I once proposed that the small box by the fire escape should carry the message “In case of boredom, break glass for key”. But on that day the room must have been charged with a frisson of anticipation: de Valera himself chose to attend. The subsequent over-reaction of the press in Ireland and abroad, though hardly a “press war”, was an embarrassment, and Schrödinger went on the retreat as his relationship with Einstein cooled. They renewed their friendship in due course, but the rest of the story is one of continued decline and fall.

Of more enduring interest to most of us is the scepticism both men shared regarding probabilistic interpretations of quantum mechanics. Their standpoint is outmoded today, but we are still uneasy about the relation of the quantum and classical worlds. Accordingly, John Bell makes an entrance towards the end of the book, which brings the quantum/classical conflict up to the present day. I once had the pleasure of accompanying Bell to a dinner in Trinity College Dublin. Referring to the implications of his theorem, I asked “Is there still a problem?” Not entirely sure what I meant by this conversational gambit, I did not expect a direct reply, but he was unequivocal. “Yes,” he said. “Very well,” I said. “Who will resolve it, the physicists or the philosophers?” “The physicists,” he replied.

Bell, like these two great predecessors, is no longer with us. We still wait for whoever will finally put Schrödinger’s cat at rest, not to mention the one who will solve the small problem of unifying gravity and electromagnetism. In the meantime, this book can be put on the reading list of those who have enjoyed The Theory of Everything and want to know more.

2015 Basic Books £18.99/$27.99hb 288pp

Carbon nanotubes bring aircraft manufacturing out of the oven

A paper-thin carbon-nanotube film that can heat and solidify the composite materials used in aircraft wings and fuselages, without the need for massive industrial ovens, has been developed by a team of researchers in the US. The film can be rolled onto industrial components to deliver uniform, controllable and efficient heating via conduction. When connected to an electrical power source, the heated film stimulates the polymer to solidify. The technique should provide a more direct, energy-saving method for manufacturing virtually any industrial composite, according to the researchers.

Large industrial components such as aircraft wings are often made of composite materials that have layers that must be bonded together. Bonding typically involves curing the composite materials at high temperatures in expensive, immobile ovens known as autoclaves. Heating metre-sized components to temperatures of several hundred degrees in autoclaves – large industrial vessels that treat materials using elevated pressure and temperature – is an energy-inefficient process: the ovens waste significant amounts of energy as they themselves must be heated before the thermal energy is carried to the components by convection.

Beyond the oven

The inspiration to use carbon nanotubes (CNTs) as conductive microheaters was based on previous studies, explains Seth Kessler, president of Metis Design Corporation in Boston, a spin-out company from the Massachusetts Institute of Technology (MIT). Together with researchers at MIT’s department of aeronautics and astronautics, the team developed a CNT-based microheater that can effectively be rolled over an arbitrary surface to provide direct heating. “We had been using carbon-nanotube-based resistive heating for de-icing applications and had then considered the possibility of using the same principle for curing,” he says.

The team’s “out-of-oven” approach avoids the use of autoclaves entirely, thereby allowing composite materials to be efficiently cured regardless of their size or shape, and irrespective of the availability of a nearby autoclave. Similar microheaters are already commercially available, but the researchers caution that “it’s not as simple as just buying material and pushing it on the surface”. Rather, significant engineering is required for each curing project to determine the appropriate resistivity and current-flow paths.

A thin sandwich

The researchers, led by Brian Wardle of MIT, first created a mesh of aligned CNTs, where each nanotube was approximately 400 microns long. Aligning the nanotubes ensured better electrical stability, which was critical since current needs to pass through the mesh to provide resistive heating. The group then added a copper mesh to create electrical contacts, and a composite surfacing film to ensure electrical insulation. Wardle and his team tested a roughly postage-stamp-sized sample of the film on a commercial, laminated composite commonly used in aerospace manufacturing. They attached a 30 V power supply directly to the two electrodes of the microheater and manually adjusted the input voltage to modulate the film’s temperature to yield a complete cure.

“We found about a 1000-fold difference in energy used for curing, resulting in a 50% cost reduction in the final production part,” says Kessler. Even though the group only tested a small piece of the mesh, the researchers envision that scaling up the size of the mesh to cover an entire aircraft wing will not present a challenge. “The larger the part, the more opportunity the current has to achieve a uniform front,” Kessler notes. “As long as the current flow is intelligently designed, the scale of the part is irrelevant.”

Furthermore, the extremely low surface density of the films (5^–10 g/m²) means that they can be simply left on the material after curing without worrying about the extra weight. Kessler told physicsworld.com that by leaving the films in situ, other multifunctional capabilities may be realized, such as damage-detection based on resistance changes.

As different composites require different temperatures in order to fuse, the researchers also tested how hot the CNT film could actually get before it failed. The team found that the film’s failure point was at more than 537 °C. In comparison, some of the highest temperature aerospace polymers require temperatures up to 399 °C in order to solidify. “We can process at those temperatures, which means there’s no composite we can’t process,” Wardle says. “This really opens up all polymeric materials to this technology.” The researchers are now working with industrial partners to find ways to scale up the technology to manufacture composites large enough to make airplane fuselages and wings.

The research is described in Applied Materials and Interfaces.

Amazing science demo four: Ball River Bobsleigh

This is the fourth in a series of “five amazing physics demonstrations” presented by science-demo guru Neil Downie and his adept assistant Matthew Isbell.

In this fourth demo from the series, Downie and Isbell go head to head as they race their homemade vehicles along a track lined with ball-bearings. Downie’s vehicle is a propeller-driven half-pipe, while Isbell has constructed a pressure vessel from a soft-drinks bottle and a valve. Watch the video to find out whose vehicle wins!

Ball River Bobsleigh

So what’s this all about? Believe it or not, ball-bearings make the world go round. Drive a car, ride a bike or travel by bus or train, and you’re rolling along on ball-bearings. Run an electric motor and – unless it is very small – rotation is assured by ball-bearings. In fact, these tiny metal spheres have been so important to industry that ball-bearing factories have been wartime military targets. This project shows how a vehicle moves on a bed of loose ball-bearings.

What bits and pieces do I need? You need a long length of plastic guttering that’s normally used to collect water draining off a roof – the kind that’s got a U-shaped cross section. Spread evenly inside the guttering lots of small, round glass beads about 3–4 mm in diameter and then tape up either end of the guttering to stop the balls getting lost. The simplest “vehicle” is a plastic soft-drinks bottle. Put a car-tyre valve in the base of the bottle and a pinhole in the screw cap so that, once you’ve pumped it up, air will be released in a continuous stream over a minute or two. Place a small weight in the bottle – such as toy figures as passengers – so that it doesn’t topple over. Another option is to make a vehicle from a smaller piece of guttering, to which you attach a propeller powered by a small electric motor.

How do I get going? Simply place the vehicle on the ball-bearings at one end of the guttering – and let go. With luck, the vehicle will quickly accelerate to a few metres per second, possibly swaying from side to side and with a few beads occasionally bouncing into the air.

And what physics will I learn? Ordinary “sliding” friction is huge – the force needed to move an object on a horizontal surface can be 30–60% of the vertical force (i.e. its weight). But “rolling” friction, which this experiment shows, is hundreds of times less and can be engineered to be less than 0.1% of the vertical force. The experiment also illustrates the complex physics of ball-bearings, which concerns how the surface of the bearings and the surface they’re rolling along deform. Ball-bearings come in many different forms, and to keep friction down, the key is to use materials such as steel that are hard yet spring back efficiently. Big balls are also better. As for the top speed of the vehicle, it depends on various factors, including rolling friction, air drag, the pitch of the propeller and the kinetic energy needed to accelerate the balls that it rolls over, which it doesn’t get back in full.

If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine

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Amazing science demo five: Double Doppler with Train Set

Double Doppler with Train Set