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Sounding out swarms

Nicholas Ouellette likes midges. Yes, these tiny flies are infuriating and can bite, but Ouellette, who’s a physicist by training, is intrigued by how and why these insects form giant swarms, sometimes thousands strong. We know the swarms are composed entirely of male midges, which have long antennae and beat their wings at nearly twice the frequency of the females. Attracted by the high-pitched sounds, the females fly towards the swarm in the hope of reproducing, which makes swarming an elaborate midge-mating ritual.

The sensitivity of midges to sound was allegedly discovered by a Finnish ecologist in the 1960s while out walking in the woods. As he sang local folk songs, the ecologist noticed swarms of these flies being irresistibly drawn into his path, seemingly by the sound of his voice. Ouellette knew, however, that he’d need something more scientific than singing folk songs if he was to study swarming using sound. He therefore got one of the postdocs in his lab at Stanford University – Rui Ni (now at Pennsylvania State University) – to track midges with a microphone and record the beating of their wings.

When Ni and Ouellette blasted the buzzing of swarming midges back at them through a loudspeaker, they noticed some unusual things. If they alternated the level of sound played back through the speaker – loud, soft, loud, soft – the region of highest midge density shifted with the change in volume. And when they played just the sound of a female through the speaker (you can easily spot the females as they lack antennae), the entire male swarm flew over and sat on it.

Fascinated by flocks

Ouellette is one of a growing number of scientists seeking to understand how such “collective behaviour” comes about. Having previously worked on pattern formation and coherent structures in fluid flows, Ouellette became fascinated by biological swarms after noting the unusual patterns formed by flocks of starlings in flight. “It looked to me like turbulence,” he recalls, pointing to the large eddies that form in the Gulf Stream as an example of the phenomenon. “They dramatically appear out of the flow itself. So there must be some kind of mechanism that leads to the formation of these kinds of structures in animals.”

But swarm scientists are still puzzling over how such a mechanism might work and whether there’s a set of universal laws dictating such collective behaviour across a wide range of biological systems. That’s the nagging question, because while flocking and swarming are common in nature, each species that exhibits such collective behaviour does so just a little bit differently. A flock of starlings, a swarm of midges or a group of fire ants linking together to form floating rafts, for instance, are not quite the same thing.

Even the terminology is woolly. “Many people use the words flocking and swarming interchangeably,” admits Ouellette. “But since I work on both, I distinguish them by using ‘flocking’ to mean a group with net, ordered motion and ‘swarming’ to mean collective motion without any overall order or net motion. Much as I try, though, that’s still not standard usage.” Nevertheless, Ouellette is convinced there is something universal about all these systems – some aspects that don’t depend on exactly which animal you are studying.

For Chad Topaz – an applied mathematician from Williams College in Massachusetts who works independently of Ouellette on models of locust swarms – it all boils down to three questions. What do the individuals do? What does the group do? And how are those two things related? “These are such simple questions, and yet they are very difficult to answer,” Topaz says.

Gaining a better understanding of swarms is not just an intellectual challenge. It could also reap dividends for society by, for example, leading to improved crowd-management strategies for subways, concerts, rallies and other places where lots of people gather. Such work could in addition give scientists a unique approach to designing complex networks that are resistant to failure. An electric power grid, for example, can fail catastrophically if just a single pylon is down, while all it takes for planes to be delayed throughout the US is for a winter storm to knock out one key node in the airline industry’s hub-and-spoke flight network.

“We are not good as human beings at designing controlled and distributed systems,” Ouellette admits. Nature, in contrast, seems to have solved this conundrum in flocks and swarms, which don’t have the same choke points. If a few birds fall out of formation as a big flock of starlings migrates, the overall dynamics do not change. There is no central node – no top-down mechanism – and yet a form of controlled order does emerge in such collective systems. “It’s bottom-up instead of top-down control,” says Ouellette.

Swarm into action purlieu

Swarming research has been firmly in the wheelhouse of observational biologists for decades, when they would monitor swarm behaviour in the field and carefully write down their observations. It wasn’t until the 1980s, however, that computer graphics specialist Craig Reynolds developed what became the canonical computational model for collective behaviour: the so-called “boids program” – an agent-based simulation that gets its name from “bird-oid” (or “bird-like”) objects. The program became a staple in Hollywood, being used to model the movement of groups of computer-generated bats in Tim Burton’s Batman Returns, as well as the movement of combatants in major battle scenes in the Lord of the Rings trilogy.

Schematic diagrams illustrating Craig Reynold’s computer model of co-ordinated animal motion — **1 Model behaviour** In 1986 the software engineer Craig Reynolds wrote a pioneering computer model of co-ordinated animal motion, such as bird flocks and fish schools, based on 3D computational geometry. He called the generic simulated flocking creatures boids, which is short for “bird-oid” (or “birdlike”) objects. The basic flocking model consists of three simple steering behaviours that describe how an individual boid (green triangle) moves based on the positions and velocities of nearby flock mates (blue triangles within the circle). The boids can (top) steer away to avoid crowding, (middle) steer towards the average heading of local flock mates to create alignment or (bottom) move towards the average position of local flock mates to create cohesion. The program was later used to model the movement of groups of computer-generated bats in various Hollywood films, including *Batman Returns*.

The basic concept is simple (figure 1). First, treat each individual in the swarm as a dot (or particle) initially moving in a straight line at constant speed. Then programme in a few simple rules governing interactions between those dots. For instance, if two dots move too close together, they must move apart to avoid colliding, but if the distance between them becomes too great, they must get nearer again. When the collection of dots becomes sufficiently dense, a flocking pattern will form. Tweak the rules, and a pattern emerges that resembles a swarm of midges or locusts. Yet another set of rules will give you a pattern similar to a raft of fire ants.

These kinds of models have dominated research into collective behaviour, but Ouellette thinks such an approach is insufficient – making an exact solution well-nigh impossible – given that such systems are highly nonlinear and have anywhere from several hundred to a billion moving parts. But even knowing which pair of variables have a nonlinear dependence isn’t clear since, as Ouellette puts it, “we don’t know what parameters matter in the first place”. The bottom line is that the combined effect of all those individuals in a nonlinear system adds up to more than the just the sum of those parts. “You get something other than a simple average over their individual states.”

Ouellette considers swarming a classic inverse problem. Scientists have collected huge amounts of data on flocks and swarms, and are now trying to work backward to ferret out the underlying rules. But even if you have the rules, it doesn’t mean you understand how animals behave. “You can say, if I make these modelling rules, it looks kind of like a flock of birds,” explains Ouellette. “But there’s a very big divide between that statement and saying ‘Birds behave this way.’ ”

His solution? A macroscale, big-picture approach that draws as much from thermodynamics and materials science as it does from particle modelling and statistical mechanics. “I’m starting from a stereotypical physicist’s approach: everything is probably the same stuff in an underlying way if you tease it apart and cast it in the right way,” Ouellette says. But rather than turning to computational modelling, Ouellette instead works with actual midges, treating the swarm like a chunk of material – probing it by doing something to it and seeing how it reacts. As he points out: “You don’t test materials by observing them. You do something to the material and measure its response.”

Photograph of fire ants working together to form a bridge — Joint effort: Fire ants work collectively in a group showing behaviour almost like a liquid. (Courtesy: Ant Laboratory, Georgia Institute of Technology)

It’s an approach shared in part by David Hu, a physicist at Georgia Tech in the US, whose lab is justly famous for its experiments with swarms of fire ants. The swarms, which can consist of 100 or more individuals, have both solid and liquid properties. By linking their bodies together, the ants form floating rafts, towers and other solid-like structures, but collectively they also flow like a fluid. One YouTube video from Hu’s lab even shows the ants pouring themselves out of a teapot into a teacup. The lab uses standard materials tools such as rheometers to apply various kinds of forces to the ant swarms to see how they collectively respond (Nature Mater. 15 54).

Midge master

In his lab, Ouellette works with small swarms of non-biting midges Chironomus riparius, which are an ideal choice from a physicist’s perspective as they are so simple. Lacking digestive systems, adult midges don’t eat and instead conserve all their energy for their swarmy mating rituals. What’s more, these particular midges don’t spread disease and, should they escape, they die within a few days.

They are also bred commercially in labs that provide customer support – a surprisingly useful feature if you’re a physicist like Ouellette who’d never dealt with living specimens before.

“The first couple of tries, I’d call them up and say, ‘Okay, everything died again, what did we do wrong this time?’ ” he recalls. “And they’d say ‘Well, did you oxygenate the water?’ Because little things like that matter.” Eventually, Ouellette mastered the art of breeding his midges, and his experiments began in earnest. Since swarming is triggered by light, he’s fixed up a lamp that switches on automatically for an hour twice a day. Each time the light flicks on, gangs of 25–30 midges swarm for the full hour until it turns off again. “It’s a really robust system,” says Ouellette. “Twice a day we get swarms.” Pieces of black cloth simulate the terrestrial features that midges like to swarm over, such as stumps or tree roots or small pools of water.

Experiments typically involve manipulating the conditions to see how swarms respond, like blasting those recordings of male or female midges – the changes in volume mimicking the application of an oscillating magnetic field to a material. Another experiment involves placing two black cloths together to draw a swarm and then slowly pulling them apart – the swarm splits into two distinct swarms, exhibiting a property akin to elasticity or mechanical strength in materials. In each case, Ouellette uses high-speed cameras running at 100 frames per second to determine the position, velocity and acceleration of each insect, and uses a particle-tracking computer program to recreate the individual trajectories. Then it becomes a matter of analysing that data in such a way as to extract the large-scale swarm features.

Trajectories of 30 midges tracked over 20 seconds — Buzzing around: 20-second clips of the trajectories of 30 midges in one particular swarming event taken by high-speed tracking cameras in Nicholas Ouellette’s lab. (Courtesy: Nicholas Ouellette)

Ouellette’s most recent analysis of the data from his lab-based swarms yielded a result strongly analogous to a liquid vapour-phase coexistence ( Eur. Phys. J. Special Topics 224 3271). The core of the swarm is consistently in a condensed phase (like water molecules in liquid form), with a more dilute “vapour” phase around the edges (like water molecules in gaseous form). Individual midges can, however, wander back and forth between the two phases. Such findings are intriguing, but fall short of the ultimate goal of a universal set of rules for swarms.

Craig Tovey, who works with Hu at Georgia Tech modelling swarming behaviour in fire ants, honey bees and other systems, has observed a couple of common principles that show up in multiple cases. First, he has found that the models do not rely on past history to make predictions for the future, depending instead just on current conditions. That not only makes life easier when analysing such systems, but also makes sense biologically since ants, bees, fish, midges and other swarmy creatures do not have long memories. They would naturally respond to cues in their immediate vicinity to determine their next move.

Second, Tovey has also noticed that randomness plays a balancing role in many swarming models. With fire ants, for example, each individual insect travels in a random direction, but eventually the group forms a roughly circular raft. “You’ll get roughly the same number going in different directions to form these circular shapes – without any individual ant knowing what it or the others are doing,” he says. That randomness explains how the fire ants can build complicated structures without any of them having any sense of the overall structure of the tower or raft or whatever else they’re building.

Tovey also suggests that evidence of scaling in the data could provide a vital clue to the question of whether a universal set of rules might exist governing swarms. Indeed, Andrea Cavagna and Irene Giardina – physicists at the Institute for Complex Systems in Rome, Italy – have already found evidence of this kind of scaling in their own studies of midge swarms ( Nature Phys. 13 914). Unlike Ouellette, they study midges in the wild, where the swarms are much larger (with up to 1000 individual midges), providing a complementary approach. By tracking the midges in 3D with high-speed cameras, Cavagna and Giardina found that as individual midges group together and get larger and denser, they interact more, with the correlations increasing sharply with density. When enough midges gather in sufficiently close proximity, a swarm develops. This observation means that swarming behaviour is an “emergent” property and could therefore be described by scaling laws.

If, for example, the number density of insects in a swarm remains constant as the swarm grows, which is roughly correct for the swarms Ouellette has studied, then there would be a scaling relationship between the number of individual midges and the swarm volume. Such a relationship would be useful as it would then let you predict the volume of a swarm that would be formed by a different number of individuals, which you could, say, use to design an enclosure to hold swarms of a particular size.

“If we were to find a scaling law, that would be a powerful thing for interpreting the data, because it would allow us to make a prediction about how a swarm of different size, for example, would behave, even though we haven’t measured it,” says Ouellette. Nevertheless, he is cautious about whether such laws actually exist. “I think the evidence for scaling in swarms is still pretty tenuous,” he admits. “A lot more work needs to be done before I would trust scaling in collective behaviour too much.”

That hasn’t held back Cavagna and Giardina, who say their experiments found that scaling holds across different swarms of three different species – each with different sizes and densities, recorded on different days over the course of a year. Their analysis, however, doesn’t amount to a true universal class of behaviour, since the existing models do not describe the specific dynamical behaviour of swarming midges. Whatever is regulating how each individual moves and interacts with others in the swarm at the microscopic level remains hidden. “We have found these systems obey these very non-trivial laws, which means we can hope to treat [swarms] with simple models in the same way as is done in physics,” says Giardina. “And we also learned that there is still something we have not yet captured in the modelling. That is the next step.”

As for Ouellette, he thinks his approach could yield even more progress by connecting the various lines of ongoing swarm research. While he acknowledges there may be some scepticism from certain quarters, he thinks he can win people over to his viewpoint: “If you’re loud enough, people eventually take notice.” A bit like buzzing midges in fact.

Ocean surface could be plastic-free within three years once littering stops

Once a plastic bag reaches the ocean, how long does it hang around at the surface? And when the bag starts to disintegrate, what happens to it next? Questions like these are notoriously hard to answer, because Earth’s oceans are huge, and plastic rubbish is unevenly spread and comes in a variety of shapes and sizes. However, after modelling ocean plastic a study indicates that the ocean surface could be plastic-free within an average of three years if we were to stop plastic entering the oceans tomorrow.

Recently it was revealed that just ten river systems transport more than 90% of plastic waste to the world’s seas. These large rivers, including the Nile and the Ganges, run through heavily populated regions where littering is common. The rivers act as superhighways, transporting plastic waste swiftly out to sea.

But once this plastic gets to the sea we have very little idea of what happens next. The huge whirlpool of plastic found in the North Pacific Gyre – often called ‘the Great Pacific garbage patch’ – shows that waste concentrates in some areas. Meanwhile, marine surveys indicate that plastic breaks down and sinks at different rates, depending on the size, density and shape of the piece of plastic. Beyond this very little is known about the fate of plastic entering our oceans, or indeed how much of the plastic we see bobbing the waves today is rubbish from the distant past.

Albert Koelmans from Wageningen University in The Netherlands and his colleagues took a systems engineering analytical approach to create a whole ocean mass balance model of plastic in the oceans. Using estimates of the amount of plastic entering the oceans each year, along with mathematical equations to calculate how quickly plastic fragments and sinks, the scientists gained a global perspective of what happens to the plastic in our oceans.

Their results suggest that 99.8% of all plastic that had entered the oceans since 1950 had sunk below the surface by 2016. For the year 2016, the model predicted that of the 0.309 million tons of plastic in the surface layer, 83.7% was macro-plastic, 13.8% micro-plastic and 2.5% nano-plastic – less than 0.335mm in diameter. If all plastic disposal to the oceans was stopped tomorrow, the model predicts that ocean surfaces would be near plastic-free within three years.

“It is important to emphasize that these figures are averages,” said Koelmans. “Small plastics will sink earlier, whereas floating sun-loungers will take much longer to fragment into settleable particles.” The findings are published in Environmental Research Letters (ERL) .

Getting a handle on what happens to plastic waste when it reaches the ocean is important if we want to optimise our strategies for tackling the plastic problem in future. “Our model suggests that the response time for plastic in the ocean is actually quite short, which favours strategies to reduce emissions of plastic, over strategies that aim to clean up the plastic that is already there,” said Koelmans.

However, Koelmans and his colleagues stress that more data is needed to understand plastic behaviour in the water better. They intend to explore the impact of cleaning up ocean plastic in their next version of the model.

Could bee glue reduce infections on replacement joints?

Hydroxyapatite (HA), a mineral naturally found in bone, is used therapeutically to replace bone and to coat prosthetics, but can become infected once in the body. Eliana Cristina da Silva Rigo and her research group have investigated antibacterial compounds from propolis, a bee product used in traditional medicine, transferred onto HA. Rigo and her team extracted antibacterial compounds from red and green Brazilian propolis, applied it to HA, and evaluated the antibacterial activity (Biomed. Mater. 13 025010).

Propolis is a glue that bees produce to fix their hives, seal alternative hive entrances and ward off microbes such as fungi and bacteria, and also mites. It has been used in traditional medicine for a long time and has only recently been analysed scientifically. Of its various antimicrobial activities, the antibacterial activities are understood best. The team of researchers from the Universidade de São Paulo found that antimicrobial peptides extracted from propolis can be applied to HA used in implants.

Can propolis prevent infections?

HA is a bioceramic material that is mainly used in the clinic to fill bone defects. New knees, hips, teeth and the like are necessary to maintain quality-of-life in old age, as well as after diseases and injuries, but they bear a risk of infection. Antibiotics are given to avoid infections, however many bacteria, especially the prevalent Staphylocci and Escherichia coli, have recently acquired multidrug resistances. HA loaded with antimicrobials such as those extracted from propolis might provide a way of preventing these infections.

The authors found that antibacterials released from propolis-loaded HA can kill Staphylococcus aureus, which – together with Staphylococcus epidermis – accounts for 50 to 60% of infections in joint replacements. In their experiments, all bacteria were killed within one hour. Whether this holds true for other bacteria and in vivo experiments remains to be investigated.

As opposed to a previous study that used HA pellets, in this work, the team used a HA powder that allows the antimicrobials to be applied by spray drying rather than by immersion. This seemed to cause a difference in the timing of the release of the antimicrobial compound. Therefore, different packaging modes of loaded HA might provide a means to control the speed of release of antimicrobial activity.

Red or green propolis?

While propolis is generally brown, its colour can veer towards red, green, black or white according to the season, hive and geographical location of the bees. These colour differences seem to be due to different tree resins that the bees find and use to make propolis. Of the red and green propolis investigated in this study, red propolis had the stronger bactericidal activity, although green propolis also exhibited potent bactericidal effects.

As propolis in general, and specifically red propolis, exhibits potent antimicrobial activity, it will be exciting to see future work that tests these effects in vivo and that moves from HA powder to prosthetics, such as joint replacements.

Satellite galaxies of Centaurus A defy dark-matter model

Most of the small satellite galaxies orbiting the large galaxy Centaurus A rotate in the same direction in a well-defined plane. This coordinated motion is contrary to predictions made by the cold dark matter (CDM) model of structure formation in the universe. The finding was made by an international group of astronomers and is consistent with previous observations in the Milky Way and Andromeda galaxies. The team suggests these puzzling observations should prompt astrophysicists to consider alternatives to CDM. Other astrophysicists, however, are cautious, and suspect that the results will ultimately be explained within the dark-matter paradigm.

Conventional wisdom says that a network of filaments of dark matter pervades the universe. Large galaxies form and grow as multiple dwarf galaxies are drawn in along these filaments. “The satellite [galaxies] will fall into the potential of their host galaxies from different directions,” explains team member Oliver Müller of the University of Basel in Switzerland. “They have all these different initial directions, so they will have all different orbits,” he adds. Observations, however, show that most satellite galaxies around the Milky Way, and the nearby Andromeda galaxy, orbit in step in single planes. “There were explanations saying ‘Oh, well: our local group – which consists of the Milky Way and Andromeda – is just a peculiar case,'” explains Müller.

Rich assembly

In the new research, Müller and colleagues in Germany, the US and Australia, looked at satellite galaxies in the Centaurus group. This is a rich assembly of satellite galaxies orbiting the large elliptical galaxy Centaurus A, which is 13 million light-years from Earth – about four times distant as the Andromeda galaxy. Previous research had established that the satellite galaxies lay in a single plane. To measure the motions of the satellites, the team used optical measurements from the Hubble Space Telescope to locate the position of several satellite galaxies in this plane. They then used ground-based radio astronomy measurements of specific atomic transition lines in the galaxies’ emission spectra.

Of the 16 galaxies that the researchers analysed, 14 followed a pattern whereby those on one side of Centaurus A were red shifted relative to Centaurus A, whereas those on the other side were relatively blue shifted. This suggests that the 14 were all rotating in the same direction. If the orbits had been random, the chance that 14 or more would have ended up rotating in the same direction was just 0.42%.

Simulations suggest that the apparently common occurrence of synchronized orbits among the satellite galaxies in galaxy groups could be explained if the host galaxy was formed by the collision and merger between two other galaxies. In this model, the satellite galaxies would not be leftover primordial material from the formation of the host galaxy, but rather debris thrown out by tidal interactions during the merger. Researchers have only recently begun to consider whether such a process could fit within the dark-matter model or whether an entirely new model of cosmological structure would be required, says Müller.

Perpendicular motion

“I think that their findings are interesting and extend some hints of problems that we’re seeing with other results into a new regime,” says astronomer Michael Boylan-Kolchin of the University of Texas at Austin. However, he cautions against assuming that the dark-matter model necessarily predicts the distribution of satellite galaxies would be random: “In [the dark-matter model] the structure around Centaurus A also depends on the details of the structures on much larger scales,” he explains. He suggests a useful follow-up study would be to look for motion perpendicular to the planes to find out whether the planes are stable over time: “It’s very difficult to do,” he says, “but it’s certainly possible for Andromeda with the Hubble Space Telescope”.

“It’s very puzzling, and very interesting – and it may possibly have something to say about the dark sector,” agrees astronomer Rodrigo Ibata of the Observatory of Strasbourg in France. He suggests that astronomers will still want further confirmation in other systems, but that, assuming the results hold up, he suspects a tweak to one of the underlying assumptions about galaxy formation will make the results seem less improbable. “But the whole thing may be much more interesting,” he says. “It may be that there’s something inherent about the way gravity works that we’re missing here, and on galactic scales, there’s some funky stuff that happens.”

The research is described in Science .

Physicists create droplets inside bubbles

Physicists in China have invented a way of encapsulating a liquid droplet within an air bubble. The technique could lead to the development of new ways of delivering drugs and processing materials.

The formation of bubbles and droplets has long fascinated physicists and understanding the processes involved has guided the development of technologies as varied as inkjet printers and marine propellers. Bubbles and droplets can occur together in several configurations. A soap bubble in air, for example, can be thought of as an air bubble encapsulated within a droplet of soapy water.

New configuration

Now, Xin Fu and colleagues at Zhejiang University claim to be the first scientists to create and characterize a new bubble-droplet configuration that they have dubbed “drop encapsulated in a bubble”. In this scenario, a droplet is formed within an air bubble, which itself is formed within a liquid (see figure).

The structures are made by driving liquid (a glycerol-water solution) and air along a tiny tube and into a tank of fluid. The liquid and air undergo “Taylor flow”, which means that alternating “slugs” of liquid and bubbles of air travel along the tube. When a bubble emerges into the tank, it is pushed off the nozzle by the following slug of liquid in the tube. Under certain conditions, the slug will penetrate the bubble and become encapsulated within it.

Whether or not encapsulation occurs depends upon the Ohnesorge number of the liquid (the ratio of surface tension to viscosity) and the height-to-width ratio of the liquid slug when it enters the bubble. Fu and colleagues also found that encapsulation is robust against sheer flow and that the liquid is released when a bubble adheres to a solid surface.

The encapsulation process is described in Physical Review Letters.

The February 2018 issue of Physics World magazine is now out

By Matin Durrani

You’ll be delighted to know that the February 2018 issue of Physics World is now out in print and digital format.

In our cover feature this month, science writer Jennifer Ouellette discovers how tiny midges can shed light on collective behaviour in physics. You can also discover from Helen Gleeson how physicists are seeking to create liquid-crystal contact lenses – and learn from Stephen Ornes more about the technological challenges of going to Mars.

Don’t miss either our interview with the physicist who heads Japan’s leading funding agency and find out from astrophysicist Elizabeth Tasker about her experiences as an overseas scientist in the country.

And our veteran North America correspondent Peter Gwynne assesses the impact that Donald Trump has had on US science, one year into his presidency.

Remember that if you’re a member of the Institute of Physics, you can read the whole of Physics World magazine every month via our digital apps for iOS, Android and Web browsers.

Let us know what you think about the issue on Twitter, Facebook or by e-mailing us at pwld@iop.org

For the record, here’s a run-down of what else is in the issue.

Physics World February 2018 cover — *Physics World* February 2018

• Donald Trump: one year on – When Donald Trump took over as US president, many scientists feared for the worst. One year into his presidency, those concerns look set to
continue, as Peter Gwynne reports

• Japan’s funding supremo – Yasuhiro Iye, a condensed-matter physicist who is executive director of the Japan Society for the Promotion of Science, talks to Matin Durrani about the pressures and challenges facing Japan’s biggest funding agency

• Protecting innovation – Mark Saunders argues that in a post-Brexit Britain, the UK
economy should be safeguarded by providing sufficient protection of innovation

• The business of physics – In this new series of columns, James McKenzie aims to raise the profile of industrial physics and explore the value and relevance of physicists to industry

• Collecting books – An important document collection is about to transform the history of physics holdings at the Niels Bohr Library, says Robert P Crease

• The long road to Mars – With ambitious plans being dreamed up to send astronauts to the red planet, what challenges would face them if they ever went – and what technology would they need? Stephen Ornes finds out

• Looking – into the future – Having to wear reading glasses as you get older may soon be a thing of the past. Helen Gleeson describes her team’s research into liquid-crystal contact lenses that will be able to switch focus and restore youthful vision

• Sounding out swarms – Swarms of midges might be an annoyance if you’re out walking in the woods, but for physicists they offer unique insights into collective behaviour, as Jennifer Ouellette finds out

• Boldly going to a galaxy far, far away – Andrew Glester reviews Treknology: the Science of Star Trek from Tricorders to Warp Drives by Ethan Siegel and The Physics of Star Wars: the Science Behind a Galaxy Far, Far Away by Patrick Johnson

• Of graphs and giggles – Tushna Commissariat reviews Festival of the Spoken Nerd: Just for Graphs – a DVD from Laughing Stock Studio/Trunkman Productions

• Tales from a British physicist in Japan – From cultural differences to grant glitches, Elizabeth Tasker describes the good, the bad and the confusing of working as a physicist in Japan

• Once a physicist – meet Anna Starkey is the creative director of We The Curious
– an interactive science centre in Bristol, UK

• The positive side of negative marking – Colin White from the University of Portsmouth recalls an unusual experience as a physics examiner

And don’t forget, if you have any thoughts on the issue do let us know on Twitter, Facebook or by e-mailing us at pwld@iop.org.

Folded graphene boosts energy storage in lithium-ion batteries

The energy storage capacity of lithium-ion batteries can be significantly increased thanks to an anode made from folded graphene. A device with a mass loading of 5 mg/cm² of the graphene has an areal capacity of more than 4 mAh/cm², which is well above that of commercial graphite anodes, and it can withstand at least 500 cycles of battery charging/discharging without any loss in its performance. The simple folding strategy could easily work for other electrode materials too, say the researchers in the Republic of Korea who report on this work.

“Small-sized, high-energy density batteries will increasingly be needed in applications such as portable electronics as well as in large-scale energy-storage devices, such as electric vehicles,” explains co-team leader Rodney Ruoff and Soojin Park of the Institute of Basic Science (IBS) in Ulsan and the Ulsan National Institute of Science and Technology (UNIST) and the Center for Multidimensional Carbon Materials (an Institute for Basic Science at UNIST), South Korea. “To reduce battery size, electrodes with high areal and volumetric capacities are crucial, but the problem here is that much space in these structures is currently being ‘wasted’ by inactive materials or by pores that are present in traditional electrode configurations.

“These electrodes can be made of graphene, for example, and contain freestanding graphene composite films comprised of densely stacked layers but they suffer from low areal energy storage because of poor electron/ion transport kinetics. Our folded electrode, which is also made from graphene composite films, shows much improved electron/ion transport and thus a high areal capacity over multiple recharging cycles.”

New paths and channels

Compared to traditional thick film electrodes, the paper-like folded electrode contains many continuous layered films. The folds at the edges of these films provide new paths through which free electrons can flow and the small gaps that exist between the folds form multiple channels through which lithium ions can diffuse. These paths and channels significantly increase areal/volumetric energy density. Indeed, a folded electrode made of tin oxide/graphene contains as much as 5 mg/cm² of active material while being just 20 microns thick. It also has a high areal density of 4.15 mAh/cm², which is much higher than that of commercial graphite anodes that have areal capacities of between just 2.50 and 3.50 mAh/cm².

The folded electrode is stable over 500 cycles of battery charging/discharging and boasts improve rate capability compared to thick graphene electrodes with the same mass loading of active material, but without folds.

No binders, carbon additives or metal current collectors required

“Folding the graphene electrode several times is in fact a way to thicken the electrode,” Ruoff and Park tell nanotechweb.org. “By increasing the electrode thickness in this way, while maintaining the electron transport efficiency of the electrode along the folds, is the reason why it has such as high areal/volumetric energy density.”

And that is not all: the researchers say that they can assemble their freestanding electrodes in a battery cell without having to resort to traditional methods, such as slurry coating, for making these electrodes. They can also completely do without binders, carbon additives and metal current collectors.

Lighter battery and higher energy density

“Our approach greatly lowers the weight of the final battery and increases its energy density,” explains team member Bin Wang. “For example, in a full cell (containing both an anode and cathode) made with the standard graphite anode and a standard LiCoO₂ cathode, the energy density is around 271 Wh/kg, but in our system of folded graphene anode and the same LiCoO₂ cathode, it goes up to 452 Wh/kg. Such a cell is also stable over 300 charge/discharge cycles and has an areal capacity as high as 2.84 mAh/cm².”

The good news is that the folding technique can be extended to other types of active materials and even other kinds of rechargeable batteries, not just Li-ion ones. What is more, new more intricate folding configurations, and not just simple back and forth folding as was done in this study, could also be possible, says Ruoff.

“In fact, the folding approach may even be useful for electrodes in supercapacitors,” adds team member Jaegeun Ryu.

“And in a separate research project on composite materials, we are trying to employ this type of folding to make thick samples from microscale films, including those made of composite materials. Such a bottom-up approach could be useful for controlling the structure and properties of the as-built materials,” adds Wang>

The new folded graphene anode is detailed in ACS Nano DOI: 10.1021/acsnano.7b08489.

Nano-ink-based sensors detect an eye blink

Through developing a graphene-nanosheet-based ink, collaborators at the University of California – San Diego and the National Taiwan University of Science and Technology have produced flexible, wearable, ultrathin sensors. Fabricated by printing multiple layers directly onto medical tape, the printed graphene sensors can be easily affixed to skin. Due to their high strain sensitivity, deformations to the sensor caused by subject movement can allow non-invasive health monitoring. Further, through combining multiple small sensor patches to make different geometries, the sensors accurately follow hand motions and can even monitor eye-blinking and pulse.

The development of wearable sensors utilizing nanostructured materials has been key to progress in achieving accurate physiological measurements. The main reason being that nanomaterial-based sensors exhibit greater flexibility and dexterity akin to the skin, unlike the more rigid electronics-based alternatives.

The sensor developed by Long Wang at the University of California and colleagues detects strains and deformations from the resulting measurable change in the resistance across it, which corresponds to the “amount” of deformation. Previous work had explored the strains involved when bending a finger covered in these sensors. Now by printing the sensors directly onto medical tape the researchers have shown how the sensors can monitor other parts of the body.

One example is the use of a rosette-shaped arrangement of several rectangular 12×1 mm multi-layer sensors to measure the change in strain distribution across the hand when using different muscle groups. Because the combination of different strain measurements across the group of sensors within the rosette is unique to each finger, the sensors can identify different finger motions.

The distinction between muscle groups is possible due to the high sensitivity of these devices, where sensitivity is defined as the normalized change in resistance compared with the corresponding change in applied strain. The printed graphene sensors also exhibit reproducible, reversible trends in their sensing ability for up to 1000 strain cycles, making them extremely durable.

How the sensors are made

Graphene nanosheet ink is key for the production of this multilayered sensor, as it makes patterning directly onto the medical tape possible. The researchers prepared nanosheets by following a highly efficient, low-cost and environmentally friendly water-assisted liquid-phase exfoliation procedure. They mixed graphite in a co-solvent solution and then sonicated for several hours at a fixed temperature and then centrifuged samples to separate out any undesired solids. They then mixed this graphene nanosheet solution to obtain a printable ink, which they printed directly onto the medical tape in 2D patterns with a resolution of 60 μm using a micropatterning instrument called SonoPlot.

What they can do

These sensors have also successfully demonstrated their capacity to monitor eye blinking; where the change in resistance arises due to sensor deformation as the eye closes. By monitoring deformations in the skin the researchers were able to measure the heart rate accurately enough to distinguish the increase during exercise compared with the resting baseline pulse.

Integrating this ultrathin and conformable sensing technology with medical tape presents a neat solution for the attachment of sensors to the skin. This research avenue is set to have a significant impact on human monitoring due to the versatility the approach offers for using these graphene-based sensors anywhere on the body. More information on this paper can be found in Nanotechnology.

Scientists discover new link to El Niño

Variations in Arctic stratospheric ozone can affect the tropospheric climate at middle-to-high latitudes in the Northern Hemisphere and all the way down to the tropics, according to researchers in China.

The result suggests a key stepping stone in the known connection between Arctic stratospheric ozone and the El Niño-Southern Oscillation (ENSO), and so could help to predict the ENSO, the researchers say.

“Oceanic, tropospheric, and stratospheric [variations] are all tightly coupled,” said Jianping Li of Beijing Normal University. “[Our study] reinforces the need for climate models to include fully coupled stratospheric dynamical-radiative-chemical processes if they are to more accurately simulate and predict future climate variations.”

The ENSO is a change in sea-surface temperatures and circulation patterns that takes place every so often over the tropical eastern Pacific. It is one of the biggest sources of inter-annual climate variability, and affects ecosystems, agriculture, freshwater supplies, hurricanes and other severe weather events worldwide.

Predicting the onset of the ENSO is difficult, as there is no known phenomenon that serves as a reliable signal. Last year, Li and colleagues reported a connection between Arctic stratospheric ozone and the ENSO, hinting that Arctic stratospheric ozone could serve as a predictor. But the connection appeared to take place over 20 months, whereas atmospheric signals are in general believed to be delayed by a few months at most. As a result, the finding raised the question of what other links might be in the chain.

Now with a slightly different group, Li has compiled data from multiple sources and performed model simulations to investigate the possibility that Arctic stratospheric ozone is connected to the ENSO via the North Pacific sea-surface temperature (SST). The researchers found that it is Arctic stratospheric ozone changes in March that are most strongly connected with the North Pacific SST, with a delay of about a month.

According to the theoretical analysis, the connection is causal. When the Arctic stratospheric ozone falls, the lower stratosphere in the Arctic cools, the researchers believe, boosting the southerly temperature gradient and, in turn, strengthening the stratospheric circulation. This circulation probably impacts the circulation in the troposphere downwards, they say, ultimately generating variations in the SST.

“The prediction of ENSO variations is very challenging work since it is different to find[ing] a predictor of the ENSO,” said Li. “This established connection suggests that the ASO [Arctic stratospheric ozone] may be able to serve as a potentially effective predictor.”

The connection does not come as a surprise – the depletion of stratospheric ozone in the Antarctic is already thought to influence the Southern Hemisphere tropospheric climate. Even so, Li and colleagues believe more work is needed to unravel the process. For instance, it is known that North Pacific SST variations lead the ENSO by about 12 months; even combined with the one month lag-time between Arctic stratospheric ozone variations and North Pacific SST variations, this cannot explain the 20-month lead time of ASO to ENSO.

“The complete mechanism for Arctic stratospheric ozone modulation of tropical SST still deserves thorough analysis… It is one subject of our next study,” said Li.

Li and colleagues published their findings in Environmental Research Letters (ERL).

Photon MLC collimates electron beams

Electron beams offer dosimetric advantages for treating superficial targets while sparing underlying organs-at-risk (OARs). Standard electron treatments, however, are still performed using generic or patient-specific cut-outs placed in the electron applicator. Replacing these cumbersome cut-outs with a multileaf collimator (MLC) would profoundly improve clinical workflow and enable advanced delivery techniques.

Previous studies have shown that electron collimation using a photon MLC (pMLC) can achieve similar quality plans to cut-out collimation, for treatments with a reduced source-to-surface distance (SSD) of 60–70 cm. The increased in-air scatter of electrons makes a short-as-possible SSD dosimetrically preferable. However, it could also lead to collisions for certain beam directions, making support of SSDs greater than 70 cm essential. With this aim, researchers in Switzerland have investigated the use of a pMLC for electron treatments with SSDs of up to 100 cm (Phys. Med. Biol. 63 025017).

“For standard electron treatments, the main benefit of switching to a pMLC is the improved clinical workflow, because patient-specific collimation would no longer require fabrication of cut-outs and won’t need to attach an accessory to deliver electron beams,” explained Silvan Mueller from Inselspital and the University of Bern. “On a bigger scope, it would lay the foundations for the implementation of advanced techniques such as modulated electron radiotherapy (MERT) or mixed beam radiotherapy (MBRT).”

Calculation accuracy

To predict dose distributions of pMLC shaped electron beams, the researchers used a multiple source Monte Carlo beam model, ebm70, and a Monte Carlo dose calculation. The ebm70 beam model was originally commissioned at an SSD of 70 cm and designed for electron beams shaped with a Millennium 120 pMLC. Here, the team investigated its use with SSDs of up to 100 cm, and three treatment units: a Clinac 23iX and a TrueBeam equipped with a Millennium 120 pMLC, plus a Novalis Tx with a high-definition MLC.

Measured versus calculated dose — Measured and calculated depth-dose curves (left) and crossplane dose profiles at a depth of 1 cm (right), for a field size of 9.8 × 9.8 cm, for the 6 MeV beam of the Novalis Tx. The dose differences between measurements and calculations are shown below.

They found no substantial differences in dose calculation accuracy among the three treatment units. Comparing ebm70 with the commercial dose calculation eMC, used clinically for cut-out collimated electrons, revealed similar accuracy between the two. Measurements in a water tank generally agreed with ebm70 dose calculations to within 3% or 2 mm.

Clinical considerations

The researchers next considered a series of clinical cases. For each case, they created a cut-out plan with typical clinical settings, plus a deliverable pMLC plan with a similar planning target volume (PTV) dose homogeneity. They examined two breast boost cases, using SSDs of 70–100 cm.

For a left breast boost using a single electron field (1E) at 18 MeV, pMLC plans failed to achieve a homogeneity index (HI) within 3% of the cut-out plan, though dose coverage of the boost PTV was similar for all plans. At SSDs of up to 90 cm, pMLC plans delivered similar mean doses to the ipsilateral lung and the heart as the cut-out, increasing slightly at 100 cm (by up to 0.3 Gy). The volume of normal tissue exposed to the low-dose bath (V_10%) was similar for the two plans at an SSD of 70 cm, increasing for the pMLC plan as SSD increased.

For a right breast boost, with a 6 MeV 1E field, the pMLC plan only achieved an HI within 3% of the cut-out plan at an SSD of 70 cm. The mean dose to the ipsilateral lung was slightly increased (by up to 0.3 Gy) for all pMLC plans compared with cut-out plans. V_10% of normal tissue was also higher for the pMLC plans, by up to 299% at an SSD of 100 cm.

For the breast boosts, the researchers also created MBRT plans using one electron field and two tangential 3D conformal photon fields (1E2X). The 1E2X plans for the left breast boost (using 12 MeV electrons) exhibited similar dose homogeneity, dose to OARs and V_10% of normal tissue for all pMLC and cut-out plans.

1E2X plans for the left breast boost — Dose distributions (left) and DVHs (top right) of TrueBeam 12 MeV 1E2X plans created for the left breast boost case, and dose profiles (bottom right) indicated by a white arrow on the dose distributions. The legend of the DVHs also applies to the dose profiles. The contours of the boost PTV (red), primary PTV (orange), ipsilateral lung (blue), contralateral lung (green) and heart (magenta) are visible on the transversal CT images.

“For most of the cases, we believe that MBRT is superior to electron-only treatments,” said Mueller. “The reason for this is the fundamentally different characteristics of photon and electron beams, which allows advantages of one particle type to compensate for disadvantages of the other.”

The team also created pMLC plans for sternum and testis (1E) and parotid gland (1E2X) treatments, with SSDs of 75, 80 and 80 cm. For these three cases, pMLC plans yielded similar or only slightly degraded values for HI and mean dose to OARs, compared with the cut-out plans.

The authors conclude that for each case, a pMLC treatment plan with similar quality to a cut-out plan could be found. They note that for cases with OARs located close to the target in the lateral direction (such as the sternum case), or for 6 MeV electron beams used at an extended SSD, OAR doses were slightly increased.

Mueller and colleagues are now working on dynamic mixed beam radiotherapy. “This advanced MBRT technique combines photon dynamic trajectories with modulated electron beams,” he explained. “Thus, even more degrees-of-freedom are accessed with dynamic couch and collimator rotation. We believe this technique could bring plan quality for targets with at least some superficial part to the next level, using only hardware available on conventional treatment units.”

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