Lasers that switch on and off billions of times per second are the backbone of optical communications networks, but this feat is only possible at certain laser frequencies. A team of researchers from the University of Leeds and the University of Nottingham in the UK has now taken a step towards extending this frequency range by using sound waves to modulate the emission of a terahertz (THz) quantum cascade laser. In this new technique, the modulation rate is limited only by the duration of the acoustic pulse applied, meaning that rates of up to hundreds of gigahertz (GHz) are possible. This could allow data to be transmitted at 100 gigabits per second – around a thousand times faster than current wireless systems.
Although THz signals have a shorter effective range than the microwave signals used in today’s wireless data systems, their higher frequency means they can carry more information over the same time frame. That makes THz radiation promising for ultra-fast, short-range data exchange – for example across hospital campuses, between research facilities at universities or in some satellite communications.
For such a system to be practical, however, the THz laser would need to be modulated extremely quickly – around 100 billion times a second. That has proved challenging, but researchers led by John Cunningham in Leeds and Tony Kent in Nottingham say they have found a way to do it using acoustic waves in THz quantum cascade lasers (QCLs).
Applying acoustic waves
Unlike standard semiconductor lasers, which generate photons when electrons and holes combine inside a material with a given band gap, QCLs consist of dozens of thin layers of semiconductors. Each electron that travels through the device “cascades” through a series of quantum wells in these layered semiconductors, emitting multiple photons as it does so, at frequencies set by the structure of the semiconductor layers. It is this emission process that must be controlled in order to modulate the laser output.
One option is to modulate the output electronically, by applying a bias electrical current to the QCL. However, the maximum modulation rate for such a system is roughly 30 GHz at best. The Leeds and Nottingham team instead used an approach based on acoustic waves. “We generated the acoustic waves by impacting a pulse from another laser onto an aluminium film in the QCL,” explains Aniela Dunn, a research fellow at Leeds and the study’s lead author. “This caused the film to expand and contract, sending mechanical waves through the device.”
Perturbation theory analysis
In effect, Dunn says that these mechanical waves cause the energy levels of the electrons within the QCL’s quantum wells to vibrate. The team explain this process using perturbation theory: by assuming that the effect of the acoustic pulses is small, they can treat the changes to the energy levels as a perturbation to the QCL’s behaviour under normal operating conditions. This enables them to predict how the number and frequency of the QCL’s emitted photons will change in response to the applied acoustic wave.
“By approximating the amplitude of the acoustic (strain) pulse generated in the QCL, we can calculate the effect the sound pulse will have on the energy levels of the QCL,” Dunn tells Physics World. “It is thus possible for us to determine the subsequent effect of the pulse on the voltage across the device and the THz power emitted from it.”
Although the team could not stop and start the flow of photons from the QCL completely, they could control its light output by a few percent – a “great start”, according to Cunningham. In the future, they hope to refine the technique to gain more control over the photon emissions. Dunn adds that they also hope to integrate the sound-generating structures into the THz laser itself, so that no external sound source is needed.
In addition to high-speed communications, Dunn thinks acoustic modulation could come in useful in areas such as high-resolution spectroscopy, active mode-locking and frequency comb synthesis. “Ultimately, it allows us to explore the interaction between THz sound and light waves, something that could have real technological applications,” she says.
Last week I was at the Grandvalira ski area in Andorra, having a great time whizzing down the slopes in glorious sunshine. Sitting on the chairlifts, my mind naturally turned to the physics involved in maintaining the pistes – and I became fascinated by two things: the moguls that quickly form on an initially smooth slope; and the snowmaking systems that help keep the slopes full of snow.
Nights are very busy at a ski area. Snow cannons hiss as they take advantage of the colder night-time air while huge bulldozer-like “piste bashers” smooth-out the slopes for the next day’s skiing.
After a few hours of being skied on, however, a smooth slope can be covered in moguls – bumps that are about a metre or so across and tens of centimetres high. These tend to form on steeper slopes where skiers control their speed by weaving back and forth across the piste, rather than plunging straight down. Each turn pushes some snow up into a pile and it is easier for successive skiers to weave around these piles, further building them up into a mogul field.
Uphill motion
Like sand dunes, moguls struck me as being ripe for investigation by physicists who study granular materials. A quick Internet search during a coffee break revealed a remarkable thing about moguls – they tend to migrate uphill. That is the conclusion of physicist and skier Dave Bahr and colleagues, who a decade ago trained a camera on an ungroomed mogul field in Colorado for an entire ski season.
The teams’s explanation for this apparent defiance of gravity is that most skiers turn as they go over the top of a mogul. This is because turning involves straightening your knees to lift your upper body, which is much easier to do with the upward boost of a mogul. After the turn, the skier will usually skid along the downhill side of the mogul, pushing some snow down the slope and onto the next mogul. This has the effect of moving both moguls uphill.
In the study, Bahr (along with Tad Pfeffer and Ray Browning) found that the moguls move uphill at a rate of about eight centimetres per day.
Unusually warm
Today, most large ski areas worldwide rely on snowmaking technology to supplement natural snowfall. While snowmaking is seen as a potential solution to global warming, it is not a panacea. It was unusually warm when I was in Andorra, and for several nights the temperature remained well above freezing – even at an altitude of 2100 m. This meant that the snowmaking cannons lining the slopes were not used overnight.
That got me wondering about how snowmaking works – and whether it could be used at air temperatures at or even above 0 °C? Apparently, the answer is yes, under the right conditions.
(Courtesy: Hamish Johnston)
There are several types of snow cannons but the basic principle involves combining cold water with compressed air and blasting out a mist at high velocity. The sudden drop in pressure at the nozzle cools the water, causing tiny ice crystals to form and grow.
Pellets, not flakes
This nucleation and growth are like what happens in a snow cloud, but at an accelerated rate. It therefore produces a pellet of snow, rather than a flake. While this is no good for powder skiers, this type of snow is apparently better than natural snow for making groomed pistes.
According to the Italian snow-cannon maker TechnoAlpin, snow can be made when the ambient “wet bulb temperature” drops below −2.5 °C. The wet bulb temperature accounts for relative humidity, which is an important factor in snowmaking. The higher the relative humidity, the harder it is to make snow. A wet bulb temperature of about −2.5 °C occurs at about 0.5 °C at 50% relative humidity so snowmaking is possible without freezing air temperatures. Indeed, if the relative humidity is a desert-like 25%, a snow cannon could operate at a balmy 2 °C.
While snowmaking won’t help when temperatures remain above freezing for sustained periods of time, the technology does allow ski areas to make large amounts of snow when it is cold. This snow can then be used to extend the skiing season into the spring. Indeed, some ski areas are looking at storing large quantities of snow over the summer so they can open earlier in the autumn.
There are downsides to snowmaking, however. It takes large amounts of energy to operate cannons and some systems use chemical additives to boost nucleation. And then there is the huge amount of water required – which is often supplied by building alpine reservoirs that can disrupt local hydrology.
Start-up: Chris Monroe co-founded a company that is developing quantum computers using trapped ions. (Courtesy: Chris Monroe, IonQ)
Chris Monroe is a physicist at the University of Maryland, US, and the co-founder and chief scientist of IonQ, a start-up that is developing quantum computers using trapped ions as qubits. He recently spoke to Margaret Harris about the rise of quantum computing, and how his previous experiences – including stints in the labs of two physics Nobel laureates – fed into his decision to start the company in 2015.
How did you get interested in quantum computing? Because you really got into the field at the very beginning…
Yes, I’ve been in this field for more than 25 years, and I have to say it sort of landed in my lap. I did my PhD work on cold atomic gases at the University of Colorado, Boulder, in the group of Carl Wieman and Eric Cornell, who went on to share the physics Nobel prize in 2001 for making the first Bose–Einstein condensate. But I always knew that at some point I might want to get a “real job”, and atomic physics is good in that respect because it involves working with practical things like optics and lasers and photonics. There’s a lot of equipment involved, and I was attracted to the technical nature of the work.
After I got my PhD, I went on to do a postdoc. The postdoc system is a little stressful, because it’s a temporary job, and you’re in your late 20s, and everyone else you know is building their career. But postdocs are also wonderful opportunities to try something random, because there’s very little at stake if it doesn’t work out. And in my case, I didn’t have to move very far. I stayed in Boulder and went down the road to work with David Wineland at the National Institute of Standards and Technology (NIST).
In the early- to mid-1990s, Wineland’s lab was basically the atomic-clock division of the US government. He is an amazing researcher, and NIST allowed him to do academic-type research within this government lab. So instead of building the clocks that people use as a real time standard, we were doing research on how you might make better clocks. One of the crazy ideas we had was that by entangling multiple atoms or ions, we could make our clock run faster (and therefore more accurately), so we came up with a scheme to entangle two ions.
As it turns out, that meant we were building a quantum gate for a tiny quantum computer. But we didn’t know those terms at the time. I didn’t hear about quantum computing until the summer of 1994, when I learned of Peter Shor’s algorithm for factoring large numbers using a quantum computer.
When Wineland and I saw Shor’s article, it entirely changed our direction of research. We were still at NIST doing atomic clocks, but now we were also doing quantum computing, and government agencies got very interested in seeing what we needed to do to scale it up. Everything we did in that laboratory was ground-breaking, and Wineland went on to win the Nobel prize in 2012 largely based on his work in the 1990s. It was a pretty cool beginning to my career.
Several years passed from when you first heard about quantum computing to when you set up IonQ. What made you decide “This isn’t just a research topic anymore, I’m going to start a company”?
For the first 10 years or so, there was a lot of research to do. Picking out the best type of quantum gate. Deciding which atomic species to use. Working out how well the lasers perform and how big our quantum systems could be before they got killed by noise. It took a long time, not just for me, but for the whole community to do those experiments. And in terms of scaling things up from a handful of qubits or gates, really nothing happened for a long time apart from high-level proposals for scaling. Although we were starting to understand the limitations of the physics, we weren’t ready to do the engineering.
Beginning in 2010, though, we started to narrow things down and make decisions, and by 2014 or 2015 we had our first tiny quantum computer. And that was interesting, because after we initialized and calibrated the system in the morning, we stopped doing atomic physics in the afternoon. Once the system was seeded, we could stop tinkering with the lasers, go over to the PC that was controlling the experiment and run algorithms.
After that, a couple of things happened. I had a long-standing collaboration with a colleague from Duke University, Jungsang Kim. He’s an engineer, and we recognized that we kind of filled each other’s gaps. I’m a physicist, and I’d been in this field for a long time, but he has great experience in what’s called systems engineering, and he thinks differently about physical systems than I do. We realized that, together, we could do some amazing things.
Around that same time, in mid-2016, IBM built a five-qubit superconducting quantum computer and put it in the cloud so that people could use it. At first, that seemed a little goofy to us, because five qubits is really small – we’re not going to learn anything from that. But it was more than just a publicity stunt. I mean, it was a publicity stunt, but it also allowed anybody to use the system, which was huge – a genius move.
As it happened, the system we were building was also exactly five qubits, but in terms of performance it was much better than IBM’s. This is because atomic qubits are nearly perfect and exactly replicable; because we could connect a pair of the atom qubits with reconfigurable laser beams; and because we could run “deeper” circuits. So people started approaching us, saying that they’d tried to use the IBM cloud, but it didn’t work for what they wanted to do – could we help? Initially, it was more like a scientific collaboration: we started running applications and algorithms that other people would send us. But we realized that to go to the next step required such a serious dose of engineering that it probably couldn’t be done at a university. And that was the genesis of IonQ.
What do you know now that you wish you’d known when you started IonQ?
One thing I’ve learned has to do with the computer science aspect of our systems. Moving the operations around in our algorithms so they’re mapped to our system in an optimal way turns out to be incredibly powerful – much more powerful than I imagined. If I’d known that two or three years ago, I would have hired more computer-science theorists.
As an analogy, the first PC I ever used had four kilobytes of memory. Now we have hundreds of gigabytes, and that means we waste it – we take pictures that are way too high resolution and store them on our hard drive because memory is a commodity. It’s cheap, it’s easy. There’s no reason not to waste it. But at the early stages of any technology, including quantum computing, you have to squeeze out every ounce of efficiency you can, because it might mean the difference between running an application and not being able to. In 10 or 20 years, I hope that qubits and gates will be more of a commodity, and then we can be more wasteful with them. But to get there we have to extract as much efficiency as possible. That’s not really physics – it’s quantum computer science, and it’s a very rare skillset right now.
In 10 or 20 years, I hope that qubits and gates will be more of a commodity, and then we can be more wasteful with them
That leads nicely to my last question. Do you have any advice for today’s physics students?
With a physics degree, you can pretty much do anything. The challenge is that the doors are not open for you to the same extent as they are in some other fields. When you study engineering, for example, it’s almost like going to business school. You make connections, there are job fairs and the doors open for you to go work for these big engineering firms. You can still do that as a physicist. It just won’t come to you. You have to go find it.
So the advice I would give is to keep your options open. If you do a PhD, it may appear like you’re narrowing your options, because you’re working for several years on just one thing. But if you can solve a problem at the forefront of your field, even if it’s a narrow problem, you learn how to do that in any field. Going in-depth in physics will help you no matter what you want to do, even if it’s something unrelated, such as finance.
We all learn quantum physics as physics students, but in recent years this field has taken on a whole new life. It’s not an esoteric theory anymore, something that only describes tiny effects in extreme forms of matter. It’s going to form the basis for a whole new type of technology. So I think that, because physicists have a bit of a leg up in this area, they should go all-in.
A new mathematical model that describes how proteins self-assemble into stacks known as amyloid fibrils could aid the search for drugs to treat diseases like Alzheimer’s and Parkinson’s. The model, developed by researchers at the University of Cambridge in the UK and Lund University in Sweden, is based on rate equations and confirms that the microscopic reaction steps behind protein aggregation occur in a catalytic manner, similar to reactions involving enzymes.
The self-assembly of proteins into amyloid fibrils is a natural biological process known to be involved in several diseases, including Type II diabetes as well as Alzheimer’s and Parkinson’s. By unravelling the reactions that lead these biofilaments to proliferate, researchers hope to gain insights into how to treat such diseases. Ultimately, such knowledge could even make it possible to prevent these illnesses by developing drugs that inhibit the protein self-assembly processes at their heart.
Two-step catalytic process
Researchers know that biofilaments assemble from protein monomers in a slow “primary” nucleation-reaction process, followed by a faster process that allows the filaments to elongate. Secondary reaction processes – including nucleation of new filaments on the surface of existing ones, filament breakage and branching – frequently accompany these first two steps.
Previous work has shown that the elongation step is best described as a two-step catalytic process controlled by the so-called Michaelis-Menten rate law, which was first employed in 1913 to describe the rates of enzyme-driven reactions. The secondary nucleation step – which is known to be crucial for the aggregation of an Alzheimer’s-associated protein called Ab40 – is described by a similar rate law and is also catalytic.
Catalyzed aggregation
A team led by Tuomas Knowles and Alexander Dear has now built on these insights to develop simple, highly general mathematical equations that model the kinetics of amyloid fibril formation and changes in protein aggregate concentrations over time. The team also applied this model to experimental data on the aggregation of Ab40.
Experiments had hinted that Ab40 begins aggregating at interfaces – for example, the surface of a liquid solution or the glass wall of the test tube in which the experiments were performed. Dear says that the close match between their rate-law model and earlier in vitro results supports this hypothesis, indicating that interfaces may play an enzyme-like role in promoting protein aggregation. “The new model sheds light on the nature of aggregation by showing that the individual steps in the aggregation process (filament growth, and both primary and secondary nucleation of new filaments) are typically all catalytic in nature,” he tells Physics World. Previous models, he adds, did not take this into account.
Enzyme-like effects
A simpler form of the model also enables the researchers to see how enzyme-like “saturation” in each step affects the overall proliferation of amyloid fibrils, as the various catalytic surfaces become fully occupied at high protein concentrations. “We can thus determine the protein concentrations at which each reaction step saturates from the measured fibril growth curves,” Dear says.
The discovery that primary nucleation of Ab40 is catalysed by interfaces also has implications for the way proteins aggregate in the human body. Since such heterogenous nucleation is highly environment-dependent, Dear says that relatively small changes in the biochemical environment in the brain or body could dramatically change the propensity for amyloids to form. Ultimately, the researchers, who report their work in the Journal of Chemical Physics, believe that their result could be used to develop potent inhibitors of amyloid formation, and thus new treatments for diseases like Alzheimer’s.
The paper-based sticking plaster is engineered to behave like a traffic light, appearing green under normal conditions, yellow if drug-sensitive bacteria are present and red if drug-resistant bacteria are detected. (Courtesy: ACS Cent. Sci. 10.1021/acscentsci.9b01104)
Antimicrobial resistance is a serious threat to global health, a phenomenon that is largely driven by incorrect treatment regimens, which result in misuse and overuse of antibiotics. Therefore, it is important to develop rapid and cheap ways of detecting bacteria, along with their sensitivity or resistance to antibiotics. This would allow rapid diagnosis of infections, tailored prescription of drugs and, in turn, a more informed and sustainable use of antibiotics.
Sense-and-treat
In response to these demands, a team of researchers from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, has developed a paper-based adhesive plaster, or band-aid, based on a “sense and treat” approach. The plaster senses the presence of bacteria by changing its colour and releases antibiotics when necessary. In addition, the paper is able to distinguish between certain drug-susceptible and drug-resistant bacteria, therefore informing the best disinfection strategy (ACS Cent. Sci. 10.1021/acscentsci.9b01104).
The plaster works like a traffic light. It appears green under normal conditions, while it turns yellow in the presence of drug-sensitive bacteria and automatically releases antibiotics to kill them. If the bacteria are drug-resistant, the paper becomes red and photodynamic therapy (PDT) can be used instead. PDT is performed by shining 628 nm light onto the plaster, which induces the production of reactive oxygen species that kill or weaken the resistant bacteria.
Ad-hoc chemistry
The plaster exploits chemical compounds that change colour in the presence of bacteria. In particular, the material is soaked in bromothymol blue, a pH indicator that turns from green to yellow when exposed to the acidic environment created by bacterial metabolism. The paper also contains a pH-sensitive metal–organic framework, a compound that acts as a cage containing antibiotic molecules. Upon increased acidity, the framework breaks open and the encapsulated antibiotic is released.
A wide class of resistant bacteria produce β-lactamase, an enzyme that destroys certain types of antibiotic molecules. In response to this, the plaster is also equipped with nitrocefin, an antibiotic that shows a distinct colour change from yellow to red when interacting with β-lactamase, hence signalling the presence of drug-resistant bacteria and the need for a therapy other than antibiotics.
Based on these principles, the researchers have shown that the plaster accelerates wound healing in mice infected with both sensitive and resistant bacteria. They monitored the status of the wound over three days, and clearly observed improved tissue regeneration following the disinfecting action of either antibiotics or PDT.
The team also demonstrated the potential of the paper device in a fruit preservation model, in which it was attached onto an infected tomato that successfully recovered after three days of sensing and treatment.
The future of diagnosis and treatment
The team’s novel adhesive plaster offers great potential for the future of diagnosing and treating wound infections. It is cheap, easy to use, and effective against certain types of bacterial infections. Extending the method to practical and point-of-care applications will be the next challenge towards widespread use. Technologies like this could contribute significantly to the fight against antimicrobial resistance.
On Saturday I headed to Oxford for a one-day meeting about big science in physics that was organised by the St Cross Centre for the History and Philosophy of Physics at Oxford University. Held in the Martin Wood Lecture theatre at the Department of Physics, the meeting covered the past, present and future of big science. The audience was made up of academics as well as the general public, with 200 people having registered to attend.
First up was Helge Kragh from the Niels Bohr Institute in Copenhagen, who gave a fascinating talk about what we define as big science and how that term has changed over the past century. Kragh’s focus was on the Manhattan atomic-bomb project and what followed regarding the development of large particle accelerators.
Continuing the particle-physics theme was Isabelle Wingerter-Seez from the Laboratoire d’Annecy-le-Vieux de Physique des Particules, which belongs to the French National Centre for Scientific Research, who spoke about the beginnings of CERN and the discovery of the Higgs boson in 2012 at the lab’s Large Hadron Collider.
Frank Close from Oxford University gives a summary of the day’s proceedings. (Courtesy: Michael Banks)
Big science doesn’t get much bigger that the ITER experimental fusion reactor, which is currently being built in Cadarache, France, and ITER director general Bernard Bigot was on hand to give a status update about how the facility is on track for first plasma in December 2025 (though actual deuterium-tritium plasma won’t be performed until 2035). Bigot noted that this year and the next will be critical for ITER as it begins assembling all the components that have been manufactured by its eight member states, all of which has to be carried out “to millimetre precision”.
In the afternoon session, Carole Jackson, director the Netherlands Institute for Radio Astronomy (ASTRON), discussed big projects in astronomy, particularly the Square Kilometer Array and the European LOFAR network.
I gave the final presentation of the day, in which I spoke about the coming decade of new facilities and the possible science that may result. I presented my “top 10 facilities to watch”, which were, in no particular order: the James Webb Space Telescope; ITER; The European Extremely Large Telescope; the European Spallation Source (ESS); the Extreme Light Infrastructure; Hyper Kamiokande; the Square Kilometre Array; the Long Baseline Neutrino Facility; the Electron-Ion Collider; and a future electron-positron collider.
I also offered a few predictions for the decade ahead (“brave”, one person said to me afterwards), including that the International Linear Collider will definitely get the go ahead, that CP violation in neutrinos will be measured at the Hyper-Kamiokande facility in Japan and at the Long Baseline Neutrino Facility in the US, and that astronauts will return to the Moon. I was the odd one out in being a journalist, not an active academic or researcher, but I hope that gave the audience a somewhat independent perspective.
One common theme in questions from the audience (of which there were many) was how big-science facilities could become, well, more green. There are some facilities that are working towards this, notably the SESAME synchrotron in Jordan and the ESS, both of which use or will use renewable energy to power the accelerator complex.
But given that big science is getting bigger and ever-more important, energy sustainability needs to become a much greater consideration for those planning, designing and building these future facilities.
Dunes of sand and other granular materials can be found in large fields on Earth, Mars and beyond. Dune fields tend to self-order into spatial patterns and the physics underlying this self-organization is poorly understood. Now, Karol Bacik and colleagues at the University of Cambridge in the UK have shown that as dunes move, they repel dunes ahead of them via fluid turbulence, preventing collisions and stabilizing dune fields.
Theoretical models usually assume that dunes act as autonomous, self-propelled agents, that exchange mass with other dunes remotely – via air or water flow – or through collisions. But the latest research, described in Physical Review Letters, suggests that dunes can interact without exchanging mass.
Bacik told Physics World that the dynamics of dune fields are an intriguing and fundamental physical problem, “because we’ve got these large landscapes with isolated dunes, and the question is how would they evolve”. He adds that for their simplified experiment the team took the dune field apart and “did a pair wise interaction with two dunes only”.
Rotating paddles
They set-up their two-dune system in a circular water-filled tank mounted with a set of rotating paddles submerged near the surface. Two cameras recorded changes in dune shape, sediment transport and water flow. To create the dunes, they placed two 2.25 kg piles of glass beads on the base of the tank (see video). When they rotated the paddles the flow quickly shifted the piles into a characteristic dune shape – a steep downstream face and shallow upstream face. The dunes then started to migrate downstream.
The researchers found that although the dunes had equal mass they did not move at the same speed. The upstream dune – the one first hit by the flow – moved at a constant speed, but the downstream dune did not. Initially it moved faster than the upstream dune, but then as the gap between the dunes increased it began to slow. Eventually an equilibrium was reached, with both dunes moving at the same speed.
Upstream wake
Analysis showed this dune-dune interaction was created by the fluid flow and the turbulent structures generated in the wake of the upstream dune. The wake created by fluid flowing over the upstream dune pushes the downstream dune away.
“It looks like repulsion, because this interaction means they space out, rather than the opposite, which would be that they collide,” Bacik says. He adds that although the interaction is there all the time “the strength of this wake decays away from the dune, so as the downstream dune keeps escaping, the interaction is weaker and weaker”. And, when the dunes initially shift, or there is a surge in the flow, the interaction increases and pushes the downstream dune a bit harder– “it gives it an extra kick”.
The experiment revealed that dune-dune repulsion is a robust phenomenon. If the dunes were close enough to start with, dune separation occurred at all flow rates explored. It even persisted when the dunes were different sizes. When the researchers created a downstream dune that was 2.5 times larger than the upstream dune, they found that initially the smaller upstream dune migrated faster, but as the gap closed the wake-induced repulsion grew stronger and the larger downstream dune sped up. Again, the two dunes eventually began to move at the same speed.
Satellite images
Although the experimental dunes were underwater, Bacik is confident that dunes on land behave in the same way, with the interaction mediated by air turbulence. “Both gases and liquids are fluids from the physical point of view, so they obey the same equation,” he explained. Bacik adds that satellite images studied by other researchers suggest a similar interaction between desert dunes. These movements, however, occur over decades, not hours. “The bigger the dunes the slower they move,” Bacik explains.
Hans Herrmann, a physicist at ESPCI Paris, is not surprised by the findings, but is impressed by the work. He told Physics World, “I never had any doubt that there exists a hydrodynamic interaction between dunes irrespective if they are in water or in air. But it is extremely difficult to include such an interaction in a model. Therefore, all models do neglect them.” Herrmann adds that the work “is a beautiful experimental measurement of the consequences of these forces and thus a very useful and very important contribution”.
Next, the team hope to conduct more thorough analysis of satellite images of dune fields to see how prominent this interaction is – and how it works – in more complex, real-world environments.
Progress on establishing a space agency for the African Union (AU) has come to a standstill over a lack of funding and questions about how it will operate. That’s the message in a new progress report on Agenda 2063 – the AU’s blueprint for development and transformation in the region. It has revealed that progress on the agency is being restrained by delays in “financial and structural” planning by member states.
Legislation to establish the African Space Agency was passed by the AU in 2017 and two years later Egypt was chosen to host its headquarters, with the agency supposed to be up and running in 2023. Plans for an ASU were included as a flagship programme for Agenda 2063, but the new Agenda 2063 progress report, released on 8 February ahead of the AU’s 33rd heads of state summit in Addis Ababa, Ethiopia, says that little progress has been made since then.
It might be instructive to take a step back and ask what exactly is it that the African space agency will do once it is established
Peter Martinez
The AU’s space programme has managed to set priority areas for a prospective space programme, while two of the four “baseline studies” – gaps on navigation and positioning in Africa and studying the African private sector – have been completed. But it is still not clear what implications the agency will have on the budgets of African countries.
Costs and benefits
Peter Martinez, a space-policy analyst and executive director of Secure World Foundation, says that the lack of progress could result in delays beyond 2023. with the agency perhaps even being scaled down in scope. “The pace at which the AU can make progress is determined by how fast member states are willing to move on an issue, and this is particularly the case when there may be budgetary implications for them,” Martinez told Physics World. “If countries can see direct benefits flowing back to them, they are much more likely to support the agency financially. On the other hand, if the agency becomes an aggregator of the space technology requirements for African countries, which are then met by entities from outside the continent, the agency will have failed, in my view.”
Martinez adds that countries may just come together to collaborate on specific projects without the need to set up a new continental entity. “It might be instructive to take a step back and ask what exactly is it that the African Space Agency will do once it is established. What is its value proposition for African countries, ranging from those with established space capabilities, to those that still have to take their first steps.”
The AU says that it will now work to tackle the issues raised in the report. It says it will also commit resources to help African countries train scientists in earth observation, satellite communication, navigation and positioning as well as astronomy.
Trace amounts of salt can help control the “coffee-ring effect” that occurs when a solvent evaporates from a solution containing non-volatile particles. The technique, which was developed by researchers in China, works for a variety of substrates, including graphene, graphite and polymers, and could help engineers deposit more uniform coatings and dyes.
Although the coffee-ring effect gets its name from the familiar stain left behind when coffee drips down the side of a cup and spreads around the base, it occurs in many other liquids, too. In this well-studied process, the edges of a drop of liquid on a surface become pinned to that surface, preventing the drop from shrinking as the liquid evaporates. Instead, the drop flattens out, pushing the liquid – and anything suspended in it – to the edges. By the time the drop evaporates completely, most of the suspended particles have reached the edges and remain behind as a dark ring.
As well as being an unsightly nuisance on your desk or table, the coffee-ring effect is the bane of many an industrial process (including printing, coating, dyeing, complex assembly and micro- and nanofabrication) because the ring structures it produces are so non-uniform. Researchers have explored many strategies for avoiding the effect, from designing paints and inks that produce an even coating when they evaporate to changing the shape of the suspended particles. Various groups have also tried adding surfactants, polymers, sol-gel inducers, co-solvents and even proteins to the solutions, while others have experimented with using external optical and electrical fields to modify the ring-forming process.
A new and efficient technique
A team of researchers led by Haiping Fang of East China University of Science and Technology and Guosheng Shi of Shanghai University has now developed a new and efficient technique that involves adding trace amounts of various salts to the solution. When the researchers tested their technique on a solution containing dyes placed on graphene, polymers and other substrates that contain chemical structures known as aromatic rings, they found that the colour of the dye remained even across the substrates. Molecular dynamics simulations revealed that strong interactions (known as cation-π interactions) between the hydrated cations in the solution and the aromatic rings in the substrates work to inhibit the coffee-ring effect, since they promote a more uniform adsorption of suspended matter onto the substrates.
In their experiments, Fang, Shi and colleagues created solutions with different concentrations of sodium chloride (NaCl) by mixing the salt into aqueous suspensions of polystyrene microspheres. They then placed drops of the salty microsphere mixtures onto a graphene substrate grown via chemical vapour deposition. As a control, they tested solutions without NaCl. In separate experiments, they also tried the technique on substrates such as glass, which do not contain aromatic rings.
No ring effect with 8.0 mM NaCl
After allowing the drops to evaporate at around 10° C, the researchers used optical microscopes and greyscale analysis to study the dried patterns left on the substrates. In suspensions without NaCl, they observed ring-like patterns with a dark rim and a light grey centre on the graphene. For the salty suspensions, however, the rings were absent, and the image contrast between the rim and the centre of the pattern gradually diminished for mixtures with increasing NaCl concentrations. At a concentration of 8.0 mM NaCl, the pattern appeared completely uniform.
The team, who report their work in Chinese Physics Letters, found the same effect with other salts such as lithium chloride (LiCl), potassium chloride (KCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2) at different concentrations. They also observed similar behaviour with other aromatic-ring substrates, including natural graphite and a common thermoplastic resin called polyethylene terephthalate (PET). When the experiments were repeated with glass substrates, however, the coffee-ring effect lingered despite the addition of salt, confirming the importance of the hydrated cation-π interactions with aromatic rings.
Cation-π interactions remain strong
Cation-π interactions are known to play crucial roles in the structure, dynamic processes and functions of both living and non-living systems. Until recently, researchers believed that these interactions weakened when the cations were hydrated, so they usually neglected them. Earlier work by Fang and Shi’s team, however, suggested that this approach was incorrect. “Our previous theoretical and experimental studies showed that these interactions remain strong enough to result in the strong adsorption of hydrated cations on graphitic surfaces (such as carbon nanotubes, graphene, graphite and graphene oxide),” Fang explains. “This is because the polycyclic aromatic ring structures of these materials themselves include more π electrons.”
Shi notes that since the cation-π interaction also exists between other cations and aromatic rings, metal ions such as Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+ might prove just as effective as salt for controlling the coffee-ring effect. The team’s future studies will focus on choosing the best cations for different applications, he tells Physics World.
Metallic glasses are promising materials for structural engineering, but their poor ductility makes them brittle, limiting their applications. Researchers in China have now shown that these glasses can be made much softer by reducing their size down to the microscale.
As their name implies, metallic glasses have the properties of both metals and glasses – they contain metallic bonds and are thus conducting, but their atoms are disordered like in a glass. These metastable materials are produced by rapid quenching from the liquid state and their physical properties depend on how they have been processed. They can be made more plastic to some extent by applying stress or high temperatures, but the effect – which is related to structural disordering in the material – is limited.
A marked rejuvenation effect
Researchers led by Bao-An Sun and Hai-Yang Bai of the Institute of Physics at the Chinese Academy of Sciences in Beijing are now reporting on a marked “rejuvenation”, or softening, of metallic glasses that have been drawn while still hot into micron-sized wires. Compared to their bulk counterparts, the modulus and hardness of these wires are much lower, decreasing by 26% and 17%, respectively.
“Such pronounced rejuvenation is unusual for metallic glasses,” explains Sun, “with previous studies reporting on only a few percent decrease in modulus and hardness.”
Higher free volume content
According to the researchers, the hot-drawn micron-sized metallic glass wires become softer and more plastic for two reasons. First, the severe thermomechanical shearing involved in the hot-drawing process provides the material with energy, thereby increasing its “free volume” (a measure of the spacing between molecules). Second, the material’s subsequent rapid cooling “freezes” this induced free volume in place. As the size of the wires decreases, more free volume is induced, and this, coupled with a higher cooling rate, produces more pronounced rejuvenation.
“Our results clearly suggest that we can significantly rejuvenate metallic glasses through a proper combination of temperature, shear stress and size reduction,” Sun tells Physics World. “Our technique thus provides us with a new way of tuning and designing the structure and properties of these materials.”
The researchers, who report their work in Chinese Physics Letters, say they will now focus on controlling the rejuvenation of the metallic glass wires in a more quantitative manner – by determining the exact role of temperature, stress and size reduction in the process they have developed
