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Frozen detonation could enable hypersonic flight

Scientists in the US have produced a detonation that is fixed in space for the first time. This standing wave detonation was created in a prototype engine and the researchers say that such a system could one day power aircraft at up to 17 times the speed of sound.

Most fires are deflagrations. This form of combustion creates a subsonic reaction wave and powers much of our transport technology. But you can get a much more powerful and efficient release of energy from a detonation. This type of combustion produces supersonic shock waves driven by energy release from closely coupled chemical reactions. These waves travel at many times the speed of sound, with those produced by igniting a hydrogen–air fuel mix, for example, often reaching speeds of Mach 5.

This intense energy release is highly unstable and difficult to control. If harnessed, however, it could be channelled to achieve hypersonic flight for future interplanetary travel and ultrahigh-speed intercontinental travel on Earth. Estimates suggest that an engine operating with a Mach 5 flow path, such as that produced by a hydrogen–air fuel mix, could enable vehicle speeds of Mach 6 to 17. That would allow you to fly from New York to London in just half-an-hour.

There are three main detonation engine concepts. Pulse detonation engines create repeated explosions, while in a rotating detonation engine, detonations continuously travel around a circular channel with the shockwave from each setting off the next. Both of these have been previously demonstrated. Now, researchers at the University of Central Florida and the US Naval Research Laboratory say they have demonstrated the third concept: a standing or oblique detonation wave engine. They report their results in the Proceedings of the National Academy of Sciences.

The idea behind an oblique detonation wave engine is to produce a continuous detonation that is fixed in space, so that the resulting shockwave is stable and remains in the same position. “We want to feed [the detonation] with the right propellant mixture, at the right speed and freeze it in space,” Kareem Ahmed tells Physics World.

To create such a detonation, Ahmed and his colleagues developed a prototype engine named HyperReact (high-enthalpy hypersonic reacting facility). This device is divided into three sections. In the first, the mixing chamber, a jet of hydrogen and air is ignited. This creates hot high-pressured air that flows into the next section, the converging–diverging (CD) nozzle, which has an axisymmetric square cross-section along its entire length. As the air enters the CD nozzle a jet of ultrahigh-purity hydrogen is added. The shape of the CD nozzle is designed to then accelerate the mixture to speeds of around Mach 4.5 as it enters the test section. In this final chamber there is a 30-degree turning angle ramp.

The team found that by manipulating the fuel mix, temperature and air flow through the chambers they were able to produce an oblique shock on the ramp that created a detonation that was stabilized on the ramp. This standing wave detonation lasted for the duration of active fuelling, which was around three seconds. This is significantly longer than a normal detonation and demonstrates proof-of-concept, the researchers say.

Air-breathing rocket engines: the future of space flight

Ahmed explains that the main benefit of detonation based propulsion would be much higher efficiency than deflagration-based systems. “Achieving hypersonic speeds is critical because we don’t currently have a propulsion system that can do that,” he adds. “The only propulsion system that can give you hypersonic speeds is a rocket motor. Now rocket motors are not efficient. We know that because otherwise we’d all be flying to outer space. They are very expensive.”

Ahmed tells Physics World that now the researchers have proved that an oblique detonation wave engine is possible they want to explore the different conditions, such as fuel type and speed of the fuel flow, under which the detonation can remain stable. This is necessary to achieve the levels of control that would be required to realise hypersonic flight.

The bots are not as fair-minded as they seem

Artificial intelligence (AI) technologies are designed to replicate human capabilities, and in some cases improve upon them. Lifelike robots are physical examples of AI technology, but it is the digital AI systems that already have a ubiquitous influence on our daily lives – from facial recognition software to decision-making tools used by banks, recruiters and the police. Too often, these systems can reflect preexisting social inequalities.

In this episode of the Physics World Stories podcast Andrew Glester investigates the ethical issues that can plague AI and machine learning technologies. He finds out about the concepts of deep learning and neural networks, why these systems can amplify problems in society, and who are the people adversely affected by these flaws.

It turns out that the physics community is part of the problem and potentially part of the solution. Directly and indirectly, physicists are involved in developing AI technology so are ideally placed to raise awareness of the issues. Featuring in the episode:

Alan Winfield, a robot ethics researcher at the University of the West of England
Julianna Photopoulos, a science writer based in Bristol, UK
Savannah Thais, an experimental particle physicist at Princeton University, US

To find out more about the issue of bias in AI systems, take a look at this feature article by Photopoulos, which is summarised in the video below.

Doped antiferromagnets switch faster

Physicists at the Massachusetts Institute of Technology (MIT) in the US have found a new way to switch antiferromagnetism on and off. Their technique, which involves doping antiferromagnetic materials with extra electrons, could allow for faster, higher-density and more stable memory storage.

Riccardo Comin, who led the research. Courtesy: Comin Photon Scattering Lab

Modern hard disk drives encode data by using magnetic field pulses to flip the spins of electrons (representing binary zeros and ones) in ferromagnetic materials within computer chips. While researchers have found ways to switch these binary bits at ever-smaller scales, the magnetic pulses require a relatively large electrical current, so the data-writing process dissipates a lot of energy. It is also relatively slow, with a complete spin flip taking tens of nanoseconds.

Promising candidates

Antiferromagnetic materials are promising candidates for future high-density memory applications because the spins in antiferromagnets flip much faster, at frequencies in the terahertz range. These rapid spin flips are possible because of strong interactions between electron spins, which come about because the spins in antiferromagnets tend toalign so that they are antiparallel to each other (unlike conventional ferromagnets, which have parallel electron spins).

Antiferromagnets also lack magnetization on the scale of a single bit (>10 nm), making them robust to external magnetic perturbations. This means that data encoded in a memory device based on antiferromagnetic bits could not be erased magnetically. A further advantage is that antiferromagnets could be made into smaller transistors (and thus be packed more densely onto chips) than is possible with conventional transistor materials like silicon.

Creating oxygen vacancies

In the new work, MIT researchers led by Riccardo Comin set out to determine whether they could manipulate the antiferromagnetic properties of 100-nm-thick films of samarium and neodymium nickel oxide (SmNiO₃ and NdNiO₃) by doping them with extra electrons. While doping usually involves introducing impurities in a material to alter its electronic properties, in this case Comin and colleagues achieved what’s known as carrier doping by stripping the SmNiO₃ and NdNiO₃of oxygen atoms. This process leaves two electrons behind for every oxygen removed, and the left-behind electrons redistribute themselves among the remaining oxygen atoms and the nickel.

During these experiments, the researchers monitored the films using advanced magnetic X-ray crystallography techniques to determine whether the material’s magnetic structure remained intact – in other words, whether the atomic spins retained their orderly anti-alignment and therefore their antiferromagnetism. Any reduction in the previously ordered magnetic structure, they explain, would be evidence that the doping was enough to switch off the material’s antiferromagnetism.

Sharp magnetic transition

The researchers say the antiferromagnetic order in rare earth nickelates like the ones they studied is remarkably robust to carrier doping – up to a point. Then, at a critical doping threshold of 0.21 electrons per nickel atom, the order suddenly collapses. This magnetic transition is sharp, similar to switching a transistor’s state from zero to one, and can be reversed by adding oxygen back into the material.

Comin says that, like conventional ferromagnetic bits, antiferromagnetic bits might be switched using voltage gates (a process in which a small voltage is applied to a bit to alter its electrical conductivity). He tells Physics World that this would allow rare earth nickelate materials to be employed in field-effect devices that could directly read/write operations on antiferromagnetic bits.

The researchers, who report their work in Physical Review Letters, say they now hope to gain better control over the magnetic switching process, optimize it for device applications and explore more exotic physical properties in the rare earth nickelate family for functional applications. They also plan to study other materials with potentially higher magnetic transition temperatures as potential platforms for a new magnetic storage technology.

Ultracold circuits for quantum computing, graphene boosts sustainability, secrets of supercooled water

Creating a quantum computer that integrates a large number of components is a huge challenge for many reasons. One is that most quantum bits (qubits) used today must be chilled to near absolute zero and therefore isolated from the room-temperature electronic components used to control them. This makes it extremely difficult to have large numbers of qubits and controllers packed into a small space. But researchers in the Netherlands may have a solution, as the science writer and educator Karmela Padovic-Callaghan explains in this episode of the Physics World Weekly podcast.

The “wonder material” graphene is a sheet of carbon just one atom thick that has extraordinary physical properties. One promising use for graphene is to make products more sustainable. On hand to talk about how graphene can do its bit to improve the environment is Andrew Pollard, who specializes in the metrology of 2D materials at the UK’s National Physical Laboratory.

Water is another substance that has wonderous properties that have proven very useful for sustaining life on Earth. Recently, researchers used a laser technique to study supercooled water in a previously inaccessible temperature regime. Katherine Skipper, a PhD student contributor to Physics World joins us to chat about this breakthrough. Skipper also talks about her research at the University of Bristol, which is focussed on active matter that comprises two-faced Janus particles.

More information about the characterization and standardization of graphene can be found at these pages on the National Physical Laboratory website: “Structural and chemical characterisation of graphene”; “Graphene and 2D materials”; and “The importance of international standards for the graphene community”.

Automation and standardization: redefining best practice in stereotactic radiosurgery

Indication-specific: Elements Multiple Brain Mets SRS dose planning enables the delivery of highly conformal single-session treatments for multiple metastases. (Courtesy: Brainlab)

Simplicity, automation, efficiency and dose targeting with sub-mm accuracy: these are the operational reference points for the radiation oncology team running the stereotactic radiosurgery (SRS) programme at Scripps MD Anderson Cancer Center in San Diego, California. Underpinning that stereotactic clinical workflow is a portfolio of enabling technologies from Brainlab, a German healthcare equipment maker specializing in hardware and software products for SRS, surgical navigation and the next-generation “digital operating room”.

“Think automated treatment planning, high-precision patient positioning plus online imaging and thermal surface guidance,” says Steve Kirsner, director of medical physics at Scripps MD Anderson. “Our established suite of Brainlab products makes it easy for physicists to do what they have to do, allowing us to deliver high-quality care to our cancer patients every time. You can’t ask for more than that.”

Kirsner, for his part, heads up a 16-strong multidisciplinary clinical team (eight medical physicists and eight dosimetrists) working across three radiotherapy centres in the greater San Diego area. Right now, those three clinics treat around 160 patients a day, with a fourth facility being commissioned ahead of coming online for initial patient treatments in August. “We run a busy SRS linac schedule for diverse brain indications,” adds Kirsner. “That’s complemented by an extensive programme of stereotactic body radiotherapy (SBRT) to treat a range of cancer types, including prostate, lung, spine, liver, pancreas and oligometastasis.”

End-to-end workflows

In developing its SRS programme, the Scripps MD Anderson team has worked closely with Brainlab engineers to implement indication-specific, end-to-end workflows for a range of brain stereotactic treatments. That specificity means the algorithms in Elements, Brainlab’s à la carte treatment planning software, are able to address the unique clinical challenges surrounding a given disease indication. Elements Cranial SRS, for example, automatically creates SRS treatment plans for complex cranial indications – including, but not limited to, vestibular schwannomas, meningiomas and large metastases – while Elements Multiple Brain Mets SRS dose planning enables the delivery of highly conformal single-session treatments for multiple metastases, minimizing dose spread to adjacent healthy tissue and critical structures.

“Elements is one of the most user-friendly platforms for SRS treatment planning,” explains Kirsner. That ease of use is particularly important when running a medical physics service across multiple sites and multiple teams. It’s straightforward, for example, to create a set of standard planning templates for a given clinical indication. Those templates can then be shared across every facility to give treatment planners a common baseline and reference point from which to develop individualized SRS plans on a patient-by-patient basis. “Put simply,” adds Kirsner, “Elements has enabled us to create a unified framework for treatment planning, driving standardization and best practice across multiple Scripps MD Anderson treatment facilities.”

It helps, of course, that Elements offers a streamlined, automated process to arrive at a high-quality SRS treatment plan, while verification of the plan and translation to the treatment machine are equally friction-free. All of which equates to a win-win for healthcare providers and patients. “Efficiency and patient throughput are key metrics for any oncology centre,” says Kirsner, “while there’s a growing demand from patients for the benefits associated with SRS treatment schedules – not least fewer visits to the clinic and a faster return to family and friends.”

Alongside Elements for treatment planning, Kirsner’s team is also taking advantage of Brainlab’s in-room X-ray-based ExacTrac positioning and monitoring system. ExacTrac X Ray enables robust verification of patient positioning – to isocentre with sub-mm accuracy – as well as intrafractional monitoring of patient motion and internal anatomical shifts during treatment. The latest iteration of the platform, ExacTrac Dynamic, is able to track patient position along an additional coordinate – using a 4D thermal camera to create a hybrid thermal image that correlates the patient’s heat signature to their reconstructed 3D surface structure.

ExacTrac Dynamic is currently being installed as part of the commissioning of the fourth Scripps MD Anderson radiotherapy clinic, with the three established treatment centres all due to upgrade to the new imaging system by the end of this year. “We like the intuitive user experience with ExacTrac,” says Kirsner, “and the fact that it allows us to image what we treat at all times, regardless of couch angle or gantry position.” In addition, the new thermal surface guidance capability will allow Kirsner and his team to automatically track both internal and external motion throughout the entire treatment of the patient. “ExacTrac imaging also integrates seamlessly with our Varian machines,” Kirsner adds. “This will allow treatment to be paused if either the X-ray imaging or surface guidance are deemed out of tolerance – so we’re excited to see the clinical impact of the latest surface-tracking functionality in ExacTrac Dynamic.”

Clinical validation

While the near-term priority for Kirsner and colleagues is the commissioning and go-live of their new radiotherapy facility, the team is also focused on achieving Novalis-Certified status by year-end. Novalis Certification is an independent accreditation programme that offers formal recognition of the highest clinical standards in SRS/SBRT treatment delivery. As such, a key outcome for cancer treatment centres is the implementation of standardized protocols for continual self-assessment and quality improvement, further reinforcing the access that Brainlab customers get to Novalis Circle, a global network of oncology centres dedicated to best practice, knowledge-sharing and collaboration on all aspects of SRS/SBRT.

“I see the independent audit of Novalis certification as a great way to strengthen the SRS/SBRT programme at Scripps MD Anderson,” concludes Kirsner. “We are all doing the best we can for our patients, but you can always learn something new from colleagues running their own radiotherapy programmes. It’s my belief that our practice here is already Novalis-compliant, so the next step is to formally document those processes and workflows for external review.”

Sandwich strategy makes solid-state lithium battery last longer

Researchers in the US have created a new solid-state lithium-metal battery that can charge and discharge over a record-breaking number of cycles at a high current density. The proof-of-concept device, which is fundamentally different from existing liquid electrolyte lithium-ion batteries, could extend the lifespan of electric vehicle batteries to 10–15 years, similar to that of petrol and diesel cars.

The ideal battery for an electric vehicle would last a long time and store up a lot of charge very quickly. While lithium-metal batteries score well on the second of these criteria thanks to their high capacity and high energy density, their longevity leaves something to be desired. This is because, during charging, lithium ions move from the cathode to the anode. When this anode is made of lithium metal, needle-like structures called dendrites form on the electrode surface and grow into the electrolyte. Eventually, these unwanted structures pierce the barrier separating the anode and cathode, causing the battery to short or even ignite.

To overcome this problem, researchers have previously tried replacing the liquid electrolyte in these devices with a solid-state one that is more difficult for the dendrites to grow through. In practice, however, lithium dendrites can still burrow through the barrier via micron- or submicron-sized cracks produced when the battery is assembled.

Sandwich effect

Researchers led by Xin Li, a professor of materials science at Harvard University’s John A Paulson School of Engineering and Applied Sciences (SEAS), have now developed a solid-state battery in which lithium dendrite penetration is no longer a problem. Rather than stopping the dendrites dead in their tracks with a single barrier, their new battery design takes a multi-layered approach. The design, which is detailed in Nature, incorporates a less-stable electrolyte sandwiched between layers of more-stable solid electrolytes. Together, these layers keep dendrite growth under control.

The full battery “sandwich” consists of six layers: a lithium-metal anode, a graphite coating, the first electrolyte, the second electrolyte, another layer of the first electrolyte, and finally the cathode. The first electrolyte, which has the chemical formula Li_5.5PS_4.5Cl_1.5 (LPSCI), is prone to dendrite penetration. The second electrolyte, Li₁₀Ge₁P₂S₁₂(LGPS), is more robust. The idea is that dendrites are permitted to grow through the graphite and electrolyte no. 1 but stop when they reach electrolyte no. 2 – and, crucially, before they short out the battery.

“Incorporating instability to stabilize the battery might sound counterintuitive,” says team member Luhan Ye. “But just like an (expansion) anchor can guide and control a screw going into a drywall, so too can our multilayer design guide and control the growth of dendrites. The difference is that our anchor (the LPSCI) quickly becomes too tight for the dendrite to drill through, so the dendrite growth is stopped.”

Superior cycling performance

The researchers found that when they paired their lithium metal anode with a LiNi_0.8Mn_0.1Co_0.1O₂ cathode, the cycling performance was very stable, with the battery maintaining 82% of its capacity after 10,000 cycles at a 20C rate (8.6 mA/cm²) and 81.3 % of its capacity after 2000 cycles at a 1.5C rate (0.64 mA/cm²). The design allows for a specific power of 110.6 kW/kg and a specific energy of up to 631.1 Wh/kg from the cathode material.

The test device cycled for 1800 hours at 0.25 mA/cm², which is substantially better than the single-electrode-type batteries the researchers tested. It can also cycle at an extremely high current density of 20 mA/cm² with a low “overpotential” (that is, the potential difference between a theoretical or thermodynamically determined voltage and the actual voltage under operating conditions) of ~0.5 V without significant signs of short-circuiting, even at a temperature of 55 °C.

Longer lifetime and faster charging

According to the researchers, the new battery technology could make the lifetime of electric vehicles comparable to that of petrol- or diesel-powered ones – 10 to 15 years – without the need to replace the battery. With its high current density, the design could also “pave the way for electric vehicles that can fully charge within 10–20 minutes,” they say.

“This proof-of-concept design shows that lithium-metal solid-state batteries could be competitive with commercial lithium-ion batteries,” Li adds. “And the flexibility and versatility of our multilayer design makes it potentially compatible with mass production procedures in the battery industry.”

The researchers say they now plan to scale up their device to a pouch cell the size of an ID card. “We expect that the attractive performance we have presented in our work will hold, since our innovation focuses on the scalable factors of materials chemistry and their combinations,” Li tells Physics World. “The design also opens the door to many new fundamental studies in solid-state battery research that were hitherto not possible without a stable lithium-metal anode.”

As MRI strength increases, so do concerns about magnet safety

As the push toward stronger and faster MRI scanners continues, so does concern over magnet safety, according to Filiz Yetisir, who discussed the potential effects MRI has on patients at the recent International Society for Magnetic Resonance in Medicine virtual meeting (ISMRM 2021).

There are three main components of MRI scanners – the main magnet, gradient coils and radiofrequency (RF) coils – that are essential to a machine’s function. However, each of these components carries unique risks for patients and operators, namely the projectile effect, nerve stimulation, hearing damage and tissue heating.

Yetisir, a postdoctoral graduate student at Harvard University School of Medicine, discussed these potential effects and provided tips for reducing their impact during procedures in a May 16 session at ISMRM 2021.

The main magnet

Consider that junkyard magnets capable of lifting cars have strengths of 1 T. Most of the MRI scanners used in clinical settings today have strengths of 1.5 T and 3 T, while scanners used mainly in research have strengths of 7 T – and in some cases up to 11 T. Recently, 7 T scanners, such as the Magnetom Terra scanner by Siemens Healthineers and the Signa 7.0T scanner from GE Healthcare, for instance, have been cleared for clinical marketing by the US Food and Drug Administration.

The main safety risk associated with the main magnet is the projectile effect, or the “missile effect,” according to Yetisir. MRI magnets attract iron particles with tremendous force. This can include other ferromagnetic objects in the scanning room, such as chairs or oxygen tanks.

MRI safety — An office chair caught in an MRI scanner. (Courtesy: of Frank Shellock, MRISafety.com)

“If there’s a person inside the scanner or in the path of the flying object, the projectile could result in serious injuries or even death,” she said.

In 2001 in New York, for instance, 6-year-old Michael Colombini died after being hit in the head with an oxygen tank. “The most important thing is to remember that the magnet is always on,” Yetisir said.

Gradient coils

The magnetic field produced by the gradient coils in the MR scanner can induce an electric field in the patient’s body due to Faraday’s Law of Induction. This electric field may form “hot spots”, which can create enough electric potential to stimulate large nerves. This phenomenon is known as peripheral nerve stimulation (PNS).

PNS causes discomfort for subjects due to a tingling or tapping sensation, and its severity depends on several factors, including the gradient coil type and wire pattern, the patient’s body size, shape and tissue distribution, and nerve orientation and diameter, for instance.

“Similar to PNS, the electric fields induced by the gradient coils can stimulate the heart, leading to cardiac arrhythmia,” Yetisir said.

Acoustic noise due to mechanical vibration is another safety concern associated with gradient coils because it can be discomforting for the patient and may damage hearing, especially with fast sequences such as echo-planar imaging.

Yetisir recommended communicating with patients regarding tingling or tapping sensations and monitoring the subject closely. If the patient experiences discomfort, one solution is to switch from first-level mode to normal mode or to select a slow gradient mode, if it is available.

Earplugs should be used by anyone in the scan room, extra protection should be used for vulnerable subjects, such as neonatal or paediatric patients, and for mechanical vibrations, table pads can be used for vulnerable patients, Yetisir suggested.

“If the patient is still too uncomfortable, the scan should be stopped,” she said.

Radiofrequency coils

The radiofrequency coils (the “antennae” of the MRI system) have a magnetic field that can induce an electric current in conductive human tissue. These currents can lead to power deposition, which eventually leads to temperature elevation and may cause heat stress or burns.

While vendors control specific absorption rate limits in the scanner itself, patients need to be carefully screened for metals and tattoos, as well as permanent makeup, which can increase the risks. No cables should touch the patient, Yetisir said.

Moreover, avoid “limb loops,” which are caused by hands touching hips, or by clasped hands, for instance, because they may increase local RF heating by up to a factor of three, Yetisir said.

In addition to the potential effects associated with the three main components of MR scanners, patient implants may pose additional safety risks, Yetisir said. Implants interact with all three magnetic fields produced by the MR scanner’s components. Metallic implants have higher conductivity than human tissue and can lead to increased nerve stimulation, heating and vibration, and ultimately malfunction of the implant.

Check the MRI labelling from the manufacturer on the patient’s implant and pay specific attention to the main field strength and gradient limits, gradient field amplitude and slew rate limits, exposure time limits and exclusion zones, Yetisir suggested.

“New implant designs are being developed to address some of the limitations in MRI due to implant safety,” she concluded.

This article was originally published on AuntMinnie.com. ©2021 by AuntMinnie.com. Any copying, republication or redistribution of AuntMinnie.com content is expressly prohibited without the prior written consent of AuntMinnie.com.

Catherine Heymans appointed first female Astronomer Royal for Scotland

Astrophysicist Catherine Heymans has become the first woman to be appointed Astronomer Royal for Scotland since it was created almost 200 years ago. Heymans, who is based at University of Edinburgh, will become the eleventh person to hold the role after it became vacant in 2019 following the death of the incumbent John Campbell Brown, who held the position since 1995.

Heymans has a DPhil in astrophysics from the University of Oxford and, following postdoctoral positions in Germany and Canada, moved to the University of Edinburgh in 2008. Since 2019 she has held a joint position at Ruhr-University Bochum as director of its German Centre for Cosmological Lensing.

I hope to be able to demonstrate that science is for everyone
Catherine Heymans

Created in 1834, the position of Astronomer Royal for Scotland was originally held by the director of the Royal Observatory in Edinburgh. Since 1995, however, it has been awarded as an honorary title. Heymans was recommended to the Queen for the role by an international panel that was organized by the Royal Society of Edinburgh. She is expected to officially begin the position in the coming days following the signing of the official Royal Warrant.

Astronomy for all

Heymans told Physics World that it is an “honour” to have been appointed not only the first female Astronomer Royal for Scotland but also the first female for all the Astronomer Royal positions in the UK. Heymans adds that she wants to use the position to show girls that science is an “amazing, creative and innovative career, and absolutely for them”.

Heymans, who has given numerous popular talks about her research including in 2013 to celebrate the 25th anniversary of Physics World, says that John Campbell Brown especially used the role to engage with the public, something that she would like to carry on. “Science is such a thought-provoking subject, yet so many people are scared of it,” she says. “I hope to be able to demonstrate that science is for everyone, irrespective of gender, age, (dis)ability, race, religion or belief, or sexual orientation – it doesn’t matter who you are or where you come from – the universe is for everyone.”

Indeed, one of Heymans’ first initiatives will be to install telescopes at all of Scotland’s remote outdoor learning centres that are visited by school pupils. “My hope is that once that spark and connection with the universe is made, children will carry that excitement home with them and develop a life-long passion for astronomy or, even better, science as a whole,” she says.

UK Astronomer Royal Martin Rees says that Heymans’ appointment is “excellent news”. “She is a distinguished successor to her predecessors in this role and I wish her a long and successful tenure,” he adds.

A free-to-read Physics World Discovery ebook by Catherine Heymans about the dark universe is available here.

Triboelectricity can boost the efficiency and lifetime of filter masks

TENG powered mask — More effective: diagram of the mask design showing the polymer fibres and the TENG. (Courtesy: LingyunWang *et al.*/*Nano Energy*)

A new design for a self-powered air filter mask with enhanced removal efficiency for nano and microparticles and an extended lifetime of at least two days of continuous service has been unveiled by researchers in China.

Filter masks play an important role in our wellbeing as the presence of airborne particulate pollutants is widespread and has a negative impact on human health. Most recently, the COVID-19 pandemic has shown us the importance of using face masks as protective equipment, to prevent the respiration of hazardous particles in the environment. However, current disposable masks are unable to remove extremely small particles effectively. Another shortcoming is that they can only be used for a few hours, after which, the efficiency of filtration decreases.

In this latest research, Lingyun Wang and colleagues at City University of Hong Kong and the Chinese University of Hong Kong have discovered that the inclusion of polymeric fibres with a small diameter within the range of nanometres are the key to enhancing a mask’s ability to eliminate suspended particles. They have created a hybrid air filter using the fibres that has a porous structure, with varied pore sizes to increase the amount of filtered material.

Charge separation

However, the capture efficiency of these fibres alone is still relatively low. This is because filtration relies on the passive accumulation of particles due to the physical impact of the particles on the fibres and Brownian motion. To boost the efficiency, Wang and colleagues took advantage of the fact the polymers have a marked separation of positive and negative charges due to their large dipole moment, which can attract more particles through electrostatic interactions.

To boost the long-range attraction of the filters to negatively charged and dipolar particles in air, the team applied an external electrical field to the hybrid air filter using a triboelectric nanogenerator (TENG). Today’s N95 masks exploit a similar strategy, using charged polypropylene to attract airborne particles. However, N95 masks can only achieve this attraction for a few hours, after which their ability to attract particles decreases. By actively applying a voltage using triboelectricity, the team enhanced these electrostatic forces and increased lifespan of their filter masks to up to two days.

The researchers also harnessed the TENG to give the filter antiseptic properties by incorporating silver nanowires with the fibres. Silver has well-known antibacterial properties because it releases ions that damage bacterial proteins. This effect can be enhanced using an electric field and this allowed the team to achieve a 35% sterilization rate.

Low-cost and green energy

The TENG provides low-cost and green energy, which is harvested from the movement between two materials that generate opposite charges by contact friction. One of the materials gains electrons while the other loses them, generating an electrical current that is used to power the filter masks. Therefore, a mask can be self-powered by the wearer’s movement only, without the need for batteries or additional energy sources.

In the new device, the team connected the polymer fibres to the TENG via an electrically conductive substrate. The hybrid filtering material was tested using a custom-made set-up that produced a continuous air flow while monitoring the numbers of particles in air before and after the contact with the filtration masks.

The researchers found that the filters can operate for two days under a continuous airflow, while maintaining a high removal efficiency even for small particles. They now hope to incorporate their technology into practical lightweight facemasks that are charged using human motion.

The findings are reported in a paper in Nano Energy.

Chemistry gets put to a quantum test

Chemical reactions are complex. Even if only a few molecules are involved, the final configuration will depend on a huge number of parameters – including, in principle, all the possible locations each electron in each atom can occupy as the reaction takes place. Calculating these trajectories is beyond the power of today’s best computers, but researchers led by Kang-Kuen Ni of Harvard University in the US have now demonstrated an alternative. By cooling molecules of potassium and rubidium down to a fraction of a degree above absolute zero, they reduced the number of possible reaction outcomes to just 57. They then probed all these outcomes down to the level of individual quantum states, paving the way for a better understanding of chemistry.

When molecules are cooled close to 0 K, they enter their absolute quantum mechanical ground state, with the lowest possible levels of electronic, vibrational and rotational energy. Additionally, their kinetic energy, which governs their movement through space, becomes vanishingly small. Ultracold molecules are therefore an ideal test bed for validating quantum-dynamical models of reactions.

Ultracold KRb reactions

In their study, which is published in Nature, Ni and her colleagues focused on the reaction that occurs when ultracold potassium-rubidium (KRb) molecules exchange atoms to form K₂ and Rb₂. This reaction releases energy (it is exoergic), but the energy released is not sufficient to excite vibrational motion in the product molecules. This means that all the energy must instead go into rotational and translational motion (recoil energy).

Under these conditions, a quantum statistical model that includes conservation of energy conservation and angular momentum predicts that the number of possible joint quantum state pairs in the product molecules is relatively small, at 57. However, measuring these states is still no easy task and requires special techniques.

The researchers detect the rotational states of their K₂ and Rb₂ molecules by ionizing them with a laser pulse. The ionization process is selective, occurring only when the laser frequency matches the gap in the molecules’ energy levels, which is different for each rotational state. The ionized molecules then fly through a series of circular electrodes (all integrated into the reaction chamber), at which point electric fields propel them towards a position-sensitive detector. Once the researchers mapped how often a particular rotational state originated from the reaction, they changed the laser frequency to match the energy levels of the next rotational state and repeated the process until the survey was complete.

Ni and colleagues also needed to determine whether the molecules they detected were produced in the same reaction event. For this, they applied the principle of conservation of linear momentum. When two KRb molecules of similar momenta collide, the resulting K₂ and Rb₂ molecules fly away from each other with equal but opposite momenta. This means the product molecules arrive at the detector at different locations, but their arrival can be correlated. This correlation enabled the researchers to distinguish reaction products from molecules that originated in other processes.

Quantum-dynamical model

In a series of experiments (most of which were run remotely to comply with COVID restrictions), the Harvard team measured the scattering probabilities for all 57 allowed rotational state-pairs of the products. They found that while 50 of these probabilities matched theoretical predictions, the remaining seven did not. In one of these non-matching outcomes, the reaction took place close to the threshold of the exoergic energy, where most of the energy couples only to rotational motion. This leaves very little recoil energy for the reaction products to fly apart and get detected, meaning that the lack of theoretical agreement could be an artefact of the team’s measurement system. The other deviations, however, have no straightforward explanation, and contradict the prediction that all product states should have an equal probability of forming.

“For many physical chemists it’s been a long-term dream to be able to follow a chemical reaction from start to finish at the quantum state resolved level,” Ni says. “In this work, we achieved that.” Ni notes that the deviations they observed “may not be understood for another 10-20 years, until perhaps a quantum computer solves the quantum reaction dynamics calculations”. However, she concludes, “the data is there for future benchmarking of theory for when that becomes available”.

“A significant achievement”

Balakrishnan Naduvalath, a physical chemist at the University of Nevada in Las Vegas, US, who was not involved in the study, says that precisely mapping the quantum states of reaction products is “a significant achievement that no other research group has accomplished for a bimolecular chemical reaction in the ultracold regime”. The experiment, he adds, “provides much deeper insights into chemistry and at the same time challenges theory. The availability of experimental results surely would motivate theorists to develop new methodologies (and benchmark them) to study this class of four-atom reactions that might also benefit other areas of chemistry.”

As a next step, members of the Harvard team plan to put their system to work on molecule-to-atom reactions, which are less complex, and for which the available theoretical predictions are more accurate. They also hope to leverage such reactions to generate quantum entanglement resources.