Chess is a seemingly simple game, but one that hides incredible complexity. In the standard game, the starting positions of the pieces are fixed so top players rely on memorizing a plethora of opening moves, which can sometimes result in boring, predictable games. It’s also the case that playing as white, and therefore going first, offers an advantage.
In the 1990s, former chess world champion Bobby Fischer proposed another way to play chess to encourage more creative play.
This form of the game – dubbed Chess960 – keeps the pawns in the same position but randomizes where the pieces at the back of the board – the knights, bishops, rooks, king and queen – are placed at the start while keeping the rest of the rules the same. It is named after the 960 starting positions that result from mixing it up at the back.
It was thought that Chess960 could allow for more permutations that would make the game fairer for both players. Yet research by physicist Marc Barthelemy at Paris-Saclay University suggests it’s not as simple as this.
Initial advantage
He used the open-source chess program called Stockfish to analyse each of the 960 starting positions and developed a statistical method to measure decision-making complexity by calculating how much “information” a player needs to identify the best moves.
He found that the standard game can be unfair, as players with black pieces who go second have to keep up with the moves from the player with white.
Yet regardless of starting positions at the back, Barthelemy discovered that white still has an advantage in almost all – 99.6% – of the 960 positions. He also found that the standard set-up – rook, knight, bishop, queen, king, bishop, knight, rook – is nothing special and is presumably an historical accident possibly as the starting positions are easy to remember, being visually symmetrical.
“Standard chess, despite centuries of cultural evolution, does not occupy an exceptional location in this landscape: it exhibits a typical initial advantage and moderate total complexity, while displaying above-average asymmetry in decision difficulty,” writes Barthelemy.
For a more fair and balanced match, Barthelemy suggests playing position #198, which has the starting positions as queen, knight, bishop, rook, king, bishop, knight and rook.
The Compact Muon Solenoid (CMS) Collaboration has made the first measurements of the quantum properties of a family of three “all-charm” tetraquarks that was recently discovered at the Large Hadron Collider (LHC) at CERN. The findings could help shed more light on the properties of the strong nuclear force, which holds protons and neutrons together in nuclei. The result could help us better understand how ordinary matter forms.
In recent years, the LHC has discovered tens of massive particles called hadrons, which are made of quarks bound together by the strong force. Quarks come in six types: up, down, charm, strange, top and bottom. Most observed hadrons comprise two or three quarks (called mesons and baryons, respectively). Physicists have also observed exotic hadrons that comprise four or five quarks. These are the tetraquarks and pentaquarks respectively. Those seen so far usually contain a charm quark and its antimatter counterpart (a charm antiquark), with the remaining two or three quarks being up, down or strange quarks, or their antiquarks.
Identifying and studying tetraquarks and pentaquarks helps physicists to better understand how the strong force binds quarks together. This force also binds protons and neutrons in atomic nuclei.
Physicists are still divided as to the nature of these exotic hadrons. Some models suggest that their quarks are tightly bound via the strong force, so making these hadrons compact objects. Others say that the quarks are only loosely bound. To confuse things further, there is evidence that in some exotic hadrons, the quarks might be both tightly and loosely bound at the same time.
Now, new findings from the CMS Collaboration suggest that tetraquarks are tightly bound, but they do not completely rule out other models.
Measuring quantum numbers
In their work, which is detailed in Nature, CMS physicists studied all-charm tetraquarks. These comprise two charm quarks and two charm antiquarks and were produced by colliding protons at high energies at the LHC. Three states of this tetraquark have been identified at the LHC. These are: X(6900); X(6600); and X(7100), where the numbers denote their approximate mass in millions of electron volts. The team measured the fundamental properties of these tetraquarks, including their quantum numbers: parity (P); charge conjugation (C); angular momentum, and spin (J). P determines whether a particle has the same properties as its spatial mirror image; C whether it has the same properties as its antiparticle; and J, the total angular momentum of the hadron. These numbers provide information on the internal structure of a tetraquark.
The researchers used a version of a well-known technique called angular analysis, which is similar to the technique used to characterize the Higgs boson. This approach focuses on the angles at which the decay products of the all-charm tetraquarks are scattered.
“We call this technique quantum state tomography,” explains CMS team member Chiara Mariotti of the INFN Torino inItaly. “Here, we deduce the quantum state of an exotic state X from the analysis of its decay products. In particular, the angular distributions in the decay X → J/ψJ/ψ, followed by J/ψ decays into two muons, serve as analysers of polarization of two J/ψ particles,” she explains.
The researchers analysed all-charm tetraquarks produced at the CMS experiment between 2016 and 2018. They calculated that J is likely to be 2 and that P and C are both +1. This combination of properties is expressed as 2++.
Result favours tightly-bound quarks
“This result favours models in which all four quarks are tightly bound,” says particle physicist Timothy Gershon of the UK’s University of Warwick, who was not involved in this study. “However, the question is not completely put to bed. The sample size in the CMS analysis is not sufficient to exclude fully other possibilities, and additionally certain assumptions are made that will require further testing in future.”
Gershon adds, “These include assumptions that all three states have the same quantum numbers, and that all correspond to tetraquark decays to two J/ψ mesons with no additional particles not included in the reconstruction (for example there could be missing photons that have been radiated in the decay).”
Further studies with larger data samples are warranted, he adds. “Fortunately, CMS as well as both the LHCb and the ATLAS collaborations [at CERN] already have larger samples in hand, so we should not have to wait too long for updates.”
Indeed, the CMS Collaboration is now gathering more data and exploring additional decay modes of these exotic tetraquarks. “This will ultimately improve our understanding how this matter forms, which, in turn, could help refine our theories of how ordinary matter comes into being,” Mariotti tells Physics World.
When people think of wind energy, they usually think of windmill-like turbines dotted among hills or lined up on offshore platforms. But there is also another kind of wind energy, one that replaces stationary, earthbound generators with tethered kites that harvest energy as they soar through the sky.
This airborne form of wind energy, or AWE, is not as well-developed as the terrestrial version, but in principle it has several advantages. Power-generating kites are much less massive than ground-based turbines, which reduces both their production costs and their impact on the landscape. They are also far easier to install in areas that lack well-developed road infrastructure. Finally, and perhaps most importantly, wind speeds are many times greater at high altitudes than they are near the ground, significantly enhancing the power densities available for kites to harvest.
There is, however, one major technical challenge for AWE, and it can be summed up in a single word: control. AWE technology is operationally more complex than conventional turbines, and the traditional method of controlling kites (known as model-predictive control) struggles to adapt to turbulent wind conditions. At best, this reduces the efficiency of energy generation. At worst, it makes it challenging to keep devices safe, stable and airborne.
In a paper published in EPL,Antonio Celani and his colleagues Lorenzo Basile and Maria Grazia Berni of the University of Trieste, Italy, and the Abdus Salam International Centre for Theoretical Physics (ICTP) propose an alternative control method based on reinforcement learning. In this form of machine learning, an agent learns to make decisions by interacting with its environment and receiving feedback in the form of “rewards” for good performance. This form of control, they say, should be better at adapting to the variable and uncertain conditions that power-generating kites encounter while airborne.
What was your motivation for doing this work?
Our interest originated from some previous work where we studied a fascinating bird behaviour called thermal soaring. Many birds, from the humble seagull to birds of prey and frigatebirds, exploit atmospheric currents to rise in the sky without flapping their wings, and then glide or swoop down. They then repeat this cycle of ascent and descent for hours, or even for weeks if they are migratory birds. They’re able to do this because birds are very effective at extracting energy from the atmosphere to turn it into potential energy, even though the atmospheric flow is turbulent, hence very dynamic and unpredictable.
Antonio Celani. (Courtesy: Antonio Celani)
In those works, we showed that we could use reinforcement learning to train virtual birds and also real toy gliders to soar. That got us wondering whether this same approach could be exported to AWE.
When we started looking at the literature, we saw that in most cases, the goal was to control the kite to follow a predetermined path, irrespective of the changing wind conditions. These cases typically used only simple models of atmospheric flow, and almost invariably ignored turbulence.
This is very different from what we see in birds, which adapt their trajectories on the fly depending on the strength and direction of the fluctuating wind they experience. This led us to ask: can a reinforcement learning (RL) algorithm discover efficient, adaptive ways of controlling a kite in a turbulent environment to extract energy for human consumption?
What is the most important advance in the paper?
We offer a proof of principle that it is indeed possible to do this using a minimal set of sensor inputs and control variables, plus an appropriately designed reward/punishment structure that guides trial-and-error learning. The algorithm we deploy finds a way to manoeuvre the kite such that it generates net energy over one cycle of operation. Most importantly, this strategy autonomously adapts to the ever-fluctuating conditions induced by turbulence.
Lorenzo Basile. (Courtesy: Lorenzo Basile)
The main point of RL is that it can learn to control a system just by interacting with the environment, without requiring any a priori knowledge of the dynamical laws that rule its behaviour. This is extremely useful when the systems are very complex, like the turbulent atmosphere and the aerodynamics of a kite.
What are the barriers to implementing RL in real AWE kites, and how might these barriers be overcome?
The virtual environment that we use in our paper to train the kite controller is very simplified, and in general the gap between simulations and reality is wide. We therefore regard the present work mostly as a stimulus for the AWE community to look deeper into alternatives to model-predictive control, like RL.
On the physics side, we found that some phases of an AWE generating cycle are very difficult for our system to learn, and they require a painful fine-tuning of the reward structure. This is especially true when the kite is close to the ground, where winds are weaker and errors are the most punishing. In those cases, it might be a wise choice to use other heuristic, hard-wired control strategies rather than RL.
Finally, in a virtual environment like the one we used to do the RL training in this work, it is possible to perform many trials. In real power kites, this approach is not feasible – it would take too long. However, techniques like offline RL might resolve this issue by interleaving a few field experiments where data are collected with extensive off-line optimization of the strategy. We successfully used this approach in our previous work to train real gliders for soaring.
What do you plan to do next?
We would like to explore the use of offline RL to optimize energy production for a small, real AWE system. In our opinion, the application to low-power systems is particularly relevant in contexts where access to the power grid is limited or uncertain. A lightweight, easily portable device that can produce even small amounts of energy might make a big difference in the everyday life of remote, rural communities, and more generally in the global south.
Circularly polarized (CP) light is encoded with information through its photon spin and can be utilized in applications such as low-power displays, encrypted communications and quantum technologies. Organic light emitting diodes (OLEDs) produce CP light with a left or right “handedness”, depending on the chirality of the light-emitting molecules used to create the device.
While OLEDs usually only emit either left- or right-handed CP light, researchers have now developed OLEDs that can electrically switch between emitting left- or right-handed CP light – without needing different molecules for each handedness.
“We had recently identified an alternative mechanism for the emission of circularly polarized light in OLEDs, using our chiral polymer materials, which we called anomalous circularly polarized electroluminescence,” says lead author Matthew Fuchter from the University of Oxford. “We set about trying to better understand the interplay between this new mechanism and the generally established mechanism for circularly polarized emission in the same chiral materials”.
Light handedness controlled by molecular chirality
The CP light handedness of an organic emissive molecule is controlled by its chirality. A chiral molecule is one that has two mirror-image structural isomers that can’t be superimposed on top of each other. Each of these non-superimposable molecules is called an enantiomer, and will absorb, emit and refract CP light with a defined spin angular momentum. Each enantiomer will produce CP light with a different handedness, through an optical mechanism called normal circularly polarized electroluminescence (NCPE).
OLED designs typically require access to both enantiomers, but most chemical synthesis processes will produce racemic mixtures (equal amounts of the two enantiomers) that are difficult to separate. Extracting each enantiomer so that they can be used individually is complex and expensive, but the research at Oxford has simplified this process by using a molecule that can switch between emitting left- and right-handed CP light.
The molecule in question is a helical molecule called (P)-aza[6]helicene, which is the right-handed enantiomer. Even though it is just a one-handed form, the researchers found a way to control the handedness of the OLED, enabling it to switch between both forms.
Switching handedness without changing the structure
The researchers designed the helicene molecules so that the handedness of the light could be switched electrically, without needing to change the structure of the material itself. “Our work shows that either handedness can be accessed from a single-handed chiral material without changing the composition or thickness of the emissive layer,” says Fuchter. “From a practical standpoint, this approach could have advantages in future circularly polarized OLED technologies.”
Instead of making a structural change, the researchers changed the way that the electric charges are recombined in the device, using interlayers to alter the recombination position and charge carrier mobility inside the device. Depending on where the recombination zone is located, this leads to situations where there is balanced or unbalanced charge transport, which then leads to different handedness of CP light in the device.
When the recombination zone is located in the centre of the emissive layer, the charge transport is balanced, which generates an NCPE mechanism. In these situations, the helicene adopts its normal handedness (right handedness).
However, when the recombination zone is located close to one of the transport layers, it creates an unbalanced charge transport mechanism called anomalous circularly polarized electroluminescence (ACPE). The ACPE overrides the NCPE mechanism and inverts the handedness of the device to left handedness by altering the balance of induced orbital angular momentum in electrons versus holes. The presence of these two electroluminescence mechanisms in the device enables it to be controlled electrically by tuning the charge carrier mobility and the recombination zone position.
The research allows the creation of OLEDs with controllable spin angular momentum information using a single emissive enantiomer, while probing the fundamental physics of chiral optoelectronics. “This work contributes to the growing body of evidence suggesting further rich physics at the intersection of chirality, charge and spin. We have many ongoing projects to try and understand and exploit such interplay,” Fuchter concludes.
Born in 1916, Crick studied physics at University College London in the mid-1930s, before working for the Admiralty Research Laboratory during the Second World War. But after reading physicist Erwin Schrödinger’s 1944 book What Is Life? The Physical Aspect of the Living Cell, and a 1946 article on the structure of biological molecules by chemist Linus Pauling, Crick left his career in physics and switched to molecular biology in 1947.
Six years later, while working at the University of Cambridge, he played a key role in decoding the double-helix structure of DNA, working in collaboration with biologist James Watson, biophysicist Maurice Wilkins and other researchers including chemist and X-ray crystallographer Rosalind Franklin. Crick, alongside Watson and Wilkins, went on to receive the 1962 Nobel Prize in Physiology and Medicine for the discovery.
Finally, Crick’s career took one more turn in the mid-1970s. After experiencing a mental health crisis, Crick left Britain and moved to California. He took up neuroscience in an attempt to understand the roots of human consciousness, as discussed in his 1994 book, The Astonishing Hypothesis: the Scientific Search for the Soul.
Parallel lives
When he died in 2004, Crick’s office wall at Salk Institute in La Jolla, US, carried portraits of Charles Darwin and Albert Einstein, as Cobb notes on the final page of his deeply researched and intellectually fascinating biography. But curiously, there is not a single other reference to Einstein in Cobb’s massive book. Furthermore, there is no reference at all to Einstein in the equally large 2009 biography of Crick, Francis Crick: Hunter of Life’s Secrets, by historian of science Robert Olby, who – unlike Cobb – knew Crick personally.
Nevertheless, a comparison of Crick and Einstein is illuminating. Crick’s family background (in the shoe industry), and his childhood and youth are in some ways reminiscent of Einstein’s. Both physicists came from provincial business families of limited financial success, with some interest in science yet little intellectual distinction. Both did moderately well at school and college, but were not academic stars. And both were exposed to established religion, but rejected it in their teens; they had little intrinsic respect for authority, without being open rebels until later in life.
The similarities continue into adulthood, with the two men following unconventional early scientific careers. Both of them were extroverts who loved to debate ideas with fellow scientists (at times devastatingly), although they were equally capable of long, solitary periods of concentration throughout their careers. In middle age, they migrated from their home countries – Germany (Einstein) and Britain (Crick) – to take up academic positions in the US, where they were much admired and inspiring to other scientists, but failed to match their earlier scientific achievements.
In their personal lives, both Crick and Einstein had a complicated history with women. Having divorced their first wives, they had a variety of extramarital affairs – as discussed by Cobb without revealing the names of these women – while remaining married to their second wives. Interestingly, Crick’s second wife, Odile Crick (whom he was married to for 55 years) was an artist, and drew the famous schematic drawing of the double helix published in Nature in 1953.
Stories of friendships
Although Cobb misses this fascinating comparison with Einstein, many other vivid stories light up his book. For example, he recounts Watson’s claim that just after their success with DNA in 1953, “Francis winged into the Eagle [their local pub in Cambridge] to tell everyone within hearing distance that we had found the secret of life” – a story that later appeared on a plaque outside the pub.
“Francis always denied he said anything of the sort,” notes Cobb, “and in 2016, at a celebration of the centenary of Crick’s birth, Watson publicly admitted that he had made it up for dramatic effect (a few years earlier, he had confessed as much to Kindra Crick, Francis’s granddaughter).” No wonder Watson’s much-read 1968 book The Double Helix caused a furious reaction from Crick and a temporary breakdown in their friendship, as Cobb dissects in excoriating detail.
Watson’s deprecatory comments on Franklin helped to provoke the current widespread belief that Crick and Watson succeeded by stealing Franklin’s data. After an extensive analysis of the available evidence, however, Cobb argues that the data was willingly shared with them by Franklin, but that they should have formally asked her permission to use it in their published work – “Ambition, or thoughtlessness, stayed their hand.”
In fact, it seems Crick and Franklin were friends in 1953, and remained so – with Franklin asking Crick for his advice on her draft scientific papers – until her premature death from ovarian cancer in 1958. Indeed, after her first surgery in 1956, Franklin went to stay with Crick and his wife at their house in Cambridge, and then returned to them after her second operation. There certainly appears to be no breakdown in trust between the two. When Crick was nominated for the Nobel prize in 1961, he openly stated, “The data which really helped us obtain the structure was mainly obtained by Rosalind Franklin.”
As for Crick’s later study of consciousness, Cobb comments, “It would be easy to dismiss Crick’s switch to studying the brain as the quixotic project of an ageing scientist who did not know his limits. After all, he did not make any decisive breakthrough in understanding the brain – nothing like the double helix… But then again, nobody else did, in Crick’s lifetime or since.” One is perhaps reminded once again of Einstein, and his preoccupation during later life with his unified field theory, which remains an open line of research today.
Sound waves can make small objects hover in the air, but applying this acoustic levitation technique to an array of objects is difficult because the objects tend to clump together. Physicists at the Institute of Science and Technology Austria (ISTA) have now overcome this problem thanks to hybrid structures that emerge from the interplay between attractive acoustic forces and repulsive electrostatic ones. By proving that it is possible to levitate many particles while keeping them separated, the finding could pave the way for advances in acoustic-levitation-assisted 3D printing, mid-air chemical synthesis and micro-robotics.
In acoustic levitation, particles ranging in size from tens of microns to millimetres are drawn up into the air and confined by an acoustic force. The origins of this force lie in the momentum that the applied acoustic field transfers to a particle as sound waves scatter off its surface. While the technique works well for single particles, multiple particles tend to aggregate into a single dense object in mid-air because the acoustic forces they scatter can, collectively, create an attractive interaction between them.
Keeping particles separated
Led by Scott Waitukaitis, the ISTA researchers found a way to avoid this so-called “acoustic collapse” by using a tuneable repulsive electrostatic force to counteract the attractive acoustic one. They began by levitating a single silver-coated poly(methyl methacrylate) (PMMA) microsphere 250‒300 µm in diameter above a reflector plate coated with a transparent and conductive layer of indium tin oxide (ITO). They then imbued the particle with a precisely controlled amount of electrical charge by letting it rest on the ITO plate with the acoustic field off, but with a high-voltage DC potential applied between the plate and a transducer. This produces a capacitive build-up of charge on the particle, and the amount of charge can be estimated from Maxwell’s solutions for two contacting conductive spheres (assuming, in the calculations, that the lower plate acts like a sphere with infinite radius).
The next step in the process is to switch on the acoustic field and, after just 10 ms, add the electric field to it. During the short period in which both fields are on, and provided the electric field is strong enough, either field is capable of launching the particle towards the centre of the levitation setup. The electric fields is then switched off. A few seconds later, the particle levitates stably in the trap, with a charge given, in principle, by Maxwell’s approximations.
A visually mesmerizing dance of particles
This charging method works equally well for multiple particles, allowing the researchers to load particles into the trap with high efficiency and virtually any charge they want, limited only by the breakdown voltage of the surrounding air. Indeed, the physicists found they could tune the charge to levitate particles separately or collapse them into a single, dense object. They could even create hybrid states that mix separated and collapsed particles.
And that wasn’t all. According to team member Sue Shi, a PhD student at ISTA and the lead author of a paper in PNAS about the research, the most exciting moment came when they saw the compact parts of the hybrid structures spontaneously begin to rotate, while the expanded parts remained in one place while oscillating in response to the rotation. The result was “a visually mesmerizing dance,” Shi says, adding that “this is the first time that such acoustically and electrostatically coupled interactions have been observed in an acoustically levitated system.”
As well as having applications in areas such as materials science and micro-robotics, Shi says the technique developed in this work could be used to study non-reciprocal effects that lead to the particles rotating or oscillating. “This would pave the way for understanding more elusive and complex non-reciprocal forces and many-body interactions that likely influence the behaviours of our system,” Shi tells Physics World.
Heat travels across a metal by the movement of electrons. However, in an insulator there are no free charge carriers; instead, vibrations in the atoms (phonons) move the heat from hot regions to cool regions in a straight path. In some materials, when a magnetic field is applied, the phonons begin to move sideways, this is known as the Phonon Hall Effect. Quantised collective excitations of the spin structure, called magnons, can also do this via the Magnon Hall Effect. A combined effect occurs when magnons and phonons strongly interact and traverse sideways in the Magnon–Polaron Hall Effect.
Scientists understand the quantum mechanical property known as Berry curvature that causes this transverse heat flow. Yet in some materials, the effect is greater than what Berry curvature alone can explain. In this research, an exceptionally large thermal Hall effect is recorded in MnPS₃, an insulating antiferromagnetic material with strong magnetoelastic coupling and a spin-flop transition. The thermal Hall angle remains large down to 4 K and cannot be accounted for by standard Berry curvature-based models.
This work provides an in-depth analysis of the role of the spin-flop transition in MnPS₃’s thermal properties and highlights the need for new theoretical approaches to understand magnon–phonon coupling and scattering. Materials with large thermal Hall effects could be used to control heat in nanoscale devices such as thermal diodes and transistors.
Topological insulators are materials that are insulating in the bulk within the bandgap, yet exhibit conductive states on their surface at frequencies within that same bandgap. These surface states are topologically protected, meaning they cannot be easily disrupted by local perturbations. In general, a material of n‑dimensions can host n‑1-dimensional topological boundary states. If the symmetry protecting these states is further broken, a bandgap can open between the n-1-dimensional states, enabling the emergence of n-2-dimensional topological states. For example, a 3D material can host 2D protected surface states, and breaking additional symmetry can create a bandgap between these surface states, allowing for protected 1D edge states. A material undergoing such a process is said to exhibit a phenomenon known as a higher-order topological insulator. In general, higher-order topological states appear in dimensions one lower than the parent topological phase due to the further unit-cell symmetry reduction. This requires at least a 2D lattice for second-order states, with the maximal order in 3D systems being three.
The researchers here introduce a new method for repeatedly opening the bandgap between topological states and generating new states within those gaps in an unbounded manner – without breaking symmetries or reducing dimensions. Their approach creates hierarchical topological insulators by repositioning domain walls between different topological regions. This process opens bandgaps between original topological states while preserving symmetry, enabling the formation of new hierarchical states within the gaps. Using one‑ and two‑dimensional Su–Schrieffer–Heeger models, they show that this procedure can be repeated to generate multiple, even infinite, hierarchical levels of topological states, exhibiting fractal-like behavior reminiscent of a Matryoshka doll. These higher-level states are characterized by a generalized winding number that extends conventional topological classification and maintains bulk-edge correspondence across hierarchies.
The researchers confirm the existence of second‑ and third-level domain‑wall and edge states and demonstrate that these states remain robust against perturbations. Their approach is scalable to higher dimensions and applicable not only to quantum systems but also to classical waves such as phononics. This broadens the definition of topological insulators and provides a flexible way to design complex networks of protected states. Such networks could enable advances in electronics, photonics, and phonon‑based quantum information processing, as well as engineered structures for vibration control. The ability to design complex, robust, and tunable hierarchical topological states could lead to new types of waveguides, sensors, and quantum devices that are more fault-tolerant and programmable.
The boundary between a substance’s liquid and solid phases may not be as clear-cut as previously believed. A new state of matter that is a hybrid of both has emerged in research by scientists at the University of Nottingham, UK and the University of Ulm, Germany, and they say the discovery could have applications in catalysis and other thermally-activated processes.
In liquids, atoms move rapidly, sliding over and around each other in a random fashion. In solids, they are fixed in place. The transition between the two states, solidification, occurs when random atomic motion transitions to an ordered crystalline structure.
At least, that’s what we thought. Thanks to a specialist microscopy technique, researchers led by Nottingham’s Andrei Khlobystov found that this simple picture isn’t entirely accurate. In fact, liquid metal nanoparticles can contain stationary atoms – and as the liquid cools, their number and position play a significant role in solidification.
Some atoms remain stationary
The team used a method called spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (Cc/Cs-corrected HRTEM) at the low-voltage SALVE instrument at Ulm to study melted metal nanoparticles (such as platinum, gold and palladium) deposited on an atomically thin layer of graphene. This carbon-based material acted a sort of “hob” for heating the particles, says team member Christopher Leist, who was in charge of the HRTEM experiments. “As they melted, the atoms in the nanoparticles began to move rapidly, as expected,” Leist says. “To our surprise, however, we found that some atoms remained stationary.”
At high temperatures, these static atoms bind strongly to point defects in the graphene support. When the researchers used the electron beam from the transmission microscope to increase the number of these defects, the number of stationary atoms within the liquid increased, too. Khlobystov says that this had a knock-on effect on how the liquid solidified: when the stationary atoms are few in number, a crystal forms directly from the liquid and continues to grow until the entire particle has solidified. When their numbers increase, the crystallization process cannot take place and no crystals form.
“The effect is particularly striking when stationary atoms create a ring (corral) that surrounds and confines the liquid,” he says. “In this unique state, the atoms within the liquid droplet are in motion, while the atoms forming the corral remain motionless, even at temperatures well below the freezing point of the liquid.”
Unprecedented level of detail
The researchers chose to use Cc/Cs-corrected HRTEM in their study because minimizing spherical and chromatic aberrations through specialized hardware installed on the microscope enabled them to resolve single atoms in their images.
“Additionally, we can control both the energy of the electron beam and the sample temperature (the latter using MEMS-heated chip technology),” Khlobystov explains. “As a result, we can study metal samples at temperatures of up to 800 °C, even in a molten state, without sacrificing atomic resolution. We can therefore observe atomic behaviour during crystallization while actively manipulating the environment around the metal particles using the electron beam or by cooling the particles. This level of detail under such extreme conditions is unprecedented.”
Effect could be harnessed for catalysis
The Nottingham-Ulm researchers, who report their work in ACS Nano, say they obtained their results by chance while working on an EPSRC-funded project on 1-2 nm metal particles for catalysis applications. “Our approach involves assembling catalysts from individual metal atoms, utilizing on-surface phenomena to control their assembly and dynamics,” explains Khlobystov. “To gain this control, we needed to investigate the behaviour of metal atoms at varying temperatures and within different local environments on a support material.
“We suspected that the interplay between vacancy defects in the support and the sample temperature creates a powerful mechanism for controlling the size and structure of the metal particles,” he tells Physics World. “Indeed, this study revealed the fundamental mechanisms behind this process with atomic precision.”
The experiments were far from easy, he recalls, with one of the key challenges being to identify a thin, robust and thermally conductive support material for the metal. Happily, graphene meets all these criteria.
“Another significant hurdle to overcome was to be able to control the number of defect sites surrounding each particle,” he adds. “We successfully accomplished this by using the TEM’s electron beam not just as an imaging tool, but also as a means to modify the environment around the particles by creating defects.”
The researchers say they would now like to explore whether the effect can be harnessed for catalysis. To do this, Khlobystov says it will be essential to improve control over defect production and its scale. “We also want to image the corralled particles in a gas environment to understand how the phenomenon is influenced by reaction conditions, since our present measurements were conducted in a vacuum,” he adds.
Rob Farr is a theorist and computer modeller whose career has taken him down an unconventional path. He studied physics at the University of Cambridge, UK, from 1991 to 1994, staying on to do a PhD in statistical physics. But while many of his contemporaries then went into traditional research fields – such as quantum science, high-energy physics and photonic technologies – Farr got a taste for the food and drink manufacturing industry. It’s a multidisciplinary field in which Farr has worked for more than 25 years.
After leaving academia in 1998, first stop was Unilever’s €13bn foods division. For two decades, latterly as a senior scientist, Farr guided R&D teams working across diverse lines of enquiry – “doing the science, doing the modelling”, as he puts it. Along the way, Farr worked on all manner of consumer products including ice-cream, margarine and non-dairy spreads, as well as “dry” goods such as bouillon cubes. There was also the occasional foray into cosmetics, skin creams and other non-food products.
As a theoretical physicist working in industrial-scale food production, Farr’s focus has always been on the materials science of the end-product and how it gets processed. “Put simply,” says Farr, “that means making production as efficient as possible – regarding both energy and materials use – while developing ‘new customer experiences’ in terms of food taste, texture and appearance.”
Ice-cream physics
One tasty multiphysics problem that preoccupied Farr for a good chunk of his time at Unilever is ice cream. It is a hugely complex material that Farr likens to a high-temperature ceramic, in the sense that the crystalline part of it is stored very near to the melting point of ice. “Equally, the non-ice phase contains fats,” he says, “so there’s all sorts of emulsion physics and surface science to take into consideration.”
Ice cream also has polymers in the mix, so theoretical modelling needs to incorporate the complex physics of polymer–polymer phase separation as well as polymer flow, or “rheology”, which contributes to the product’s texture and material properties. “Air is another significant component of ice cream,” adds Farr, “which means it’s a foam as well as an emulsion.”
As well as trying to understand how all these subcomponents interact, there’s also the thorny issue of storage. After it’s produced, ice cream is typically kept at low temperatures of about –25 °C – first in the factory, then in transit and finally in a supermarket freezer. But once that tub of salted-caramel or mint choc chip reaches a consumer’s home, it’s likely to be popped in the ice compartment of a fridge freezer at a much milder –6 or –7 °C.
Manufacturers therefore need to control how those temperature transitions affect the recrystallization of ice. This unwanted outcome can lead to phenomena like “sintering” (which makes a harder product) and “ripening” (which can lead to big ice crystals that can be detected in the mouth and detract from the creamy texture).
“Basically, the whole panoply of soft-matter physics comes into play across the production, transport and storage of ice cream,” says Farr. “Figuring out what sort of materials systems will lead to better storage stability or a more consistent product texture are non-trivial questions given that the global market for ice cream is worth in excess of €100bn annually.”
A shot of coffee?
After almost 20 years working at Unilever, in 2017 Farr took up a role as coffee science expert at JDE Peet’s, the Dutch multinational coffee and tea company. Switching from the chilly depths of ice cream science to the dark arts of coffee production and brewing might seem like a steep career phase change, but the physics of the former provides a solid bridge to the latter.
The overlap is evident, for example, in how instant coffee gets freeze-dried – a low-temperature dehydration process that manufacturers use to extend the shelf-life of perishable materials and make them easier to transport. In the case of coffee, freeze drying (or lyophilization, as it’s commonly known) also helps to retain flavour and aromas.
If you want to study a parameter space that’s not been explored before, the only way to do that is to simulate the core processes using fundamental physics
After roasting and grinding the raw coffee beans, manufacturers extract a coffee concentrate using high pressure and water. This extract is then frozen, ground up and placed in a vacuum well below 0 °C. A small amount of heat is applied to sublime the ice away and remove the remaining water from the non-ice phase.
The quality of the resulting freeze-dried instant coffee is better than ordinary instant coffee. However, freeze-drying is also a complex and expensive process, which manufacturers seek to fine-tune by implementing statistical methods to optimize, for example, the amount of energy consumed during production.
Such approaches involve interpolating the gaps between existing experimental data sets, which is where a physics mind-set comes in. “If you want to study a parameter space that’s not been explored before,” says Farr, “the only way to do that is to simulate the core processes using fundamental physics.”
Beyond the production line, Farr has also sought to make coffee more stable when it’s stored at home. Sustainability is the big driver here: JDE Peet’s has committed to make all its packaging compostable, recyclable or reusable by 2030. “Shelf-life prediction has been a big part of this R&D initiative,” he explains. “The work entails using materials science and the physics of mass transfer to develop next-generation packaging and container systems.”
Line of sight
After eight years unpacking the secrets of coffee physics at JDE Peet’s, Farr was given the option to relocate to the Netherlands in mid-2025 as part of a wider reorganization of the manufacturer’s corporate R&D function. However, he decided to stay put in Oxford and is now deciding between another role in the food manufacturing sector, or moving into a new area of research, such as nuclear energy, or even education.
Cool science “The whole panoply of soft-matter physics comes into play across the production, transport and storage of ice-cream,” says industrial physicist Rob Farr. (Courtesy: London Institute for Mathematical Sciences)
Farr believes he gained a lot from his time at JDE Peet’s. As well as studying a wide range of physics problems, he also benefited from the company’s rigorous approach to R&D, whereby projects are regularly assessed for profitability and quickly killed off if they don’t make the cut. Such prioritization avoids wasted effort and investment, but it also demands agility from staff scientists, who have to build long-term research strategies against a project landscape in constant flux.
A senior scientist needs to be someone who colleagues come to informally to discuss their technical challenges
To thrive in that setting, Farr says collaboration and an open mind are essential. “A senior scientist needs to be someone who colleagues come to informally to discuss their technical challenges,” he says. “You can then find the scientific question which underpins seemingly disparate problems and work with colleagues to deliver commercially useful solutions.” For Farr, it’s a self-reinforcing dynamic. “As more people come to you, the more helpful you become – and I love that way of working.”
What Farr calls “line-of-sight” is another unique feature of industrial R&D in food materials. “Maybe you’re only building one span of a really long bridge,” he notes, “but when you can see the process end-to-end, as well as your part in it, that is a fantastic motivator.” Indeed, Farr believes that for physicists who want a job doing something useful, the physics of food materials makes a great career. “There are,” he concludes, “no end of intriguing and challenging research questions.”
