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Smart contact lenses power up

Flexible contact lenses that incorporate supercapacitors and wireless-charging components are now possible, thanks to newly formulated printable inks that serve as the electrode and electrolyte. Researchers in the Republic of Korea showed that a specific mixture of carbon molecules, polymers and solvent can be used to print a supercapacitor’s electrodes onto a lens with micron-scale precision via a technique called direct ink writing. The same process deposits a UV-cured ionic liquid that functions as the supercapacitor’s electrolyte. As a proof-of-concept, the work could one day lead to smart contact lenses with sensors for health monitoring, or with integrated displays for augmented reality applications (Science Advances 10.1126/sciadv.aay0764).

While smart glasses have yet to catch on, there might still be a niche for wearable electronics that project information or images directly into the user’s field of view. If such a device could be miniaturized to fit into a contact lens, it could offer the added advantage of being able to sample certain biomarkers in the wearer’s tears, which can diagnose diseases including diabetes and glaucoma.

Before that can happen, researchers must come up with a way to deliver a ready supply of energy to the sensors, displays and the information-processing and communication infrastructure that supports them. Trailing wires from one’s eyes to a battery pack is obviously unacceptable, so smart lenses will need a store of electrical charge incorporated into the lens, as well as a way to replenish it wirelessly. For Jang-Ung Park of Yonsei University, Sang-Young Lee of Ulsan National Institute of Science and Technology (UNIST) and colleagues, the solution was to integrate a miniature, flexible supercapacitor and an energy-harvesting antenna to recharge it.

“Commercial supercapacitors are composed of sheet-type components that are stacked in fixed cylindrical or rectangular cases, which make them too bulky and rigid to fit into a tiny, soft smart contact lens,” explains Park. “The breakthrough was to make the supercapacitor components printable in an ink form. The component inks were drawn around the edge of the smart contact lens, so they won’t block the optical view of the user.”

Microscale direct ink writing (DIW) is already established as a method for fabricating components with the precision required for this application. The challenge lay in deriving a set of compatible inks that could print structures with the necessary electrical and mechanical properties, but that were also fluid enough to be extruded smoothly from the print nozzle.

To form the supercapacitor’s electrodes, the researchers mixed activated carbon and multiwalled carbon nanotubes (MWCNTs) with the polymers polyvinylpyrrolidone (PVP) and polyvinylidene fluoride (PVDF), and an organic solvent. The composite ink was thixotropic, meaning that when a shear stress was applied, the ink’s viscosity was low enough for it to flow through the print nozzle, whereas under static conditions the printed structure remained solid.

For the electrolyte, the researchers combined an organic ionic liquid with a mixture of thiol-ene monomers. After the necessary electrolyte pattern had been deposited by DIW, the team cured the mixture with UV light, securing the ionic liquid within a polymer skeleton.

Smart contact lens — Overview of the wirelessly rechargeable smart contact lens. (Courtesy: Jang-Ung Park, Yonsei University)

So that the device could be recharged wirelessly, on top of the supercapacitor the team deposited an antenna consisting of an array of electrospun silver nanofibres (around 400 nm in diameter) and electrosprayed silver nanowires (15–25 nm in diameter). They fabricated a rectifier for the antenna conventionally using a silicon wafer, which made up the one rigid component in the lens.

The researchers found that the smart lens survived repeated cycles of flexing, and could withstand stretching of up to 30% in two axes simultaneously. In trials with rabbits and a human subject, the device proved comfortable and safe, maintaining a stable temperature during wireless charging and when used to power an integrated light-emitting diode.

The team’s results are an encouraging demonstration of what’s possible, but don’t expect to be wearing smart contact lenses any time soon. For one thing, their supercapacitor just doesn’t have the endurance to handle the power demands of a realistic device.

“Our integrated supercapacitor is eligible for electronic devices which need a fast power supply in a few minutes,” says Lee. “For devices with longer working times, high-energy-density batteries are required, for which further studies are needed.”

Embedded DNA used to reproduce 3D-printed rabbit

Information encoded in DNA has been embedded within ordinary objects using a new technique that could lead to self-replicating machines.

The breakthrough was made by a team led by Yaniv Erlich of Erlich Lab LLC and Robert Grass at ETH Zurich. They used the technique to incorporate replication instructions into a 3D-printed rabbit. They then reproduced five successive generations of rabbits from tiny extracted fragments containing DNA. The result paves the way for new applications in a diverse array of fields including the storage of medical information and self-replicating machines.

Humans are creating rapidly-growing amounts of data and even the latest, most compact storage systems such as hard drives struggle to keep up. DNA could be a solution because it offers a versatile, robust and efficient way of storing and replicating information. DNA, for example, is the only known storage medium that can exist as a liquid.

Tiny silica beads

The team’s technique begins with encoding information onto molecules of DNA, and then encapsulating them within nanometre-scale silica beads. Then these beads are fused into functional materials, where they could remain indefinitely. The information is retrieved by taking a sample of the DNA and sequencing it (see video).

An important feature of the technique is that all of the stored information can be retrieved from just a tiny fragment of the material. This means that data could be stored throughout an object in a highly redundant manner.

Furthermore, the protective silica minimizes any degradation of the DNA structures over the material’s lifespan. Unlike current hardware devices, whose storage capabilities can be compromised by physical damage the memory stored in these materials could never be altered.

Reproducing like rabbits

Once they had established their storage technique, Elrich, Grass and colleagues pushed the DNA paradigm further to embed an object with DNA that encodes instructions for reproducing that object – in analogy to living organisms They made a 3D-printed rabbit containing 45 kbyte of printing instructions within embedded DNA.

dna-binary-code-255618778-Shutterstock_ymgerman

Data storage in DNA gains from a larger ‘alphabet’

By extracting just tiny fragments containing DNA from the rabbit, they had the blueprint for producing a next-generation object. They were able to repeat this process to create five generations of the rabbit – each replicating the memory of the previous generation, without the need to synthesise new DNA.

The team’s achievement could lead to significant new advances in data storage. Since 1 g of DNA can store up to 215 Pbyte of data, their technique could allow for storage densities orders of magnitude higher than current hard drives. They predict applications ranging from embedding electronic health records in medical implants, to constructing buildings which contain their own blueprints. With further research it could be possible to create machines that use raw materials to automatically replicate themselves over many generations.

Physics in the 2020s: what will happen over the decade ahead

As the 2020s gets under way, what can we expect to happen over the next 10 years? To get you in the mood, let’s first look back over the decade that’s now fading into the past. We kicked off the 2010s with Barack Obama one year into his science-loving presidency, CERN physicists poring over the first 7TeV collisions from the Large Hadron Collider (LHC) and researchers wondering if the CDMS-II experiment in the US had really obtained the first direct evidence for dark matter (answer: probably not).

Many of the successes in physics over the last decade were honoured by Physics World through our annual Breakthrough of the Year. The award was a straightforward choice in some years, especially when there’d been a big breakthrough in particle physics or astronomy. In 2012 the prize went to CERN’s discovery of the Higgs boson, in 2013 to the IceCube detector in Antarctica spotting cosmic neutrinos, and in 2016 to LIGO’s momentous discovery of gravitational waves.

Quantum physics was a burgeoning area in the 2010s too, reflecting physicists’ growing ability to experimentally probe the deepest mysteries of the subject. In 2011 the award went to work on “weak measurement”, which broke the taboo that it’s impossible to gain knowledge of the paths taken by individual photons travelling through two slits to create an interference pattern. Four years later, the prize was awarded for “double” quantum teleportation, in which physicists simultaneously transferred both a photon’s spin and its orbital angular momentum to a distant photon.

One significant cultural change in physics during the 2010s was the growing realization that physicists needs to do much more to root out inequalities in the field and make it more diverse. In fact, the decade saw some high-profile dismissals and resignations in the physics community – on the grounds of unwanted harassment of women and other groups – that in the past would have been unheard of and most likely swept under the carpet. Much of those changes came to light due to the openness wrought by the digital age.

One great hope for the decade, however, went unfulfilled: the discovery of “new” physics beyond the Standard Model. Despite 10 years of the LHC, there are still no signs of supersymmetric particles, forcing particle theorists to make progress with their mathematical wits alone. But with the LHC about to embark on an ambitious upgrade programme under the stewardship of Fabiola Gianotti, who just became the first person to be awarded a full second term as CERN boss, particle physicists will surely hope they can achieve in the next decade the dreams they held at the start of this.

The future is bright

I’m not sure for how long my own optimism will last, and for sure there will be plenty of downs as well as ups over the next 10 years. But as cognitive psychologist Stephen Pinker argued in his 2018 book Enlightenment Now, the world is, overall, improving. Whether measured in terms of health, literacy, safety or prosperity, things are only getting better – and those advances are due, in no small part, to science.

Physicists often don’t get the credit, but their discoveries have transformed everyday life, not least in how we communicate. Powered by developments in semiconductor physics and optics, I can see smartphones continuing to be ever lighter, faster and more powerful over the next decade (LiFi phones anyone?). Quantum computing and communication will become mainstream, with quantum computers routinely accessed via the cloud. Physics experiments will generate ever more data and analysing that information using artificial intelligence and machine learning will become “the new normal”.

I can see environmental concerns having a bigger influence on physics. Scientific lab equipment will become cleaner and greener. Particle-accelerator labs, which used to boast about how much electricity they consumed to perform collision experiments, will either have to be more coy or, better still, put energyefficient technologies centre stage. The rising impact of climate change will see air travel increasingly frowned upon, with jet-setting physicists having to fight harder to justify those conference flights. (And before you ask, yes we are looking at changing the plastic wrappers used to post print issues of Physics World.)

Medical physics will continue to boom, from improved radiotherapy treatments to new imaging techniques. In astronomy, NASA’s James Webb Space Telescope should finally launch (though I wouldn’t bet against there being yet another delay beyond 2021). The ITER fusion experiment in France should create its “first plasma” by mid-decade – roughly at the same time as the high-luminosity upgrade to CERN’s LHC comes online. China’s underground gravitational-wave detector could be ready then too, joining a similar facility in Japan searching for these ripples in space–time, which should open this year.

Diversity matters

But perhaps the biggest change in physics over the next 10 years will be in terms of diversity and equality. Efforts under way in recent times to make physics open for all will finally pay off and, though I’m not convinced that the overall numbers studying physics will increase by much (or even at all), I do predict that those at the top of the field will, by 2030, be far more varied in background than now.

One thing is for sure: Physics World will be around to give you the liveliest and most thought-provoking coverage of the world of physics over the coming decade. So stay tuned for the ride ahead.

Study of liquid-nitrogen droplets gives new insight into cryogenic spray-cooling

Making hot stuff very cold – and doing so reliably and repeatably – is a tricky business. Now, researchers in the Netherlands have developed a better understanding of a technique called cryogenic spray cooling – which is fast emerging as a commercial cooling method.

Cryogenic spray cooling works by impacting drops of low-temperature fluids (such as liquid nitrogen) on solid surfaces, with the latent heat of the liquid-vapour phase transition delivering the temperature control. The technique has a diverse range of industrial and biomedical applications, among them high-speed materials processing of machine tools and parts, freezing of food products for long-term preservation, and direct-spray cryotherapy of premalignant and cancerous lesions in the oesophagus.

Despite its apparent simplicity, the underlying mechanisms of cryogenic spray-cooling have hitherto remained elusive, with engineers often forced to adopt costly and time-consuming trial-and-error methods to optimize their freezing apparatus and processes. Now, however, a team of scientists at the University of Twente, has gained important insights into the process by studying the fundamental physics of droplet–surface interactions at cryogenic temperatures

“The cooling performance depends on a number of parameters: the size of the droplets, the velocity of the droplets, the angle of impact, as well as the physical properties of the cryogenic fluid and the material to be cooled,” explains Srinivas Vanapalli, who heads up the research in Twente’s Applied Thermal Sciences Laboratory.

Dynamic droplets

Vanapalli and his colleagues Thomas Nes (now at CERN) and Michiel van Limbeek (now at the Max Planck Institute for Dynamics and Self-Organization in Göttingen) looked at an intriguing phenomenon known as the Leidenfrost effect, whereby fluid droplets float above a hot solid surface. The droplets are held up by the vapour they emit– the so-called Leidenfrost boiling regime. The Leidenfrost effect is highly undesirable in spray-cooling systems because the vapour forms an isolating layer that prevents effective heat transfer between the solid and the droplet.

To study the droplet–surface behaviour up close, the researchers dropped a single liquid-nitrogen drop on a smooth sapphire prism and followed its progress from below using high-speed, frustrated total-internal-reflection (TIR) imaging. “In these cryogenic conditions, the prism behaves as a perfect thermal conductor,” says Vanapalli, “while its transparency enables us to study the contact behaviour during the impact and the spreading phase of the drop.”

By varying the drop impact velocity and target temperature, the team was able to map the boiling behaviour in a phase diagram. What is more, by combining single-drop measurements with studies of a continuous stream of liquid-nitrogen droplets, it was possible to correlate the cooling rate with the wetting behaviour of a single drop – revealing good agreement between the phase diagram and the various cooling curves (as well as with the behaviours previously observed for non-cryogenic liquids in spray-cooling applications).

Impact velocity

The team’s studies – using a single droplet as a model system and scaling to a stream of droplets – show that the cooling power is proportional to the impact velocity of the droplets in the Leidenfrost boiling regime. Careful analysis of the spreading behaviour of the liquid-nitrogen droplets also reveals a highly counterintuitive finding.

Specifically, for all surface temperatures in the contact boiling regime (when the droplets make good contact with the solid and bubbles nucleate at the liquid–solid interface), the conductive contribution to cooling exceeds that from evaporation. Since the role of evaporation scales with the latent heat, this behaviour is likely to be more pronounced for polar liquids such as water, in which the latent heat of vaporization is an order of magnitude larger than for liquid nitrogen.

Taken together, these insights have significant implications for understanding the range of heat-transfer mechanisms at play in a cryogenic spray-cooling system. “When we understand the fundamental processes,” Vanapalli concludes, “we can develop a physics-based model to predict the heat-transfer rate, providing application engineers with a toolkit to design and optimize the spray-cooling system for their specific process. That’s pretty cool, if you’ll pardon the pun.”

The research is sponsored by Dutch funding agency NWO-TTW within the CryoOn project and will be described in a upcoming paper in the International Journal of Heat and Mass Transfer.

Nonvolatile charge memory device shows excellent room-temperature performance

Qinliang Li, Cailei Yuan and Ting Yu from Jiangxi Normal University, along with Qisheng Wang and Jingbo Li from South China Normal University, are developing nonvolatile charge memory devices with simple structures. Wang explains how the optically controllable devices combine the functions of light sensing and electrical storage.

The research is reported in full in Journal of Physics D: Applied Physics, published by IOP Publishing – which also publishes Physics World.

What was the motivation for the research and what problem were you trying to solve?

Nonvolatile memory devices are central to modern communication and information technology. Among various material systems, emerging two dimensional (2D) materials offer a promising platform for next-generation data-storage devices due to their unique planar structure and brilliant electronic properties. However, 2D materials-based nonvolatile memory devices have complicated architectures with multilayer stacking of 2D materials, metals, organics or oxides. This limits the capacity for device miniaturization, scalability and integration functionality.

In this work, we are trying to design a nonvolatile charge memory with simple device architecture. We also expect to explore a new type of optical control on the charge storage devices, which may bring us smart operation on data deposition and communication.

What did you do in this work?

We discovered the novel optical-tunable charge memory behaviour in In₂Se₃ nanosheets. We grew the single-crystalline In₂Se₃ nanosheets in a tube furnace with chemical vapour deposition methods, and used standard photolithography to depict the device contacts by depositing Ti (3 nm)/Au (50 nm) as electrodes. We recorded electrical signals using an Agilent B1500 semiconductor characterization system equipped with a probe station, at temperatures from 30 to 300 K. In order to observe the effect of laser irradiation on the charge memory, we used a 532 nm laser to irradiate the device.

What was the most interesting and/or important finding?

The devices showed superb memory properties at room temperature, with a large memory window, long retention time and robust endurance. Furthermore, we demonstrated optical manipulation of charge storage, with laser powered control of the numbers of stored charges and the on/off ratio. Supported by a theoretical model, we found that the nonvolatile charge storage originated from the surficial/interfacial trapped electrons, which are removed via photo-generated holes.

Why is this research significant?

Our photo-tunable nonvolatile charge memory devices combine the functions of light sensing and electrical storage. In comparison with multilayer heterostructures of 2D materials, our work facilitates device manufacture, reduces the circuit complexity, and is of benefit for the miniaturization and large-scale integration of functional devices.

At room temperature, the devices showed comparable memory properties to memory devices of 2D material heterostructures with floating gate mode. Together with the simple device structure, the photo-tunable nonvolatile charge memory devices will pave the way towards large-scale integration and high-speed intelligent electronics, such as ultrafast remote operation on data coding, artificial synapse and neurons.

What is the next stage for the research?

In the future, we plan to optimize the memory performance of α-In₂Se₃ nanosheets via surface chemical modification. Meanwhile, we will develop the wafer-scale growth of α-In₂Se₃ nanosheets, which would be useful for industrial application. The large-area growth will allow us to explore an artificial visual system based on optical-controllable data memory properties of α-In₂Se₃ nanosheets.

The full results of the study are reported in Journal of Physics D: Applied Physics.

‘Beam rider’ technology keeps solar sails aligned

Like most spacecraft, the CHEOPS mission launched last week uses a chemical propellant to adjust its position once in orbit. The high mass of such propellants has, however, prompted space scientists to explore alternatives such as “light sails” that enable spacecraft to navigate using the radiation pressure from light, much as old-fashioned sailing ships used wind to travel the high seas. A team of researchers in the US has now taken a step towards a more powerful light sail by testing a prototype design that uses lasers and diffraction gratings rather than the sunlight and mirrors employed previously.

The simplest light sail designs, such as that adopted by JAXA’s IKAROS mission or the privately-funded LightSail 2, use a large, thin mirror to reflect sunlight. Although the propelling force from sails of this type is small compared to that supplied by chemical fuels, spacecraft that rely on sails do not have to carry their own fuel supply. Another advantage is that their “fuel” will never run out as long as the sail is illuminated – making them good candidates for future long-haul space missions that require more energy than chemical propellants can supply.

Staying aligned

A laser-based propulsion system could, in principle, retain many of these advantages while also providing a much greater acceleration. The trouble is that if the sail on such a spacecraft were to drift out of alignment with the central axis of the laser beam, the craft would lose power.

A team led by Grover Swartzlander at the Rochester Institute of Technology (RIT) has now developed a way around this problem. In the RIT system, the propulsion laser beam shines onto a pair of adjacent diffraction gratings. These paired gratings are arranged such that the left-hand grating deflects incident laser light slightly to the right, while the right-hand grating deflects it slightly to the left. The result is a force that propels the craft forward, plus a pair of opposing lateral forces that increase in magnitude the further the beam strays from the centre. Hence, when the bi-grating moves away from its equilibrium position, the lateral force pushes it back, allowing the sail to remain aligned.

Nanonewton-sized force re-centres the sail

This “beam rider” technology, as it has been dubbed, has passed a preliminary test in the laboratory with the demonstration of a centimetre-sized prototype. In this device, the micron-thin gratings are made from nematic liquid crystals fixed to a 100 micron-thick polymer film for structural support. The anisotropy axis of the crystals is rotated in the grating plane and arranged in a periodic pattern to produce the required deflection.

saeo_V1e — The experiment. Courtesy: RIT

The researchers measured this deflection by placing the prototype in a sensitive force-measuring setup (known as a torsion oscillator in vacuum) and shining a laser beam onto its centre. Using time-lapse photography and force measurements, they observed that when the beam’s position shifts away from equilibrium, a nanonewton-sized force re-centres the device so that it returns to alignment with the beam.

Looking forward

The research, which is detailed in Physical Review Letters, is still in its infancy and there are many challenges to overcome. One key question is whether the technology can function in the vertical as well as the horizontal direction, and thereby control a spacecraft’s pitch as well as its side-to-side yaw. The RIT team plan to test their design in the coming months to find out.

Looking forward, Swartzlander says he hopes to use the team’s diffractive sails on a spacecraft that would fly a camera to the north and south poles of the Sun. “These sails are transparent, so they’re not going to absorb a lot of heat from the Sun, and we won’t have the heat management problem as you do with a (conventional, reflecting) metallic surface,” he says. He estimates that it would take five years for such a spacecraft to reach the solar poles and make the first direct images of these so-far-unobserved regions.

In the more distant future, Swartzlander believes that sail-driven spacecraft might occupy a prominent place in the developing space economy. “Convoys of reusable solar sails could be used to transport precious resources throughout the solar system without fuel, ferrying water and minerals to an orbiting processing facility,” he tells Physics World.

Philip Lubin of the University of California, Santa Barbara, who was not involved in the research, says that the RIT’s team’s work on diffractive reflectors for solar and laser sails is a “very important step forward”. In his view, the study, which was supported by NASA, shows that non-traditional custom materials such as diffractive and photonic crystal-based reflectors provide unique and highly desirable characteristics, including the ability to beam follow (for laser sails) and self-stabilize (for laser and solar sails). “Excellent work with much more to come,” he concludes.

The 10 quirkiest physics stories of 2019

From the physics of car parking to how to create the perfect fondue, physics has had its fair share of quirky stories this year. Here is our pick of the 10 best, not in any particular order.

Table of knots

UNESCO designated 2019 the International Year of the Periodic Table of Chemical Elements (IYPT) to celebrate 150 years since the Russian chemist Dmitri Mendeleev created the world’s first periodic table. Biochemistry graduate Jane Stewart created her own tribute to Mendeleev by making a macramé version of the table. The ancient art of macramé – the art of knotting string in patterns – originated in Arabia where the word comes from the Arabic migramah, which means “fringe” and was used to finish off a weaver’s work. Stewart used a metallic crochet thread, which is about the same thickness as embroidery thread, to create the 100 x 60 cm table. The table contains about 200 000 “half hitch” knots and with each element taking over two hours to complete, Stewart spent at least 240 hours putting it together. “I learned there are a heck of a lot of transition metals,” Stewart told Physics World, “and needed a lot of moral support to keep going through them for more than a month.”

Historic chart

Still on the IYPT, the University of St Andrews in Scotland declared that it was home to what is thought to be the oldest classroom periodic table in the world. The historic chart, which dates back to 1875, was found by accident during a storage room clearout back in 2014. The table is annotated in German and an inscription at the bottom left – “Verlag v. Lenoir & Forster, Wien” – identifies the scientific publisher in Vienna that created it. The table has since been restored and is now being kept in climate-controlled conditions, with a full-scale replica on display in the university’s school of chemistry. Records show that it was bought in 1888 by chemist Thomas Purdie who had studied in Germany. It cost him three German gold marks – about £17 today.

Entanglement: the game

Quantum mechanics is hard, right? To make it more understandable to high-school students, researchers from the University of Innsbruck, Austria, created a game that can be used as a teaching tool. It conveys basic quantum concepts without players needing to know any advanced mathematics (arXiv:1901.07587). In the game, students are split into two teams called “particles” and “scientists”. The goal of the scientists is to perform “measurements” on the particles, who are told to obey specific rules. The scientists then must try to come up with theories that explain their observations. The game aims not only to teach concepts such as entanglement and decoherence, but also to develop students’ critical thinking. It was even tested last year with three science classes at Colegio JOYFE school in Madrid, whose students described it as “fun”.

Hawking coin — Event horizon: new 50p coin honours Stephen Hawking. (Courtesy: Royal Mint)

Hawking cash

To mark the first anniversary of the death of the University of Cambridge physicist Stephen Hawking, the UK’s Royal Mint released a new 50p coin to celebrate his life and work. Designed by UK artist Edwina Ellis, it features a stylized black hole along with the Bekenstein–Hawking formula for the entropy of a black hole. Unfortunately, the 50p coin won’t enter circulation in the UK but for £10 you can get your hands on one presented “in educational packaging that brings the science of black holes to life”. There are also limited-edition versions – a silver proof coin costing £55 and a gold proof variant for £795 – both of which sold out within days. The gold version was later being resold on eBay for an eye-watering £2000 – a case, surely, of black holes sucking your money up too.

Black-hole LEGO

The unveiling of the first-ever image of a black hole earlier this year by the Event Horizon Telescope (EHT) created much excitement around the world – and it seemed to have touched the LEGO community too. At least three black-hole-inspired LEGO designs were submitted to the LEGO Ideas website, which lets fans share blueprints of their own models. Luis Peña, who previously made LEGO models of the Hubble Space Telescope and the Mars Curiosity rover, built a LEGO model of a single antenna belonging to the Atacama Large Millimeter/submillimeter Array, which played a crucial part in the observations. Peña’s design also features a small mosaic of 18 × 18 studs to illustrate the blackhole image. LEGO user “tm.bricks”, in contrast, created a design consisting of all eight EHT telescopes together with a mosaic of bricks to signify the black hole. Meanwhile, “douglasfx” has made a model of the M87* black hole featuring accretion disk and jets.

Physics of car parking

It’s the old conundrum for car drivers looking for somewhere to park. Should you plonk your vehicle far from your destination, where finding a spot will be easy but the walk is long? Or should you spend time trying to park close to the destination, where spaces are harder to find but you just might get lucky? Physicists Paul Krapivsky of Boston University and Sidney Redner of the Santa Fe Institute applied Poisson statistics to the problem, finding that a “prudent” strategy is best, in which you park in the first gap of cars you come across but take the spot nearest to the venue (arXiv:1904.06612). This is deemed better than the “optimistic” strategy, in which you drive all the way to the target location and then park in the first spot that you find as you backtrack away from your destination. Redner, however, admits that the model – based on a single road leading to the destination – is “unrealistic”. “In its present idealised form, our model is not practically useful,” he says. Better just take public transport then.

Particle serenade

CERN’s Large Hadron Collider (LHC) near Geneva is currently being upgraded to boost the collider’s luminosity, which meant that it was possible this year to go down into the tunnel and have a look around. One lucky punter who did just that was the US virtuoso cellist Yo-Yo Ma, who in September not only donned the obligatory CERN hard hat but also performed a solo of Bach’s Cello Suite No. 6. Bach was an apt choice given that the German composer was apparently fascinated with mathematics and numbers and often hid numerical messages and puzzles in his works. “I have always thought that philosophy, arts, and sciences belong together as equal partners in this thing we call culture,” Ma noted. “We must fight for this belief. Because the widening gaps between disciplines of inquiry and between culture, economics, and politics have led to increasing and frightening fractures in the world.”

Atomic tipple

Would you be brave enough to try a vodka made with grain and water from the exclusion zone in Chernobyl? Well, the gauntlet has now been thrown down in the shape of Atomik – an “artisan vodka” produced by the Chernobyl Spirit Company. Those behind the spirit say they are the first to create a consumer product from the abandoned area around the damaged nuclear power plant. In something of a glowing endorsement, environmental scientist Jim Smith from the University of Portsmouth noted that there is no danger of radioactivity from the 40% ABV tipple given the distillation process, adding that it had been tested in a lab at the University of Southampton. The team hopes to use the profits to help communities in Ukraine still affected by the economic impact of the disaster. Currently there is only one bottle, but the team hoped to have produced 500 bottles by the end of the year, selling it initially to the “nuclear tourists” who visit the exclusion zone.

Peering down the barrel

Still on alcoholic beverages, you might fancy pouring yourself a tipple to celebrate the end of the year– perhaps an American bourbon whiskey. But how can you be sure that it is genuine? Whisky (or whiskey in the US) is big business and distillers are naturally keen to protect their markets from counterfeit products. Physicists at the University of Louisville in Kentucky discovered a way to identify genuine American bourbon whiskey from the pattern of residue it leaves after evaporating. Unlike other spirits, which leave spots of residue, the US tipple apparently leaves a distinctive spiderweb pattern (Phys. Rev. Fluids 4 100511). Stuart Williams and colleagues believe that the pattern is linked to chemicals that seep into their whiskey as it is ages in newer barrels that are lined with charred wood. This is unlike other types of whisky that are aged in mature – and often recycled – barrels. After evaporating tens of different brands and ages of whiskeys, the team even found that they each had a unique, reproducible pattern or “fingerprint”. That’s one neat result.

You can be sure that next year will throw up its fair share of quirky stories from the world of physics. See you in 2020!

Our favourite images of 2019

Physics is not just equations and data plots – physicists can sometimes create fantastic images. Here are some of our favourites from 2019.

Above is a scanning electron microscope image of the world’s smallest house, according to its creator Travis Casagrande of the Canadian Centre for Electron Microscopy at McMaster University in Hamilton, Ontario. It was assembled from pieces of silicon and the decorations were carved using a beam of gallium ions. To give you an idea of how small it is, the chimney opening is about 1 micron across. The house sits on the head of a tiny silicon snowman and the sculpture was made using a focused ion beam microscope that Casagrande normally uses to prepare even smaller samples for use in the centre’s powerful transmission electron microscopes.

“I think projects like this create science curiosity,” Casagrande says. “Looking into how this was made leads to more interest in science, and that builds more science literacy, which allows everyone to make better decisions.” Watch this video to learn more about how the house was made.

Magnetically-manoeuvred guidewire — A guidewire is manoeuvred by magnets through a life-sized model of arteries in the brain. The nodules on the arteries represent aneurysms with a high risk of rupture that can cause a stroke. (Courtesy: Yoonho Kim *et al*/MIT)

Sometimes all that is needed for a memorable image is an abstract shape and a nice colour scheme. The above photograph shows a guidewire being manoeuvred by magnets through a life-sized model of arteries in the brain. Developed by team of researchers at the Massachusetts Institute of Technology, the new surgical tool can wind its way through some of the narrowest twisting networks of blood vessels to help treat stroke and aneurysm. Using hydrogels and magnetic materials, they have created a magnetically steerable guidewire that can slide easily through blood vessels to reach blood clots in the brain.

From left to right, precursor molecule C24O6, intermediates C22O4 and C20O2 and the final product cyclo[18]carbon C18 created on surface by dissociating CO masking groups using atom manipulation. The bottom row shows atomic force microscopy (AFM) data using a CO functionalized tip. (Courtesy: IBM Research)

Seeing is believing, which is why I have included the above atomic force microscope images of a ring of 18 carbon atoms and its precursors. This image of the hotly debated carbon ring allotrope was taken by researchers at IBM Zurich in Switzerland.

In August, engineers successfully connected the two halves of NASA’s $8.8bn James Webb Space Telescope (above) for the first time. Engineers at Northrop Grumman’s facilities in California used a crane to lift the mirror and science instruments onto the sunshield and spacecraft. Now that the observatory has been mechanically connected, the next steps will involve electrically connecting the halves, followed by testing those connections. Engineers will then fully deploy the intricate five-layer sunshield, which is designed to keep the telescopes’s mirrors and scientific instruments cold by blocking infrared light from the Earth, Moon and Sun.

Space snowman: the Ultima Thule planetesimal. (Courtesy: NASA)

NASA’s New Horizons mission reached a small lump of rock and ice some 6.5 billion km away in the Kuiper belt at the beginning of the year. The above images, which were taken on 1 January when the craft was around 27 000 km from the object, reveal that 2014 MU69, or Ultima Thule, is a “contact binary”, consisting of two connected spheres around 31 km long. The large sphere (Ultima) is 19 km across and the other (Thule) is 14 km long. “This fly-by is a historic achievement,” says New Horizons principal investigator Alan Stern from the Southwest Research Institute in Boulder, Colorado. “Never before has any spacecraft team tracked down such a small body at such high speed so far away in the abyss of space.”

Liquid crystal skyrmions — Fishy behaviour: clusters of skyrmions in a liquid crystal. (Courtesy: University of Colorado, Boulder)

The skyrmion is one of our favourite quasiparticles here at Physics World, so we were chuffed to discover earlier this year that the magnetic excitations can sometimes behave like a school of fish. Hayley Sohn and colleagues at the University of Colorado, Boulder, first spotted the behaviour by accident, but quickly realized that they had discovered an intriguing new form of active matter. Their work could lead to the development of new types displays with the potential to transform the ways in which humans and computers interact.

Tennis balls — Balanced hobby: Tennis-ball pyramids created by Andria Rogava, a physicist and tennis player. (Courtesy: Andria Rogava)

Another obsession we have is with the tennis ball towers made by Andria Rogava, who is a physicist at Ilia State University in Georgia. The towers are held together by friction between the balls, and Rogova keeps coming-up with new designs to see how far he can push the concept, as he explains in “Physicist creates remarkable tennis-ball towers, including one made from 46 balls“.

Europe draws up plans for plasma-based particle accelerators

Physicists from across Europe have outlined plans to build a new type of accelerator facility based on “plasma wakefields”. Named the European Plasma Research Accelerator with eXcellence In Applications (EuPRAXIA), the project envisages building high-energy plasma devices on one or possibly two sites In Europe. Its aim is to show how the size and cost of accelerator facilities – particularly those relevant to industry – could be radically reduced compared to ones employing conventional radiofrequency technology.

Plasma wakefield accelerators exploit the extremely high electric-field gradients that are created when laser pulses or charged particles travel at high speed through a plasma. Those gradients, generated by a sharp separation of electrons and ions in a pulse’s wake, can be used to accelerate electrons travelling behind the pulses to very high energies over very short distances.

Researchers have already demonstrated the principle of wakefield acceleration in the lab, having achieved gradients as high as 100 GeVm^-1 – compared to the roughly 0.1 GeVm^-1 possible with radiofrequency cavities. However, the EuPRAXIA consortium, which consists of researchers from 16 institutes in five European countries, aims to show that the technology can be used by industry and academia to carry out specific tasks.

Direct gains

In a conceptual design report released at the end of October but not yet formally announced, the consortium says that it intends to build plasma accelerators fed by both high-powered lasers and conventional electron accelerators in order to generate high-quality beams of electrons with energies between 1 and 5 GeV. Those beams, it says, could be used in a range of applications including medical imaging, positron generation, material testing and, above all, to power compact free-electron lasers (FELs).

Existing FELs use either copper or superconducting cavities to accelerate electrons to high energies, with magnets then forcing the particles around sharp bends so that they emit exceptionally bright flashes of X-rays. But the world’s leading devices – located in Europe, the US and Asia – are hundreds of metres if not several kilometers long. The much higher gradients possible with plasma accelerators could potentially make FELs small enough to fit in the grounds of a hospital, for example.

Physicist Carsten Welsch from the University of Liverpool, who is EuPRAXIA’s head of communications, adds that existing FELs are heavily over-subscribed. “If you had a facility that could produce similar quality laser beams but with a smaller footprint there would be a direct gain,” he says.

In its report, the consortium puts forward several scenarios for the new facility along with their associated costs. The most expensive option, at €320m, involves building laser-driven and electron-driven accelerators on separate sites, complete with a number of applications as well as an extensive programme of laser R&D. The construction of a single type of accelerator combined with a FEL, however, comes in at €68m if fed by electrons and €75m if a laser is used.

Duel sites

The consortium explains that it originally intended to propose a design for a plasma accelerator facility independent of any site. In the end, however, it endorsed a specific location for the electron-driven accelerator – the Frascati National Laboratory outside Rome. Researchers there have already embarked on a programme to build a plasma device that doubles the energy provided by advanced radiofrequency cavities.

In contrast, the report lists four possible sites for a laser-driven accelerator. Two of these are in Italy – the Frascati lab and a branch of the National Optics Institute in Pisa – while the others are the ELI Beamlines Lasers centre near Prague in the Czech Republic and the Rutherford Appleton Laboratory in Oxfordshire, UK.

According to Welsch, if EuPRAXIA gets the go-ahead it should be providing beams to users within about 10 years. Given how long it will take to build and commission the accelerators, he says that a site decision “would need to be taken definitely in the next five years, and probably in the next two to three”.

As for using the facility to accelerate electrons for particle-physics experiments, according to Welsch the energies involved will be too low to make significant discoveries. “At least for the initial ten-year timeframe these applications wouldn’t be possible,” he says.

Self-amplifying crystallization produces a Christmas tree of salt

Salt creep – a common phenomenon that occurs when salt crystals rapidly precipitate from evaporating solutions – can create serious problems for outdoor electronics, buildings, artwork and agriculture because of its corrosive effect. A team of researchers from the University of Amsterdam in the Netherlands has now quantitively described the creeping mechanism for the first time, showing that it is most likely to occur when relative humidity is low and solution evaporating rates high. They also say that creeping, which they describe as a self-amplifying crystallization process, can be inhibited by adding non-ionic additives such as Tween 80 to a salt solution.

The salt crystals deposited during creeping can “climb” onto solid surfaces and spread quickly away from the evaporating solution. The phenomenon is fascinating because of the complex patterns it produces, but it is also a major nuisance in many practical situations, rapidly corroding even well-protected surfaces. Indeed, the speed at which the salt spreads is so fast that it has even been likened to superfluid helium flow.

Although the first research paper on salt creep was published nearly 100 years ago, few studies have probed the driving forces behind the initiation and rapid growth of salt crystals. A team led by Noushine Shahidzadeh has now provided a quantitative understanding of the creeping phenomenon thanks to a simple experiment.

High and low relative humidity

In their study, the researchers dipped a glass rod with a diameter of around 6 mm into four types of evaporating salt solution. The solutions they tested were sodium chloride (NaCl), potassium chloride (KCl) and two types of sodium sulphate: Na₂SO₄-A, which precipitates in the form of hydrated crystals, and Na₂SO₄-B, which forms anhydrous crystals.

sodsulf352 — Salt creeping self amplifying crystallization process. Credit: M.J. Qazi; H. Salim; C.A.W. Doorman; N. Shahidzadeh

They then left the solutions to dry in a chamber whose relative humidity (RH) they could control. At a RH of 40% and a temperature of 21°C, they did not observe salt creep on the rod for any of the solutions. Instead, the solutions became super-saturated with salt and crystals nucleated either in the chamber (for Na₂SO₄-B) or at the liquid/air interface (for NaCl, KCl, and Na₂SO₄-A). These latter crystals subsequently grew and fell into the bulk solution, where they continued to grow. The solution then detached from the glass rod as the level of solution in the chamber dropped.

The situation is very different at lower humidity, however. At a RH of 6%, the researchers observed salt creep in all the salt solutions they studied. Before creeping starts, the solution first becomes saturated in salt, triggering crystal precipitation and growth in the chamber — as observed for a RH of 40%. As evaporation continues, though, creep begins to occur on the glass rod and new crystals start to precipitate at a specific angle of the liquid/air interface with the surface (substrate). Next, a chain of crystals forms and quickly moves up the rod in a chaotic fashion.

Self-amplified creep

The team monitored this cascade of crystal formation with two cameras. They also measured the total weight of the rod plus salt crystals over time. From this, they learned that creeping is self-amplifying: a deposited crystal induces the precipitation of two new ones, which in turn, produce four new ones and so on. The salt deposit grows exponentially, which explains why large distances can be covered at great speed. The result is a Christmas-tree-shaped pattern of salt, Shahidzadeh says.

Shahidzadeh and colleagues, who report their work in Science Advances, say they can control and even suppress creep by influencing the number of emerging crystals (that is, the branching of the crystallization pattern) and the size of the crystals. They did this for the NaCl-containing solution by using two different additives at very low concentration known to affect salt crystallization – namely cetyltrimethylammonium bromide (CTAB), which is cationic, and Tween 80, which is non-ionic. The cationic surfactant is a nucleation promoter and thus accelerates the rate of salt creep, while the non-ionic surfactant acts as a nucleation inhibitor and therefore completes the creeping.

Although CTAB attracts chloride ions and causes NaCl to precipitate out, accelerating the rate of salt creeping, Tween 80 continuously reduces the contact angle between crystals and the meniscus at the liquid/air interface so that it never reaches the critical angle required for salt creeping to occur.

Controlling salt spread

This strategy could come in useful in four major domains, Shahidzedah tells Physics World. The first of these is civil engineering. Salt crystallization is one of the major causes of damage to stone and ceramic artefacts such as historically important buildings. This is because salt can transport out of the mortars employed (typically producing ugly white spots) and speed up corrosion, she explains. Another potential application lies in preventing salt from coming out of the stones that frescoes are painted on, to prevent blistering of the paint, she adds. “The recent disastrous flooding in Venice will probably induce a lot of salt crystallization and creeping on the artworks and frescoes of the city in the future.”

The third area of interest is electronic devices, since salt creep greatly limits the lifetime of outdoor electronic equipment.

A final area where mitigating salt creep could make a difference is agriculture. Underground salt deposits can migrate to the soil surface by creeping, which renders the soil infertile if present in high amounts. In parts of Senegal, for example, fresh water sources have almost disappeared because of drought. Evaporation leaves behind salty remnants, and these spread to nearby territory via salt creep.

A similar problem occurs in some forests and farmland in low-lying areas of North Carolina, on the US east coast. As in Senegal, salt seeps into these areas from the nearby ocean, leaving plants unable to grow as the soil is literally eaten away by salt. As the climate warms and sea levels rise, salt water will advance even further. Finding ways to slow down salt creep will thus be vital for communities in these and other similarly-affected regions.

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