Inspired by origami, researchers at Donghua University in Shanghai, China, have built self-folding paper from extremely thin sheets of graphene oxide. The new paper bends in response to light or heat, and can be made to “walk” on a surface and even turn corners. The material could be used in a range of applications, including sensing, artificial muscles and robotics.
Origami is the ancient Japanese art of paper folding, and can transform a lightweight flat material into a strong and flexible 3D object. Its principles have inspired engineers to design a host of structures including vehicle airbags, satellite components and artificial muscles.
Self-folding structures are especially useful when they can be programmed to fold and unfold by exposing them to an external stimulant such as light. Such structures contain an active material or materials that respond to these stimuli. These active materials are usually polymer-based, which means that they respond well to changes in temperature, solvent, humidity, electricity and light. However, such materials can also be unstable and difficult to fabricate.
Strong and flexible
Now, researchers led by Hongzhi Wang and Meifang Zhu at the College of Materials Science and Engineering at Donghua have used extremely thin graphene-oxide nanosheets as the building blocks for a self-folding paper. The paper is flexible and easy to manipulate, and has a high tensile strength.
The sheets are made of several layers of reduced graphene oxide (rGO), which is very stable and does not change shape in response to external stimuli. Self-folding is achieved by coating some parts of the rGO sheet with several layers of graphene oxide that contains the polymer polydopamine (GO-PDA).
Unlike rGO, the GO-PDA layers contain water molecules that have been absorbed from the surrounding air. When the material is heated using an infrared laser, some of the water is driven out of the GO-PDA layers, which causes them to shrink. The rGO layers do not shrink, however, and the result is a sharp bending of the sheet in places where GO-PDA is present. When the light is switched off, the GO-PDA reabsorbs water from the air as it cools and the sheet flattens out.
Helping hand
By carefully patterning sheets of rGO with GO-PDA, the team made simple paper robots that can move forwards and backwards (see video above). They could even turn corners, which is a first for such a walking structure. Self-folding was also used to make a “hand” that could grasp and hold objects five times heavier than its own weight.
“We can programme how this paper bends so we can make it walk and turn around, as well as fold into pre-designed shapes simply by applying light or heat to it,” Wang explains. “We believe our work will help in the development of next-generation industrial mechanical actuators that could be used in applications like wireless remotely controlled microrobots, microfluidic chemical analysis, tissue engineering and artificial muscles, to name but a few.”
Wang and Zhu are now busy trying to make smaller versions of their paper. “As the device scales down in size, especially to the nanoscale, its folding properties will change significantly,” says Wang. “We are therefore interested in developing a nanosized all-graphene origami structure.”
Nit picker: the cold plasma lice killer. (Courtesy: Fraunhofer IST)
By Hamish Johnston
Do your children have head lice again? Now you don’t have to comb their hair until your arm goes numb or cover their head with goop. Instead, you can zap them away using a plasma. I’m not suggesting that you put your child’s head into ionized gas that’s hotter than the Sun – it turns out that a “cold atmospheric pressure plasma” will do the trick.
That’s the claim of researchers at the Fraunhofer Institute for Surface Engineering and Thin Films in Göttingen, Germany. The team has created the above prototype, which creates a plasma using a high-voltage generator that sends short pulses to the teeth of the comb. The pulses ionize air molecules surrounding the teeth, but they are so short that the resulting plasma does not heat up. The charged ions and electrons in the plasma make short work of killing lice and their eggs, but are harmless to humans – at least according to Wolfgang Viöl and colleagues, who will be unveiling their device later this month at the MEDICA trade fair in Düsseldorf.
Tiny dust particles have been found floating more than 1000 km above the surface of Mars – about 10 times higher than planetary scientists had expected to find them. The particles were spotted by NASA’s MAVEN spacecraft, and their presence suggests that the red planet is accumulating dust from the solar system. The discovery could provide important clues about how dust moves around the solar system.
Scientists already knew that dust is lifted as much as 100 km from the surface of Mars by localized “dust devils” and global dust storms. Dust that is higher than about 150 km from the surface could come from the surface erosion of the Martian moons Phobos and Deimos. However, calculations suggest that this dust would enter the atmosphere via a doughnut-shaped ring around the planet – a ring that has not been detected by MAVEN.
Plasma clouds
The mysterious dust particles were seen by MAVEN as it follows a highly elliptical orbit of Mars, which varies in altitude from 150–6200 km. MAVEN began orbiting Mars in September 2014, and its Langmuir Probe and Waves (LPW) instrument detects dust by sensing the tiny plasma clouds that are created when dust particles strike the craft.
Because the entire surface of MAVEN acts as the detector, it can measure very low concentrations of dust. The size of each particle detected is estimated from the amplitude of its LPW signal, and the data were collected over seven months, which corresponds to more than 1000 passes to within 150 km of the Martian surface.
The data reveal a relatively constant concentration of dust particles from an altitude of about 1500 km down to about 500 km. At the lower heights, the number of particles increases rapidly, and is a factor of five greater at 150 km – the lowest altitude probed. Lab-based experiments suggest that the particles are about 1–12 μm in size, but the researchers cannot think of any known mechanism that could propel the dust from the surface to beyond a height of about 150 km.
Missing process
“If the dust originates from the atmosphere, this suggests we are missing some fundamental process in the Martian atmosphere,” says Laila Andersson, who is lead author on a paper in Science that analyses data from the LPW. Writing in the latest issue of Science, Andersson and colleagues reckon that the dust is interplanetary in origin and could be particles driven by the solar wind or debris brought into the inner solar system by comets.
If so, Mars will not be alone as a collector of interplanetary dust. The Earth’s atmosphere contains it too, and the team used previous measurements done on Earth to estimate how much interplanetary dust should be found on Mars. This calculation suggests that the LPW has only seen a small fraction of the interplanetary dust that could surround Mars.
Raving over MAVEN
Also in Science are three other papers reporting MAVEN results. The orbit of MAVEN was adjusted occasionally so that it dipped to within 120 km from the surface of Mars. These “deep-dip” campaigns took MAVEN to within the planet’s upper atmosphere, where Stephen Bougher of the University of Michigan and an international team measured the concentrations of gases including carbon dioxide, argon, nitrogen oxide and oxygen.
They found that the concentrations varied substantially from dip to dip, suggesting that the upper atmosphere is a dynamic place. The team also measured variations in the magnetic field of Mars from dip to dip. Its results suggest that both the crust of the planet and the solar wind contribute to the magnetic properties of Mars.
In another paper, Bruce Jakosky of the University of Colorado Boulder and colleagues describe how they used instruments on board MAVEN to work out how many ions escape from the Martian atmosphere during solar bursts. This information could help scientists to understand how a sizeable chunk of atmosphere could have been lost by Mars early in its history.
The final paper focuses on an aurora event that occurred in the planet’s northern hemisphere. Nick Schneider of the University of Colorado and colleagues used MAVEN’s Imaging Ultraviolet Spectrograph to study this Martian version of the northern lights, which they found to be more evenly distributed and diffuse than its terrestrial counterpart.
The four papers are published in a special issue of Science.
It being the International Year of Light, guests were also treated to two spectacular stage shows. Having just settled into our seats, we first watched as three dancers performed in front of lasers, dry ice and strobe lighting (see photo above) – certainly a first for an Institute awards dinner – while after the meal we were treated to a troupe called Feeding the Fish.
Their dancers carry laser batons to create “one-of-a-kind performances that fuse tight choreography with…specialized lighting effects”, with the batons being used to show everything from triangles and butterflies to even the logo of the Institute of Physics. Quite how it all worked certainly had physicists in the audience scratching their heads.
By examining the aftermath of the collisions of gold ions at close to the speed of light, an international team of physicists has measured the strong interaction between pairs of antiprotons for the first time. The researchers found that at very short distances, antiprotons – the antiparticles of protons – attract each other just as protons do. While this result was expected, it should improve our understanding of how antinuclei are held together. It also further strengthens the idea that charge, parity and time-reversal (CPT) symmetry is a fundamental symmetry of nature.
The strong interaction binds protons and neutrons together in atomic nuclei and it is also responsible for gluing together the quarks that make up protons, neutrons and other baryons. At very short distances, the strong force is attractive and much stronger than the electromagnetic force, which tends to push protons apart.
This latest experiment was carried out by the STAR collaboration at the Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). As well as showing that the force between antiprotons is attractive, the team was also able to measure two other important parameters – the scattering length and the effective range of the interaction.
It’s the mirror we think nature obeys Daniel Kaplan, Illinois Institute of Technology
Brookhaven’s Aihong Tang explains that the team expected the interaction to be CPT symmetric, which means that antiprotons behave as protons do. CPT symmetry means that if you switched the charge and parity of a particle and reversed time, all of the laws of physics would remain the same. “It’s the mirror we think nature obeys,” says Daniel Kaplan of the Illinois Institute of Technology in the US, who was not involved in the work. No experiments, to date, violate CPT symmetry, and physicists currently accept it as “an article of faith”, he adds.
It is not surprising that the researchers did not find CPT violations, Kaplan says, because the conventional wisdom states that if such violations existed, they should be much weaker than the strong force. This means that STAR will have to make much more precise measurements if it hopes to reveal violations. “It will be some super-small interaction, so weak that we haven’t noticed it before,” Kaplan says. “The strong force is the strongest force there is. If you imagine some new mechanism that causes a CPT violation that no previous experiments have ever seen, how can it be big enough to be a significant part of the strong force?”
Lamppost search
While the results are not groundbreaking, it is useful to have another experimental confirmation of CPT symmetry, Kaplan explains. He describes the hunt for CPT violations this way: “Suppose you lost your keys at night. Where would you look? A normal person would say ‘I’d look where I think I dropped them.’ But a scientist would say ‘Let’s look under the lamppost because there’s actually light there.’.” The STAR collaboration looked for CPT violations where it had at least a chance of seeing them.
Performing the measurement is a numbers game, Tang says. The teams derived the result by observing the signature of the antiproton–antiproton interaction in 500 million gold–ion collisions. The team also made a similar measurement of the proton–proton interaction. Antiprotons are difficult to produce, and because they annihilate with regular protons to produce photons, they do not hang around, either. Each collision at RHIC produces thousands of particles but only 3–4% of these are antiprotons.
Useful parameter
This latest result will also help physicists to understand how antinuclei are bound together. Such experimental measurements are useful in quantum chromodynamics (QCD), the theory that describes the strong interaction. It is difficult to use QCD to calculate the strong force purely from theory, and this measurement of the antiproton interaction can be used as a parameter in the calculation.
In addition, by studying how antinuclei are bound together, physicists could learn how to make heavier antimatter nuclei. Several types of antinuclei have already been produced. These include antitritium – which is one antiproton bound with two antineutrons, and antihelium-4, which is two antiprotons bound with two antineutrons. However, Tang says that physicists are still a long way from producing antimatter for any sort of practical use.
Collage of the CMS detector at CERN created by Genevieve Lovegrove.
By Michael Banks
We’ve already had a LEGO model of the giant CMS detector at CERN’s Large Hadron Collider (LHC) but now University of Leicester modern literature student Genevieve Lovegrove has attempted to go one better by creating a collage of the LHC detector made from everyday objects.
Glasgow’s Technology and Innovation Centre. (Courtesy: University of Strathclyde)
By Margaret Harris in Glasgow
If you’re the first speaker after lunch at a conference, how do you make sure your audience stays awake and engaged?
For Oliver Ambacher – who occupied the dreaded post-prandial slot during Wednesday’s applied photonics conference at Glasgow’s Technology and Innovation Centre (TIC) – the answer is simple. You pretend to jump off a cliff.
Ambacher, the director of the Fraunhofer Institute for Applied Solid State Physics in Freiburg, Germany, made his leap (actually, several leaps of varying lengths) to illustrate one of the toughest challenges in applied physics: the yawning gap between what academic researchers can provide, and what industry scientists need to turn that research into innovative products. This gap is sometimes called the “valley of death”, and Ambacher’s point was that the risks of leaping across it are generally higher on the industry side. “If I, whose heart is still in physics, jump into the valley of death, I lose funding, maybe a project,” Ambacher explained. “But somebody from industry, they may lose their job. So they cannot jump so far.”
For domestic use, options abound. Pitch black, jack black, lamp black, fine black, velour black, onyx black, blackboard black, black fossil, charcoal, soot, smoke, sinner, black stillness, off black, little black dress, penny black, deep black and – should you want to be left in no doubt – black black.
There are probably 50 shades of black, if not more. But sometimes, and especially where physicists are concerned, even the blackest black isn’t black enough.
For many years, NASA has used a black paint manufactured by the international aerospace corporation Lord that has a reflectivity of just 3.5%. That’s several per cent lower than conventional black pigments. Dubbed Z306, it was applied to the Hubble Space Telescope, as well as other NASA instruments, to reduce stray light from the Sun, Moon, Earth and indeed the telescope’s own housing. Without it, all those images of distant galaxies and novae might not have appeared quite so spectacular.
Black is important in the design of the best telescopes, but it is also key in other scientific equipment, including infrared detectors, black-body calibrating materials, radiators and solar panels. The military, too, requires very black materials for sensitive infrared seekers used in ballistic missile defence and satellite surveillance, because the blacker the surface, the more sensitive it is to light. With such high-stakes applications, it is no wonder that the fashion for black is being eagerly updated. After all, Z306 is so last season. Enter the new black: super black.
Dark materials John Hagopian with his nanotube-based black material. (Courtesy: Chris Gunn/NASA)
As it happens, NASA has been working on a super black for some time. In 2007 physicist John Hagopian from the agency’s Goddard Space Flight Center in Maryland, US, began experimenting with the growth of carbon nanotubes. These materials share the same hexagonal “chicken wire” structure of the layers in graphite, but are rolled up into tiny hollow cylinders with diameters ranging from fractions of a nanometre to hundreds of nanometres. Carbon nanotubes are typically grown on a substrate inside a furnace, into which a carbon-containing gas is fed. What Hagopian did was to coat a silicon substrate with iron, which acted as a catalyst, encouraging the nanotubes to grow vertically, like a forest.
Hagopian’s nanotube forests had a refractive index close to one – that of air – which meant that light striking the surface didn’t “see” a barrier. Rather than reflect, the light was absorbed by the nanotubes’ hollow interiors, and ultimately turned into heat. The material had a reflectivity of just 0.5%, making it seven times blacker than Z306.
That is very black. Look around and you will probably see several black surfaces – on computers, smartphones and televisions. Black is the designer’s go-to chic colour when it comes to consumer technologies. But this familiar black, it turns out, is hardly black at all compared with the carbon-nanotube variety. Hagopian’s coating was so black in fact that it attracted interest from a Belgian artist, Frederik De Wilde, who quickly persuaded the NASA group to allow him to use it on some of his projects. One of them, the 2010 work Hostage, is a square nanotube forest on a conventionally black background. It looks like a mysterious void: no reflections, no features to speak of.
Dark rivalry
A coating that absorbs all but 0.5% of light is surely black enough, you might say. But even while Hagopian’s group was writing up a paper on its super-black nanotube forests in 2008, another group, based at the Rensselaer Polytechnic Institute (RPI) in New York, US, announced that it had made “the darkest material ever made by man”. That material, too, was a carbon-nanotube forest, albeit with a density even better tuned to minimize reflectance. “This is the kind of field where everyone feeds off everyone else,” says Hagopian. “There’s a lot of history, a huge number of papers.”
Hostage by Frederik De Wilde. (Courtesy: Frederik De Wilde)
The RPI group, led by physicist Shawn-Yu Lin, claimed a visible-light reflectance for its nanotube forest of 0.045%, making the material more than 10 times blacker than Hagopian’s, and nearly 80 times blacker than Z306 (Nano Lett.8 446). The material swiftly became the blackest black, as recognized by the book Guinness World Records.
For a few years the RPI record seemed unbreakable. During this time, however, Ben Jensen and others at Surrey NanoSystems, a company in Newhaven, UK, had been experimenting with the growth of carbon nanotubes at relatively low temperatures to make highly conductive electrical contacts. The problem is that carbon nanotubes do not normally grow below a temperature of about 700 °C, which is above the melting temperature of aluminium, one of the most widely used metals in aerospace. To make the two materials compatible, Jensen’s group realized that it could keep the bottom of an aluminium substrate cool while locally heating the top surface with a high-powered lamp. That way, the nanotubes could grow without any disruption to the bulk metal.
In 2012 the idea caught the attention of Theo Theocharous at the National Physical Laboratory (NPL) in Teddington, UK, who immediately realized its potential for creating super-black surfaces for space applications. The result, after two years’ collaboration between NPL and Surrey NanoSystems, was “Vantablack” – the “vanta” prefix being an acronym of “vertically aligned nanotube arrays” (Opt. Express22 7290). Although the paper emphasized the importance of the aluminium substrate, the firm’s press releases stressed Vantablack’s claimed minimum reflectivity in the visible spectrum of 0.04%, later revised to 0.035%. The improvement over the RPI material may have been slight, but in due course Guinness World Records made it official: Vantablack was the new black.
Here is where the rivalry for the blackest black has shown its dark side. After Surrey NanoSystems’ 2014 announcement, lawyers acting on behalf of De Wilde reportedly contacted Dazed, a UK fashion and culture magazine, to contradict its description of Vantablack as “the world’s blackest material”. “Nobody likes to be left out and disrespected,” the artist said in a subsequent interview with the same magazine. “Unfortunately I cannot talk in detail yet about my relationship with Surrey NanoSystems…The only thing I can say is that Surrey NanoSystems contacted me and we exchanged information.”
In production UK firm Surrey NanoSystems produces Vantablack, the current record holder for the least-reflective material. (Courtesy: Surrey NanoSystems)
De Wilde did not respond to questions from Physics World about the reasons behind his contention. Jensen from Surrey NanoSystems, however, says that the artist had approached his firm with a proposal of artistic collaboration, but it had to refuse because it had already signed a separate exclusive agreement with the British-Indian Turner Prize-winning sculptor Anish Kapoor. In any case, Jensen adds, De Wilde’s claim is plain wrong: Vantablack was, and still is, the blackest black. “It’s just bad cheese because we refused to work with him,” he says.
Back at NASA, Hagopian tries to distance himself from the claims of his artist collaborator. “There’s a little battle going on – who can be the darkest,” he says. “We’re kind of foregoing that, and working on practical applications.” Still, Hagopian does not bow out of the competition altogether. “You know, the difference between RPI and Surrey NanoSystems – what is that, about 0.01%? It’s not that great. I’m sure we could do better, it’s just not what we’re trying to optimize.” In recent tests, he claims, the visible-light reflectivity of his nanotube forests has dropped below 0.1%.
Space, shock waves and solar power
Rather than creating blackness for blackness’ sake, much of Hagopian’s work has had the practical focus of applying the nanotube forests to substrates suited to aerospace. By using a patented aluminium-oxide buffer layer, his group has managed to grow nanotubes on stainless steel, titanium, copper, silicon and silicon nitride. He admits that the ability to deposit Vantablack on aluminium is interesting, but doubts how much it would be used in practice. “I think their systems are quite expensive, half a million dollars or so, versus a tube furnace that costs maybe ten thousand,” he says. “We didn’t really find it to be cost effective for what we’re doing. Titanium, the stainless steels – they’re just as good as aluminium.”
The blackest black UK firm Surrey NanoSystems produces Vantablack, the current record holder for the least-reflective material. (Courtesy: Surrey NanoSystems)
Most scientists seem to agree that when it comes to space applications, a super black’s minimum reflectivity in the visible spectrum is somewhat incidental: low reflectivity across the entire infrared, visible and ultraviolet spectrum is more important. Even then, there are other concerns, such as the material’s mass and its tolerance to extreme conditions. Hagopian points out, though, that his nanotube forests have already been tested for durability aboard the International Space Station, being exposed to the harsh radiation of outer space. Elsewhere they are being used to boost the signal-to-noise ratio of an airborne laser fluorescence experiment, tracking gases used to monitor atmospheric currents. His group has also applied its super black to an occulter used to block out starlight in exoplanet hunting, as well as to diffraction baffles for the upcoming LISA gravitational-wave experiment.
Not that the main destination for super blacks is necessarily space. In 2012 Jay Guo, an engineer at the University of Michigan in Ann Arbor, US, and colleagues reported that nanotubes could be coated with polymers onto a concave lens. When irradiated with a laser, the coating quickly transformed the light to heat, generating ultrasonic shock waves that could destroy materials placed in the lens’s focal zone (Sci. Rep.2 989). The lens, says Guo, could become an “invisible scalpel” for use in surgery.
Working with researchers at Zhejiang University in China, Guo’s group is also planning to report on an entirely new way to make a super black. Although he cannot yet give details, Guo claims that simple evaporation of materials can produce a multi-layered structure that has a reflectivity of 1% from the visible to near-infrared. This is not nearly as low as other super blacks, but he reckons that the technique can be scaled more easily to large areas.
Guo isn’t the only one to highlight scalability as another important factor. Renkun Chen – an engineer at the University of California in San Diego, US – is interested in applying super blacks to the sunlight-absorbing tubes used in concentrated solar power plants. In these plants, a field of mirrors concentrates sunlight onto a central tower containing tubes, which heat up gas to drive turbines. A super-black material made from a carbon-nanotube forest – and especially the painstaking method needed to create one – would simply not be up to this task, he says. “For a solar tower you need hundreds of tubes,” he explains. “Each tube is about 20 m long. Think about it.”
Looking forward The Abengoa Solar plant in Spain is an example of concentrated solar power, which could be much improved by the use of super blacks. (Courtesy: Abengoa)
Even if super blacks made from nanotubes were coated onto such tubes, says Chen, they would burn up in the heat of a solar tower. Current solar towers can reach temperatures of about 600 °C, but engineers want to push them even higher to improve efficiency and to make the technology competitive with solar photovoltaics. To meet this challenge, Chen and his colleagues are now developing a super black consisting of cobalt-oxide nanoparticles that can be sprayed onto the absorber tubes. Stable up to 750 °C, the nanoparticles create a rough surface that absorbs light in much the same way as a nanotube forest, although the reflectivity reaches a minimum of only 2% (Sol. Energ. Mat. Sol. Cells.134 417).
With this development, the hunt for super blacks has gone full circle. Chen’s super black is, in essence, a spray paint. But that is not to say that growing carbon nanotubes, or depositing materials by evaporation, is the wrong approach. It only means that there are other criteria to consider. Depending on who’s asking, the best black may not be the blackest black.
Hiding in the shadows
The search for super blacks could, in time, turn up ideas that bear fruit in other areas. In one 2011 experiment, Guo found that if he coated a micron-scale 3D etching of a military tank with a nanotube forest, the tank became perfectly camouflaged against its surroundings: it appeared as flat and as black as the surface it was etched upon (Appl. Phys. Lett.99 211103). The military applications were obvious. But the result also got him thinking: what if objects in outer space were hidden in this way? In principle, he concluded, a super-black coating could hide an arbitrarily large object from the ultraviolet through the infrared and into the terahertz part of the electromagnetic spectrum.
Beyond black NASA has already begun testing its super blacks on the International Space Station. (Courtesy: NASA)
“It is interesting to note that deep space itself is a perfect background without reflecting any radiations,” he wrote in his paper. “It would only take a ‘magic veil’ consisting of low density and broadband absorbing particles to render matters and objects totally invisible to our instruments based on the detection of electromagnetic waves.”
Perhaps, then, the allure of super blacks is rather more primitive than many scientists let on. It is that ultimate uncertainty, the one that has tormented every human who has ever stared into the void. Is there something there, or not?
Water droplets will bounce spontaneously on a specially designed surface by simply evaporating some of their liquid, according to scientists in Switzerland. The bounces become successively higher until the droplets freeze. The researchers believe the effect could be put to work on self-cleaning surfaces.
The bouncing is reminiscent of the well-known Leidenfrost effect, whereby a droplet of liquid can levitate slightly above a hot surface such as a frying pan. The high temperature underneath the droplet evaporates some of its liquid to create a vapour pressure that offsets the downward pull of gravity and keeps the droplet afloat. In the Leidenfrost effect, the droplet oscillates briefly with decreasing amplitude before reaching an equilibrium height where the pressure differential is just enough to balance gravity.
In the new research, mechanical engineer Dimos Poulikakos and colleagues at ETH Zurich looked at how water droplets interact with superhydrophobic surfaces at low pressure. The researchers created arrays of tiny silicon micropillars coated with water-repellent fluorosilane. Such a material is expected to be superhydrophobic because water droplets will sit on top of the micropillars. This reduces the contact area between the droplet and the surface, and so makes it more likely that the droplet will not stick.
Pressure drop
At ambient atmospheric pressure, this small contact area is sufficient to keep droplets tethered to the surface. Lowering the air pressure normally reduces the water repellence of a superhydrophobic surface. This is because with less air underneath it, a drop will sink deeper into the textured surface, thereby increasing the contact area. However, when Poulikakos and colleagues looked closely at the behaviour of water droplets in 1% atmospheric pressure, they found them bouncing up and down on the micropillars with ever-increasing height (see image). “I was very surprised, of course,” says Poulikakos.
The researchers have worked out that the bouncing is related to an additional effect of low pressure – it reduces the humidity of the air and thereby allows the droplet to evaporate faster. Liquid evaporating from the top of the droplet dissipates rapidly in the air. Underneath the droplet, however, things are different. Vapour cannot easily dissipate between the micropillars, and this generates pressure under the droplet that increases until it is large enough to detach the droplet and send it flying upwards. Once the droplet leaves the surface, the pressure is released and the droplet falls back down onto the surface. The droplet then bounces upwards with an extra kick generated by its vapour pressure. This effect is repeated again and again, and the evaporation of the droplet feeds increasing amounts of energy into its motion, causing it to bounce higher each time.
Supercool effect
Evaporation also causes the droplet to cool rapidly, putting it into a supercooled-liquid state. When the droplet does eventually freeze, the decrease in enthalpy in passing from liquid to solid causes a sudden increase in its temperature, and a consequent increase in how much vapour it sheds. The sudden increase in vapour then propels the ice crystal away from the surface.
“If the surface is vertical, it will fall off,” says Poulikakos. “If a breeze blows, it will blow it off.” Even if an ice crystal falls back down after it freezes completely, it will be much easier to remove than the crust of ice that would form if water froze on the surface, he explains.
Physical chemist Doris Vollmer of the Max Planck Institute for Polymer Research in Germany is impressed. “It’s beautiful and very amazing work – I would never have guessed this,” she says. She cautions that, at present, this is not a practicable proposal for a self-cleaning surface because it does not work at atmospheric pressure, but she concludes that “the aim of science is fundamental understanding and, from the point of view of fundamental understanding, it’s brilliant”.
“Pictures convey abstract concepts to each other and to ourselves,” explained Michael Berry last night in his talk “Optica Fantastica: Nature’s optics and our understanding of light” here at IOP Publishing in Bristol as part of the city’s Festival of Ideas. Berry, who is the Melville Wills emeritus professor of physics at the University of Bristol, began by declaring that he was going to take the “opposite approach to Einstein”. Whereas Einstein said, “I want to know God’s thoughts, the rest is just detail”, Berry is interested in those details.
Sponsored by the Institute of Physics, which publishes Physics World, the talk was held to celebrate the International Year of Light 2015 and in it Berry ran the audience through the four levels of understanding light: rays, waves, polarization and quantum. He showed beautiful pictures from his own research but also included some artworks related to the phenomena he was explaining and even quoted passages from novels.
To explain some of the effects of light on water, Berry showed the David Hockney painting “Dive in” and read an extract from AS Byatt’s Possession, in which she described “tessellation in the water’s glaze”.
A series of pictures of interference patterns between random waves allowed Berry to explain phases of light, optical vortices, knot theory and the cosmic microwave background.
The image above was taken by creating what Berry called a “black sandwich” – placing a sheet of overhead transparency foil between two polarizing filters and taking a photo through them with an ordinary digital camera. It creates a beautiful effect (one that I fully intend to try myself just as soon as I can find a supply of overhead transparency foil) and Berry assured us that he can give a full hour’s lecture on the mathematical properties that the image elucidates.
I learned some interesting nuggets, such as the controversy over whether Vikings used polarization in crystals to navigate from Norway and Iceland to Canada 1000 years ago. (Berry’s conclusion is that it’s unlikely but he’d still like it to be true.) And that the halo you sometimes see around your own shadow (or if you’re in an aeroplane, the halo you see around the plane’s shadow on a cloud) is called a Brocken spectre and is described in Thomas Pynchon’s Gravity’s Rainbow.
I also learned that while Berry appreciates art, and can name many examples of science having inspired art, he believes it is a one-way street – he does not know of any science inspired by art (despite a few tentative examples given by the audience). What do you think? Are there any solid examples of art inspiring science?
