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Diatoms bring the quantum effect to life

The exoskeleton of a tiny organism has been used as a diffraction grating by researchers in Vienna, who have carried out a molecular interferometry experiment using it. The team showed that a coherent molecular beam could be diffracted from the silicon-based cell walls of a marine alga. Algae are cheap and easily available, so replacing costly nanodevices with them in interferometry experiments would be beneficial, according to the researchers.

Contrary to classical mechanics, quantum physics states that a particle can act like a wave and vice versa – an idea that was first proposed by Nobel-prize-winning physicist Louis de Broglie back in 1923. While the idea that tiny particles such as electrons could behave like a wave came as a shock, scientists now know that even objects a million times more massive than electrons, such as complex molecules, also show quantum interference. Massive molecules have very small wavelengths and therefore a grating with extremely thin and closely spaced slits is needed to observe their diffraction. Currently, such sophisticated devices are specially fabricated using nanotechnology techniques.

Natural grating

Now physicists Michele Sclafani, Markus Arndt and colleagues from the University of Vienna have shown that a simple, inexpensive and natural grating is close at hand in the form of diatoms. These are a group of mostly unicellular marine algae that are encased within a cell wall made of silica that is known as a frustule. In the experiment, Sclafani used Amphipleura pellucida – an alga that can be found on the beach. It has a wall thickness of 90 nm and a surprisingly regular pore distance of about 200 nm. The researchers found that they could use the diatom to measure molecular De Broglie wavelengths as small as a few billionths of a millimetre by firing beams of molecules at it.

Lighting up molecules

To do this, the Amphipleura pellucida frustule was suspended in a vacuum with its pores oriented towards the coherent molecular beam of phthalocyanine molecules – a commonly used blue-green dye. Incidentally, a phthalocyanine molecule is about a million times more massive than an electron. Phthalocyanine was chosen because it gives off fluorescent light when illuminated with a laser and so can be easily detected. The beam was created by shining a 1.5 μm focused laser onto a dye-coated glass window. The tiny spot size of the laser ensured that the beam was coherent. The beam was then sent through the alga and each individual molecule was counted on a screen behind the sample. This was done by observing the location of the fluorescent light with a microscope. After many molecules were passed through the alga, the ensemble of their locations made up the diffraction pattern.

“We actually managed to delocalize and coherently propagate a particle through the pores of a biologically grown nanostructure,” exudes Sclafani, explaining that here, coherently means without loss or scrambling of quantum phases. Instead, the wavefunction of each molecule is delocalized over several of the alga’s pores. He also points out that “most of the apparatus was very easily sourced – if you are making a matter–wave interferometer in school, this might be the way to go”. Sclafani went on to tell physicsworld.com that the diatoms used were bought online and cost a mere £12. You can see more about how the researchers carried out their experiment in the video abstract (above) of their paper published in New Journal of Physics.

Compare and contrast

To dissect and understand their experimental results, the researchers took a photograph of the alga and used a set of formulae (that normally model the propagation of light in similar circumstances) to calculate the molecular intensity distribution that should appear on the screen behind the alga. Upon comparing the two, the researchers noted something odd – while their theory predicted 2D interference, experimentally they had observed a 1D pattern.

Sclafani says that the simple explanation for this dimensional anomaly is gravity: as the molecules travel from the source to the detector they “fall” to some extent, thanks to the Earth’s gravitational field. This means that molecules with different velocities end up contributing to the same spatial region along the y-axis and the pattern is smeared out. “If we repeat the interference experiment and twist by 90°, you would see a 2D pattern,” says Sclafani.

Missing orders

The researchers also noticed that in their theoretical simulation, the higher-order interference effects are almost missing, but they see many more higher-order interference effects in their experiment. They believe this can be explained by an interaction potential between the single molecules and the real material surface of the pores of the alga. Sclafani says that this interaction is described by the Van der Waals potential that comes into play as the molecule goes past the alga’s walls, inducing a small dipole moment as it does so. So, the Van der Waals potential generates an additional momentum kick.

Mathematically, this interaction can be mimicked by reducing the size of the alga’s pores, as this results in a larger population of high interference orders. As an alternative, the team also extended its theoretical model by adding a phase term to the integral that estimates the interaction constant between the phthalocyanine and the inner surface of the alga. This results in good qualitative agreement between theory and experiment.

Sclafani is keen to point out the many advantages of using diatoms. In addition to that fact that there are regular structures in most algae that could be used in matter–wave interference, one could use atoms or different molecules to probe the internal properties of the algae via their dephasing influence on the quantum-interference pattern. Diatoms are less exotic than it may appear at first glance: they have many uses including drug delivery and as natural filters, thanks to their “very nice and regular structures”, according to Sclafani.

Scalfani says that, in the long run, the focus of the group is to probe the matter–wave duality of an object that has mass and complexity. “Matter–wave duality still puzzles people – is there any limit to quantum mechanics, mass-wise?” he asks. Understanding this is crucial.

The research is published in the New Journal of Physics.

Perimeter Institute welcome speech reignites the string wars

Neil Turok at the Perimeter Institute for Theoretical Physics (Courtesy: Gabriela Secara)
Neil Turok at the Perimeter Institute for Theoretical Physics. (Courtesy: Gabriela Secara)

By Hamish Johnston

A series of four articles about people at the Perimeter Institute for Theoretical Physics (PI) has been published in the Canadian magazine Maclean’s and one article in particular has got people talking.

(more…)

New silicon devices are fast, flexible and semi-transparent

A new, low-cost and industry-compatible process to make flexible and semi-transparent silicon-based circuits has been unveiled by researchers at the King Abdullah University of Science and Technology in Saudi Arabia. The technique is an important step forward in making high-performance flexible and transparent computers, says the team.

The last decade has seen considerable progress in flexible electronics, with a host of novel applications such as electronic paper-like displays and artificial skin having already seen the light of day. However, most of these devices are made from organic materials such as polymers, carbon-based nanowires and carbon nanotubes, rather than inorganic silicon – despite the fact that this element is the basis of most modern technology. This is because silicon is intrinsically brittle and rigid, meaning that it does not easily lend itself to flexible applications.

Organics are too slow

The problem with organic flexible-electronics devices, however, is that they are much slower than their silicon counterparts. They are not very robust at higher temperatures either and cannot be formed as continuous films – unlike bulk silicon. Integration densities (the number of transistors that can be packed onto a single chip) for these materials are also much lower than those for the silicon-based transistors found in today’s microprocessors.

Although inorganic flexible electronics do exist, they rely on expensive substrates such as SOI, UTSOI and silicon (111), and the processes to fabricate ultrathin silicon or porous silicon are far from straightforward.

Now, a team led by Muhammad Hussain could have gone a long way in overcoming these fundamental problems with a new method that employs traditional state-of-the-art CMOS-compatible processes that transform silicon-based circuits into flexible and semi-transparent ones while still retaining silicon’s excellent electronic properties. Costs are also kept low because the technique makes use of the most common substrate employed in the semiconductor industry today: bulk monocrystalline silicon (100) wafers.

Deep trenches

Hussain and colleagues made p-type metal–oxide–semiconductor field-effect transistors (MOSFETs) by first gorging deep trenches less than 50 µm wide on a silicon-dioxide layer (the active layer in the circuit). Next, they added a high-dielectric/metal gate stack on top of this layer using atomic-layer deposition. The deposited films are then pattered using reactive-ion etching down to a resolution of 5 µm ready to receive the different device components.

The next step involves implanting dopant ions to form source and drain electrons in the active area and these electrodes are contacted to the substrate using nickel-silicide film. Holes are created in the contact pads and then over the entire sample surface. Deep reactive-ion etching is finally employed to create straight channels about 30 µm deep in the silicon substrate.

A thin silicon film (containing all the device elements) is then released from the bulk substrate, again using etching. This film is flexible and semi-transparent thanks to the network of etch holes.

Fast and flexible

The researchers tested the electronic and mechanical properties of their devices and found them to have a 80 mV/dec sub-threshold slope – which is the fastest ever switching demonstrated in a flexible transistor. “What is more, the devices have the lowest bending radius compared with any other flexible transistor made to date – including plastic ones,” Hussain told physicsworld.com.

John Rogers of the University of Illinois, who was not involved in this work, comments that the devices “do not only demonstrate mechanical flexibility, but also remarkable levels of optical transparency. The ideas outlined here add to the growing toolkit of approaches and materials for making flexible integrated circuits, with excellent performance.”

The results are an important step forward in realizing a fully flexible computer, adds Hussain. Indeed, the Saudi Arabia team says that it is now working on making integrated systems such as foldable computation and communication devices using its flexible and semi-transparent silicon.

The results are detailed in Nature Scientific Reports.

The seven chemical wonders

Picking the seven chemical elements that have done most to change the world sounds like a fantastic science parlour game. And with 80 stable and 38 (so far) unstable elements in the periodic table to select from, the choice might appear far from straightforward. Not so for John Browne, former chief executive of oil giant BP, who seems to have lost little sleep in selecting his top seven elements – the historical, social, scientific and economic significance of which he describes in Seven Elements That Have Changed the World.

Three elements seemed obvious to Browne from the outset: carbon, iron and silver. Carbon, of course, has powered industry through oil and coal, while iron is the ultimate building material. Silver’s influence is perhaps more subtle: it makes Browne’s list because it made photography possible. As for the remaining four elements, Browne picked these by digging out his old school copy of the periodic table, scanning the list and plumping for gold, uranium, titanium and silicon. Gold, he explains, has underpinned trade and motivated colonial expansion. Uranium led to nuclear energy and the Cold War, while silicon’s uses have stretched from glass mirrors and lenses to transistors and solar cells. Titanium is, for me, an odd choice, and I’ll come back to that later, but Browne has no such qualms: “Time and again, while writing this book,” he explains, “these seven elements stood out as having most powerfully changed the course of human history.”

Browne devotes a chapter to each element but it is his discussion of silicon that is by far the strongest and most interesting part of the book. He begins by looking at the glass-making prowess of the Venetians, who in the 17th century were so desperate to protect their trade that they threatened to kill any glassblowers who took their skills elsewhere. Glass windows were such a convenient form of taxation on wealthy homeowners that a window tax remained in place in Britain until 1845 – and is, Browne claims, the origin of the term “daylight robbery”. The removal of that tax led to a boom in the British glass industry, and the 1851 Great Exhibition at London’s Crystal Palace, which was built from 3300 iron columns and 300,000 panes of glass, showed just how versatile this material could be.

Of course, silicon also underpins more recent wonders, and Browne does a fine job at detailing its use in solar cells and transistor-based integrated circuits, infusing his descriptions with some interesting personal perspectives. As a fresh-faced employee at BP in 1970, for example, Browne was one of the first people in the oil industry to use computers to model the location of petroleum reserves in the Alaskan oilfields. Today, drilling for oil without computers is unimaginable, but back then computer-aided drilling was a much harder exercise, with computers literally crashing when their spinning disks ground to a halt. As the author points out, computers have advanced so much since then that a modern smartphone has more computing power than the whole of NASA did in the late 1960s.

The inclusion of titanium is, for me, the weakest of Browne’s choices. Discovered in 1791, it only rose to fame after the Second World War when a commercial process to extract titanium from its ore was developed. Dubbed a wonder material on account of being extremely strong, light but resistant to corrosion, titanium was used by the Americans in the Cold War to make the famous Blackbird spy plane, while the Soviet Union even manufactured a submarine built entirely from this non-magnetic material.

Yet, as Browne rather forlornly admits, titanium as a metal never lived up to its potential. It is 10 times more expensive to make than steel, and production of titanium today is barely one-thousandth of the level of its rival, being restricted to specialist areas such as limb implants, tennis rackets and oil rigs. Yes, titanium – in the form of nanoparticles of titanium dioxide – is also a key ingredient in sun cream and is used as a whitening agent in foods, paint and toothpaste, but as Browne himself writes, “today titanium has a limited role”.

Perhaps for this reason, Browne devotes just 12 pages to this element, followed by silver (19 pages), uranium (25), iron (28), gold (30), silicon (33) and carbon (56). The chapter on carbon strays too much into the economics of the oil industry for my taste, while those on gold and silver are well researched but occasionally read, dare I say, like Wikipedia entries. As for uranium, Browne covers all the bases – from Hiroshima and the arms race to Chernobyl and Fukushima – without saying anything stunningly new.

Although the book is as much about economics and history as it is about science, when Browne does get into the physics and chemistry of the seven elements, he has a real knack for clear scientific descriptions. That is perhaps not surprising: Browne did, after all, study physics at the University of Cambridge in the 1960s. It is therefore disappointing that so much science is booted into the extensive 43 pages of footnotes at the end of the book. In fact I found myself constantly flicking back and forth between the main text and the footnotes (of which there are almost 500). Where did this trend for vast numbers of footnotes come from? And do we really need to flip to the end to learn nothing more than “viscosity is the friction between molecules in a liquid”?

Selecting seven elements about which to write a book is unlikely to have been the toughest choice Browne has ever faced in his long career. In fact, the ability to make clear decisions – and stick by them – is probably a pre-requisite for being top dog at one of the world’s biggest energy firms for a dozen years. Still, I felt Browne could have done more to justify his choices – why do, say, copper, zinc or technetium not make the cut? What about aluminium, hydrogen or germanium?

Anyone keen, as I was, to find out more about Browne himself through this book will have slim pickings, although he does reveal a life-long passion for photography, including a set of Leica cameras, and – bizarrely – a collection of more than 100 toy glass elephants. In the end, though, I was won over by Browne’s enthusiasm and I was sorry when the book finished, leaving me wanting to read more. It would be hard to imagine many business leaders writing a book as interesting, accessible and well researched as this one.

Web life: 94 Elements

 

So what is the site about?

94 Elements is a documentary film project about how people interact with the chemical elements. In its format (short films) and its structure (one film about each element), the site owes an obvious debt to the University of Nottingham’s excellent Periodic Table of Videos site, which was the subject of the very first Web life column (January 2009 p35). However, the resemblance between the two sites ends there. While the videos that make up Nottingham’s periodic table concentrate on the elements’ chemical properties and applications, the films in 94 Elements have a much more human and environmental slant. The film “Copper: acid and dust”, for example, shows a group of men from Bihar, one of India’s poorest states, extracting copper from old printed circuit boards by steeping them in barrels of nitric acid. Their work is dangerous and illegal, made possible by bribes to Indian officials and (the film implies) by a “throwaway” culture that nevertheless places more value on things than it does on human beings.

Which other elements are covered?

Copper is one of only five elements to receive the cinematic treatment so far (the others are oxygen, germanium, gadolinium and osmium) but another five films are currently in the works and the project’s leader, Mike Paterson, hopes that the site will eventually contain films about all the elements from hydrogen to plutonium (element 94) on the periodic table. Paterson has made the films on copper and germanium himself, while the other three are by different directors from the UK, Georgia and the Netherlands.

What are some highlights?

Nino Kirtadze’s film “Gadolinium: scan” takes as its subject the use of gadolinium in MRI scans to improve contrast between normal and abnormal tissue. In the seven-minute film, the central figure, a Georgian man called Levan, has a scan on his shoulder, then spends the rest of the film puzzling over medical jargon and arguing with his friends and extended family about what the results mean. Some of their conversations are funny as well as poignant, as when a few of Levan’s relatives seem to revel in the possibly gruesome consequences of his injury. “There can be pus, then maybe gangrene, and then they’ll cut your arm off,” says one with apparent satisfaction. “And it’s so close to your head!” another adds helpfully as Levan squints at the gadolinium-enhanced images.

Why only 94 elements?

Good question. In a project about our use of natural resources, it probably makes sense to exclude the exotic elements high up in the periodic table; even the all-encompassing Periodic Table of Videos does not have much to say about ununoctium. But Paterson’s rationale for selecting these particular 94 elements seems flawed. The site claims that “there are 94 naturally occurring elements, from hydrogen to plutonium”, but its definition of “naturally occurring” must be either too broad or too narrow, since it includes several elements, including plutonium itself, that exist naturally only in trace quantities, while excluding others. This is a pity, because a few of the heavier elements could make interesting films. For example, the 95th element, americium, is used in some types of smoke detector.

Why should I visit?

Paterson has a good track record of making films about science and people. His previous project, Colliding Particles, was a series of 12 films about CERN researchers and the search for the Higgs boson (November 2009 p39), and it will be interesting to see how this new work develops over time. Several of the upcoming films look promising, including one about the impending shortage of indium (a key ingredient in the manufacture of solar cells and other electronics) and the intriguingly-titled “Boron: mon amour”, which is being filmed (where else?) in Boron, California. Paterson is also developing new features for the site, including the Mix Lab, which hosts films about chemical compounds rather than elements, and “a range of interactive tools and visualizations exploring our use and exploitation of the elements”. In the future, Paterson says, there will be opportunities for people to pitch their own film ideas, and to vote on which films should get priority for whatever funding becomes available. Just don’t mention americium.

Tiny switch toggles the position of a single atom

A single atom held between two sharp points has been used to create the smallest-ever memory device – according to the international team of researchers that built it. The aluminium atom operates as a two-terminal switch that can be toggled back and forth between two logical states. This is done by passing electrical currents through the atom, which results in tiny shifts in its position. The device could one day be used to create computer memories with extremely high density.

Conventional electronic switches are usually made of transistors that have three electrodes. The current flowing between two of the electrodes is controlled by applying a voltage to the third electrode. Nanometre-sized transistors based on one atom have been made before. However, the team says that its device is the first example of a single atom acting as a switch without the use of a gate electrode – the presence of which made previous devices much larger than one atom.

The device was made by researchers at the University of Konstanz in Germany and the Universidad Autónoma de Madrid in Spain. To make the switch, the team start with an aluminium wire just 1μm wide that is sitting on a flexible substrate. By gently bending the substrate, a break begins to form in the wire. The bending is stopped when the two sections are held together by just one atom at its narrowest part (see figure).

Distinct geometries

The switch is operated by sending a current through the single-atom contact. When the current is sent in one direction the switch is open and when the current is sent in the opposite direction the switch is closed. The open and closed switch positions correspond to the atom being in two different locations. These two distinct geometries have two different values of electrical conductance, which the team measured. These two values can be thought of as “0” and “1”, making the device able to store information.

“Our atomic contacts work as switches and non-volatile memories at the same time,” says one of the researchers, Juan Carlos Cuevas of Universidad Autónoma de Madrid. “Ideally, these memories could be used to replace the standard magnetic memories of our computers.”

The observation that alternating current made the switch open and close “first came as a surprise,” says team member Elke Scheer of the University of Konstanz.

Quantum interference

The big challenge for the team was to prove that the switch involved just one atom. To do this, the scientists cool their switch until the aluminium becomes a superconductor. Quantum mechanics governs the current that flows through the atomic-size wire and current-voltage characteristics in the superconducting state are sensitive to the quantum mechanical transport probabilities, which are determined by atomic positions. Since atomic contacts are so small, the conduction electrons behave as waves rather than as classical particles and the current is completely determined by quantum mechanical interference effects.

“The amazing part of our story is that we can determine how many wave functions carry the current in our switches and the probabilities for these waves to go through the contacts, which is something unique of our work,” says Cuevas.

“This is a very elegant experimental result,” says physicist Douglas Natelson at Rice University in Texas, who did not take part in the research. “It’s a great combination of techniques that provides detailed microscopic information that’s often difficult to obtain, such as the transmission coefficients of the quantum channels. That information is vital to establishing the remarkable fact that it is possible – at least under certain circumstances – to use current to toggle controllably between two particular atomic arrangements at the single-atom scale.”

Missing link

Physicist Jan van Ruitenbeek of Leiden University in the Netherlands, who was also not involved in the study, says that until now, the mechanism of the current’s effect on the atomic scale has not been understood: “Experiments that could probe [these] effects were simply missing and these results give us a first glimpse of what may be happening.”

At the moment, the switches can only function at cryogenic temperatures below 1 kelvin and clean-room lab conditions, where it is possible to limit atomic motion by freezing out the diffusion of metal atoms and thus stabilizing the switches for long periods of time. For potential future applications, the team says it will have to demonstrate that the devices can also work at room temperature, which could be challenging because metal atoms tend to diffuse around on surfaces. “Also, we would need to scale up the device to have millions of those switches working together in a macroscopic memory,” says Cuevas. But, he adds, “there are strategies to solve both problems and we believe that the applications will be possible in the near future, in 10 years or so.”

The research is described in Nature Nanotechnology 8 645.

Sleepless nights at the synchrotron

This film is about the experiences of the scientists who travel to the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to use its intense X-ray radiation for imaging experiments. For researchers, this experience can be both exhilarating and stressful, as they often face a race against the clock to get their results within the time that has been allocated to them. Scientists work long hours in beamline stations where synchrotron radiation is guided through experimental samples to illuminate and interact with the materials being studied.

In the film you will meet Rachael Hazael and Fabrizia Foglia – two researchers who are studying how life processes take place in the high-pressure environments at the bottom of oceans. On this particular trip to the ESRF, they are subjecting live bacteria to extreme pressures to gather data that can be analysed back at their home research institutions. Foglia and Hazael are gearing up for their experiments, which are scheduled to get under way the following day, and they seem to be intensely aware of the hard work that lies ahead.

“After three weeks of beamtime, you think ‘Where is my life outside of the beam?’,” asks Foglia. To which Hazael responds “There is no life outside of the beam!”

Looking inside the Earth

Another research group that features in the film is examining the geology of the Earth’s interior and how it responds to changing pressures. The group’s leader, Jean-Philippe Perillat, explains that he wants to get a better understanding of the chemical composition and structure of the Earth’s upper mantle, which is composed mainly of a mineral called olivine. Perillat and his colleague Isabelle Daniel demonstrate how a tiny sample of olivine is subjected to high pressures and temperatures in a device called a large-volume press.

“Life at the synchrotron is a bit exhausting because we work overnight to optimize the amount of beam time we have,” says Perillat. But it seems to be worth the effort, because on this particular trip the group appears to have achieved the outcome it had hoped for. “We got really interesting results just last night,” says Perillat. The reason that the scientists are so pleased to get good data is that there is stiff competition to gain access to time at the ESRF beamlines, which makes a trip to the facility an intense experience.

Artificial muscles lift heavy loads

Human-like artificial muscles that can extend to five times their original length while lifting loads 80 times their own weight have been developed by researchers in Singapore. Made from polymers, the artificial muscles mimic the operation of their natural counterparts by contracting and expanding rapidly in response to electrical stimuli. This development is a first for robotics and could pave the way towards a new generation of more efficient, greener and cheaper robots.

The core of the breakthrough comes in the use of dielectric elastomers to form the muscles. In theory, such materials can stretch over 10 times their original length without breaking – enabling them to undertake a range of operations while still carrying heavy loads of many times their own weight. This is unlike today’s artificial muscles that are based on hydraulic designs and only capable of lifting loads up to half of their own weight.

“Our materials mimic human muscle, responding quickly to electrical impulses, instead of slowly [like] mechanisms driven by hydraulics,” says lead researcher Adrian Koh, of the National University of Singapore, adding that robots with their artificial muscles would move smoothly, unlike their hydraulic counterparts. “Robots equipped with such muscles will be able to function in a more human-like manner – and outperform humans in strength.”

Energy harvesting

The muscles also have another important potential application, according to the team. While originally designed to convert electrical energy into mechanical energy, they can also work the other way by generating and storing energy harvested from mechanical movements. According to Koh, a 10-kg muscle-based “soft generator” would be capable of generating electrical energy at the same rate as a one-tonne electrical turbine. In terms of storage, the artificial muscle acts much like a capacitor that is able to reach full capacity very quickly, offering rapid charge times.

As the polymer material is comparatively inexpensive, robots made with these artificial muscles should be much cheaper than those using existing alternatives. Indeed, Koh says that an artificial muscle would cost about 5% of the price of a comparable hydraulic system. The muscles also have the potential to use much less energy than hydraulic systems. As a result, Koh believes that muscle-based robots could become popular consumer products, much like tablets and smartphones are today.

The artificial muscles could have many other potential applications beyond just robotics. For example, Koh says that the technology could be used to create a new generation of more efficient cranes.

Fully soft machines

Commenting on the research, Robert Shepherd at Cornell University says “[This] work on electrically addressable artificial muscles is a big step forward to creating fully soft machines that can operate quickly and with enough strength to perform useful tasks.”

Looking to the future, the researchers are continuing to improve their robotic muscles: their next goal is to develop a muscle that can operate repeatedly over a million cycles. In three to five years, the team expects to be able to integrate the muscles into a fully functional robotic arm that is capable of performing such tasks as picking up and accurately repositioning loads. The arm will even be able to take part in an arm-wrestling contest with a human being, Koh says, adding: “We, of course, hope our arm will win!”

Leidenfrost drops race through a maze

In this fantastic video, physics students at the University of Bath in the UK have had some fun with the Leidenfrost effect. This occurs when a liquid drop comes in contact with a hot surface that produces an insulating layer of vapour that keeps the drop from evaporating rapidly. This layer also allows the drop to glide effortlessly over the surface – and that’s where the fun begins.

It turns out that if you replace a smooth surface with the sort of asymmetrical teeth found in a ratchet, the drop will move rapidly in one direction. By using ratchet surfaces to accelerate liquid drops, the team has made the drops move uphill and even follow a predetermined path through a maze.

And if you wonder what would happen if you combined the Leidenfrost effect with the paramagnetic response of a liquid, check out the beautiful image in the article “Levitating drops controlled by fridge magnets”.

Physicists claim further evidence of link between cosmic rays and cloud formation

A Danish group that has reproduced the Earth’s atmosphere in the laboratory has shown how clouds might be seeded by incoming cosmic rays. The team believes that the research provides evidence that fluctuations in the cosmic-ray flux caused by changes in solar activity could play a role in climate change. Other climate researchers, however, remain sceptical of the link between cosmic rays and climate.

The conventional view of climate scientists, as expressed in the 2007 report of the Intergovernmental Panel on Climate Change, is that most of the warming of the Earth’s surface over the last few decades is down to the atmospheric build-up of manmade greenhouse gases such as carbon dioxide. But Henrik Svensmark of the National Space Institute in Denmark believes that an effect related to the Sun’s fluctuating magnetic fields may also play a major role in the warming.

For well over a decade Svensmark has studied how the energetic particles reaching Earth from deep space, known as cosmic rays, can influence the planet’s climate as a result of changes to the Sun’s output. The idea is that cosmic rays seed clouds by ionizing molecules in Earth’s atmosphere that draw in other molecules to create the aerosols around which water vapour can condense to form cloud droplets. The low-lying clouds that result then have the effect of cooling the Earth by reflecting incoming sunshine back out to space. Since the Sun’s magnetic field tends to deflect cosmic rays away from the Earth, the planet will be warmer when solar activity is high and, conversely, cooler when it is low.

Laboratory tests

In 1997 Svensmark backed up this idea with a study showing correlations between the distribution of clouds and cosmic-ray flux around the world, as measured by satellites and neutron counters, respectively. Although subsequent studies by other scientists suggest that no significant correlations exist, Svensmark stands by his claim. But in 2007 he and his colleagues at the National Space Institute provided an alternative line of evidence in support of their theory, carrying out controlled laboratory tests showing how ionizing radiation in the form of gamma rays could stimulate atmospheric molecules to clump together into aerosols.

The researchers then supported these findings with further results published in 2011, which showed that the ionizing effects of electrons from a particle accelerator were identical to those from the gamma rays, so allaying doubts about the suitability of the latter to serve as a surrogate for cosmic rays. Independent confirmation of cosmic rays’ fecundity then came later that year, when the CLOUD collaboration at CERN in Geneva found that it could boost aerosol production at least 10-fold when it sent charged-pion beams through a 27 m3 artificial atmosphere.

Too small to form clouds

Those results were not, however, enough by themselves to prove that cosmic rays can indeed seed clouds. The particle clusters produced measured just a few nanometres across, whereas aerosols typically need to have a diameter of at least 50 nm in order to serve as so-called cloud condensation nuclei (CCN). In fact, there were theoretical grounds for thinking that such seeding would not be possible to any significant degree. Computer simulations carried out by Jeffrey Pierce of Dalhousie University in Canada and colleagues showed that competition for raw material in the atmosphere as well as merging mean that relatively few of the small aerosols brought about by ionization should go on to form CCNs.

The latest experiment was designed to find out whether – the results of this modelling notwithstanding – cosmic rays could in fact generate significant quantities of CCNs. To do so, Svensmark and colleagues measured the number and size of aerosols produced over extended periods – up to 36 hours – inside an 8 m3 stainless-steel chamber filled with varying concentrations of water vapour, ozone and sulphur dioxide. The chamber was exposed to ultraviolet radiation in order to stimulate production of sulphuric acid – one of the main components of atmospheric aerosols – as well as ionizing radiation from two caesium-137 gamma-ray sources, to mimic incoming cosmic rays.

First, the researchers carried out a control experiment in which they continually injected pre-made small aerosols into the chamber but kept the production of sulphuric acid – which is needed to feed the growing aerosols – constant, without ionization. As expected, there was not enough sulphuric acid to go around and hardly any clusters grew to more than 30–40 nm in diameter. But when the researchers let the clusters form of their own accord inside the chamber and switched on the ionization, the result was significant numbers of aerosols measuring at least 50 nm across – large enough to serve as CCNs.

More experiments needed

In their paper reporting the results, the researchers point out that this growth of larger aerosols presumably requires the generation of fresh raw material inside the experiment. They suggest that, thanks to the ionization, “the charged clusters are producing additional sulphuric-acid molecules from reactions involving negative-ion chemistry of ozone, sulphur dioxide and water”. Svensmark, however, describes this proposed mechanism as just a “guess”, and he says that the group hopes to carry out a new round of experiments in order to identify the substance being produced. “This is something that needs to be investigated in more detail,” he adds, “but it is potentially an important piece in the puzzle of how cosmic rays affect cloud formation.”

Others, however, remain to be convinced. Pierce, who carried out the earlier modelling work, describes the result as “an extremely interesting and potentially important finding”, but he questions just how many CCNs could be produced in this way in the real atmosphere. “It is not clear”, he says, “how much of an effect a 10% change in the cosmic-ray flux – which would occur from the solar minimum to the solar maximum – would affect sulphuric-acid production, CCN formation and clouds.”

The researchers have a really long way to go before they can convince anyone that this is fundamental to climate change
Gavin Schmidt, NASA

In fact, according to Gavin Schmidt of the NASA Goddard Institute for Space Studies in New York, proving the real-world applicability of its aerosol research is only one of several hurdles that the Danish group must overcome. Others, he says, relate to cloud properties, degree of radiative forcing and climate trends. “The researchers have a really long way to go before they can convince anyone that this is fundamental to climate change,” he adds.

There is also an experiment at CERN in Switzerland that is investigating the possible links between cosmic rays and clouds. In the video below, CERN’s Jasper Kirby explains the aims of the CLOUD experiment, which mimics conditions in the Earth’s atmosphere.

The research is reported in Physics Letters A.

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