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Home » Page 1686

Labyrinths appear in bead mixtures

Labyrinthine patterns can emerge spontaneously in a number of physical systems including magnetic fluids and reaction-diffusion systems of chemicals. Physicists believe that such patterns arise because the systems are driven from equilibrium, where competing forces cause fingers of one phase of the system to invade another phase.

Emerging labyrinth
Emerging labyrinth

Bjornar Sandnes and colleagues at the University of Oslo created such patterns by combining glass beads (50-100 µm diameter) with water and glycerol and injecting the mixture into the narrow gap between two circular plates. The liquid was slowly pumped from a hole at the centre of the disk. After about three hours, the first fingers of air pushed into the edges of the disk and gradually moved towards the centre. After about three days, the entire disk was covered by a labyrinthine pattern made by dried beads that were pushed aside by the air (see Emerging labyrinth).

The team found that different patterns were produced when the concentration of beads in the initial mixture was changed. When the mixture contained fewer beads, the fingers of air were very large and the regions of dried beads were widely-spaced. When the mixture contained more beads, the air fingers were smaller and the regions of dried beads were spaced closer together.

The researchers then modelled the invasion in terms of two forces – the surface tension at the interface between air and fluid and the friction encountered by the finger as it pushes the beads aside in order to grow. These two forces were calculated at each time step in the simulation and the finger was then advanced in the direction of least resistance. By repeating these steps, the team were able to reproduce the patterns seen in their experiments (see Experiment and simulation).

According the researchers, the tips of narrow fingers encounter less friction than wider fingers because they push aside fewer beads as they grow. However, the tips of narrow fingers have a very high curvature, which means that their growth is more inhibited by surface tension than wider fingers. According to Sandes, the system strikes a balance between these competing forces, which defines the width of the fingers and the overall look of the labyrinths.

While the patterns resemble labyrinths observed other systems, Sandes told physicsworld.com that the simulation is specific to the forces acting on the bead mixture and therefore could not be used to study magnetic fluids or reaction-diffusion systems.

‘Seesaw’ explains light dark-energy particles

The top-contender for dark energy is the cosmological constant, which predicts that quantum fluctuations existing in the vacuum could create a constant energy density throughout space. But another dark-energy contender is quintessence, a scalar field with a negative pressure that, on large scales, would overcome the attractive force of gravity.

Unlike the cosmological constant, quintessence would be able to vary with time, thus explaining why the acceleration has only become significant in the past five billion years or so. Unfortunately, the particle-like mass associated with quintessence’s scalar field would have to be more than 38 orders of magnitude smaller than an electron, and so would easily be wiped-out by the vacuum fluctuations.

Kari Enqvist from the University of Helsinki and colleagues from the University of Aarhus, however, say they can solve this problem by borrowing an idea from particle physics that explains why neutrinos in the Standard Model are observed to have a very small, but not zero, mass. Called the seesaw mechanism, it suggests that the observed left-handed neutrinos – that is, those spinning anticlockwise in the direction they are travelling – can get their small mass if the equations are “balanced” by much heavier, right-handed neutrinos.

But to explain quintessence with a seesaw mechanism, say Enqvist and colleagues, you would not need just two types of particles, but eight different scalar fields. Each of these fields would be associated with a large particle of mass 1 TeV or more, but would interact to produce a single “physical” quintessence particle with a very small mass. Crucially, this particle would be protected from vacuum fluctuations, because they would only act on all eight fields as a whole.

Although eight fields may sound a little far-fetched, they would also have just the right properties to explain the early “inflation” era of the universe when the expansion was exponential. According to Martin Sloth, one of the Aarhus researchers, this would mean future observations of the cosmic microwave background – radiation left from an era after inflation – could shed light on the new theory’s credibility.

“Since explaining dark energy is a very challenging problem…it is certainly worth looking into such ideas,” Josh Frieman, the physicist who originally highlighted the problem of a small quintessence-particle mass, told physicsworld.com. Speaking about the possibility of eight dimensions, he added: “That could be the right explanation, but one might still hope that in the end nature is more parsimonious.”

Big science and China hold future promise

This special Physics World supplement looks at four such “big science” projects that have extraordinary vacuum requirements. CERN’s Large Hadron Collider (p17) will be the world’s largest vacuum system, while the Laser Interferometer Space Antenna will require a vacuum system that can survive the traumas of space flight (p14). The vacuum pumps on the International Thermonuclear Experimental Reactor must remove large quantities of radioactive tritium (p7), while contamination with a few specks of dust could disrupt the superconducting cavities of the European X-ray Free-electron Laser (p11). While these projects all have their unique problems, they pose a common set of challenges to the vacuum scientist, as outlined by Joe Herbert on p5.

The past decade has seen extraordinary growth in the use of vacuum technologies in China. On p19, Jiangou Hou, president of the Chinese Vacuum Society, explains how the organization is planning for continued growth and gives his predictions for the future. China’s burgeoning hi-tech sector is also becoming a major market for vacuum equipment, and this is attracting an increasing number of foreign vacuum-equipment suppliers to China (p22).

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Thoughts of an astronomy master

You graduated in maths from Cambridge University. What inspired you to go into astrophysics?

It was Fred Hoyle’s book Frontiers of Astronomy, which I bought while I was still an undergraduate. He presented this wonderful series of unsolved problems and speculated on a solution to each. It seemed a very lively field. So after graduating I did a one-year Masters course in maths and made sure I took all the astrophysics options.

Did the discovery of the cosmic microwave background in 1965 influence you too?

Absolutely. The year before, I had just started my PhD in cosmology with Bill McCrea, who was very interested in the steady-state theory – the idea that the universe is unchanging and that new matter is created continuously to compensate for the expansion of the universe. The discovery of the cosmic microwave background (CMB) was a bit of a blow to the steady-state idea because it seemed to be evidence of a hot Big Bang. But the results didn’t immediately rule out the steady-state theory. At first the measurements of the CMB were only at one or two wavelengths, so it wasn’t clear whether the results really showed true black-body radiation, which was integral to the Big Bang theory. So some of my early work investigated whether the steady-state theory really could be ruled out.

NASA’s WMAP satellite has allowed us study the CMB in great detail. What do you hope the Planck Surveyor will reveal about the CMB when it launches next year?

The Planck Surveyor will map the CMB to even higher resolution than the WMAP satellite, so its results should allow us to measure cosmological parameters like the density of the universe or the Hubble constant to an accuracy of 1%. At this level we should really feel confident that we know what kind of universe we live in. It is also possible that it could detect the imprint of primordial gravitational waves from the very early universe. We have no idea what the levels of these waves would be – they’d have to be pretty strong to be detected.

The bit I’m working on, however, is the all-sky survey of point sources at sub-millimetre wavelengths. This will help us to understand how galaxies formed in the first place, because we will be able to observe some of the most violent star-forming galaxies at very early times.

Some astronomers have claimed to see a pattern in the CMB that they call the “axis of evil”, suggesting the hot and cold patches in the CMB are not randomly distributed. Do you think data from the Planck Surveyor will rule out these claims?

It will certainly help clarify the situation. The “axis of evil” is an effect over the whole sky, so it might suggest large-scale anisotropies. This could mean that the universe is not infinite but cellular – a finite region that is endlessly repeated. But it is more likely that the pattern is a systematic error in our measurements.

While there may be primitive life on extrasolar planets, I do not think there is a profusion of extraterrestrial intelligence

What do you think is currently the biggest question in astrophysics?

Obviously there is a big issue with dark energy, but I’m not sure whether we’re going to succeed in solving that problem. I think it might just be something terribly basic that was laid down in the Big Bang that makes gravity pull locally and push on larger scales – basically Einstein’s theory of gravity with a cosmological constant.

In the next 10 to 20 years, however, the main thrust of astrophysics may shift away from cosmology towards planetary systems such as exoplanets.

Do you think that the search for exoplanets will reveal extraterrestrial life?

Personally I think that while there may be primitive life on extrasolar planets, I do not think there is a profusion of extraterrestrial intelligence. Given that there has been such an immense time period during which other civilizations could have existed, the fact that “they” don’t seem to be signalling to us and that they’re not visiting us suggests to me that for some reason they are not common.

There are two explanations for this. One is that it is very difficult to make that first step towards any kind of intelligent organism that would be capable of communicating with us. In the case of the Earth, we don’t know how we got from organic molecules to self-replicating plants and animals – it may have been the most amazing fluke that will never be repeated.

The other explanation is that as soon life gets intelligent enough it will soon destroy itself with nuclear weapons. But I don’t think that this idea works. Not all life would be as war-like as us – if the same experiment is repeated many times, it’s bound to last longer on some occasions than others.

You recently criticized NASA boss Michael Griffin for his dismissive comments on the need to tackle climate change. Do you think engaging climate change should be scientists’ number-one priority?

I do. Michael Griffin’s comments were unacceptable. He’s allowed to have his own reservations, but it was wrong of him to voice them to the media since he represents not only US space scientists but also those international partners like ourselves who collaborate with NASA. Space scientists have played an important part in identifying the problem and in convincing politicians that they need to act now.

Big science needs vacuum innovation

The next generation of big-science experiments, such as CERN’s Large Hadron Collider (LHC) and the International Thermonuclear Experimental Reactor (ITER), will rely on vacuum systems that push the limits of today’s technology in terms of volume and pumping capacity. Others, such as the European X-ray Free Electron Laser (XFEL), will require extremely clean vacuum conditions, while the Laser Interferometer Space Antenna (LISA) will involve launching vacuum systems into space.

The key challenge for a vacuum scientist working on such experiments is how to provide a suitable vacuum environment at reasonable cost. How this is achieved can differ from project to project. This supplement looks at the specific challenges associated with each of these four projects.

While these experiments are all very different, they pose a similar set of challenges to the vacuum scientist. These involve understanding the vacuum requirements of the experiment and how its mechanical design will affect the vacuum system; dealing with the various uncertainties in the operating parameters of vacuum equipment; manufacturing and processing the vacuum components; and finding enough suitably trained people to build and operate the system.

Many early challenges arise because the designers of the experiment – the physicists who will ultimately use the facility as a research tool – may not know what is required of its vacuum systems. Researchers tend to demand the best possible vacuum, even though it may not be necessary. Overspecification leads to unnecessary expenditure, which can be avoided if the vacuum scientist has a good initial understanding of how the experiment will be influenced by the vacuum conditions. Even if the vacuum requirements are well defined, it can be very difficult to calculate to any degree of accuracy the pressure that can be expected at the important positions in a large vacuum system. Many uncertainties must be taken into account and even a small design change can make a large difference to the vacuum conditions. As a result a design will often go through several iterations before a satisfactory vacuum is achieved.

The vacuum scientist must also understand the demands and limitations that will be imposed on the vacuum system by the physical design of the experiment. For example, exotic materials may have been chosen for their mechanical, thermal or electrical properties, even though the vacuum characteristics of the materials are not well known. Measuring the thermal outgassing properties of such materials and finding processing and cleaning recipes to reduce outgassing to a satisfactory level can take a great deal of time. Outgassing is a significant challenge for those building the ITER, for example. The vacuum system for this project must handle gas loads from many thousands of components that are manufactured from specialized materials that have to cope with the harsh envir­onment inside a fusion device.

Particle accelerators, such as the LHC and the XFEL, are usually housed in tunnels where there is little space for vacuum pumps. In addition, the huge lengths of many modern accelerators (nearly 27 km in the case of the LHC) can put severe restrictions on pumping speeds. In the LHC this problem is alleviated by non-evaporable getter (NEG) coatings, which turn the walls of the vacuum chambers into pumps.

Another challenge is the vacuum equipment itself, the performance of which is often not very well defined. For example, the pumping speeds of UHV pumps may only be known for a few gas species. To complicate matters further, similar pumps may achieve different speeds, depending on how an individual pump has been used in the past. Vacuum gauges can also have a major effect on the vacuum – especially at very low pressures – and their stability often leaves something to be desired. It may be that the special requirements of a large facility cannot be met by commercial equipment and new systems must be developed. This usually involves extending existing technologies rather than having to develop something from scratch.

Once the design has been specified, the components must be manufactured and prepared, often using materials and methods at the boundaries, or even beyond the capabilities, of the vacuum industry. Quality assurance is a major challenge, and the project often has to provide the necessary protocols, personnel and equipment. Needless to say, big projects require a lot of detailed design, manufacturing and processing capacity, plus careful record keeping. The final challenge is that of finding enough suitably trained and experienced people who really understand vacuum. It’s not a black art, but it’s not an exact science either.

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Suppliers can reap big benefits from China’s hi-tech boom

The boom in China’s hi-tech manufactur­ing has made the country a major market for vacuum technology. While challenges abound for foreign suppliers, the rewards can be great for those that can master the Chinese way of doing business.

China has the fourth largest economy in the world (in terms of gross domestic product) and has enjoyed an annual growth of about 9% throughout the past decade – more than double the global average. The country is home to a booming hi-tech sector, including a massive semiconductor industry, which is a big end user of vacuum technology.

The main opportunities for foreign suppliers are in the high-end vacuum market, which serves China’s semiconductor, metallurgical, solar-cell, medical, optical and other hi-tech industries. This high-end equipment is still largely made by US, European and Japanese firms, according to Kuno Herrmann, sales and marketing manager for Pfeiffer Vacuum. He estimates that this market is worth some €80 m per annum.

Although foreign companies have been selling in China for decades, the vacuum market took off about five or six years ago, says Frédéric della Faille, vice-president of sales at Alcatel Vacuum Technology. As a result, most leading vacuum suppliers have responded by opening sales offices and service centres there.

“Having a ‘presence’ is very, very important,” said Ting Zhang, founder and CEO of China Business Solutions Ltd, a UK-based business consultancy. A Chinese presence allows a firm to build trust by being closer to potential customers, and a local office can deliver more and better training and support to customers. “Before, there was very little need for this kind of upkeep,” said della Faille, because many vacuum users in China simply threw out their pumps after a few hundred hours and just bought new equipment.

Indeed, convincing buyers to invest in higher-quality vacuum equipment that will last has been a key challenge. Many Chinese customers look for the lowest bid, even though the higher initial costs of a well designed system are usually offset in the long run by a longer lifetime and significantly more efficient operation. “We have made this argument successfully in other parts of the world,” said Mark Fitch of Vergason Technology, but “it often falls on deaf ears in China”.

Understanding and adhering to industrial regulations in China can also be a major challenge for foreign vacuum suppliers. Local interpretations of the law can vary significantly throughout the country, so a firm must develop good working relationships with people in administrative positions. “The most popular buzz word for foreign business people in China is ‘guanxi’, which means ‘connections’,” says Zhang. Oerlikon Leybold Vacuum got guanxi by retaining a business consulting firm that had been working in China for some time. “You need to know someone there who understands the regulations and knows how to haggle,” explained Christina Steigler of Oerlikon Leybold.

Another way of dealing with the intricacies of the Chinese system is to hire locally. Although there is a large pool of skilled workers in China, Steigler’s experience is that few of them are able to speak English. Also, once a qualified Chinese employee has been identified, retaining their services can be difficult because employee turnover rates are often as high as 30%, according to Zhang.

After establishing a presence in China, the next step for a company is to start a manufacturing operation there. While this could be done through joint ventures with established Chinese businesses, many firms have legitimate concerns that their intellectual property will not be respected.

To minimize the risk of patent infringement, Oerlikon Leybold has been operating its own manufacturing site in China since 1997. According to Steigler, the facility has lower production costs than plants in the West and can avoid certain import fees. In addition, she believes that the “made in China” mark is a selling point with local companies, although it is less important for multinational customers.

Making it in China is not easy, and some vacuum companies have already failed. The lack of a suitable infrastructure for supporting foreign manufacturers is still a problem, according to Herrman. However, the overall situation for manufacturers is improving in China, says Zhang, with many regulations being updated, a growing awareness about intellectual property rights and a greater number of young people learning English.

Despite the problems, Herrmann believes that the high-end vacuum will enjoy an annual growth rate of at least 10% in the foreseeable future. He is confident that more foreign companies will start production in China. “It’s only a question of when,” he said.

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Perfect lens could reverse Casimir force

The mysterious attraction between two neutral, conducting surfaces in a vacuum was first described in 1948 by Henrik Casimir and cannot be explained by classical physics. Instead it is a purely quantum effect involving the zero-point oscillations of the electromagnetic field surrounding the surfaces. These fluctuations exert a “radiation pressure” on the surfaces and the overall force is weaker in the gap between the surfaces than elsewhere, drawing the surfaces together. Tiny though it is, the Casimir effect becomes significant at distances of micrometres or less and actually causes parts in nano- and micro-electromechanical systems (NEMS and MEMS) to stick together.

Now, Leonhardt and Philbin have calculated that the Casimir force between two conducting plates can turn from being attractive to repulsive if a “perfect” lens is sandwiched between them. A perfect lens can focus an image with a resolution that is not restricted by the wavelength of light. Such a lens could be made from a metamaterial made of artificial structures that are engineered to have negative index of refraction — which means that the metamaterial bends light in the opposite direction to an ordinary material.

According to the researchers, the negative-index metamaterial is able to modify the zero-point oscillations in the gap between the surfaces, reversing the direction of the Casimir force. Indeed, the researchers believe that this repulsive force is strong enough to levitate an aluminium mirror that is 500nm thick, causing it to hover above a perfect lens placed over a conducting plate.

Since the Casimir force acts on the length scale of nanomachines, manipulating it could be important for future applications of nanotechnology. “In the nano-world, the Casimir force is the ultimate cause of friction,” Leonhardt told physicsworld.com. “Our result means we could now envision frictionless machines or novel micromotors.”

While physicists have had some success creating perfect lenses from negative-index metamaterials, the technology is still in its infancy. “The work points towards new applications of left-handed materials that are not strictly optical,” says Federico Capasso of Harvard University, who studies the effect of the Casimir force on MEMS. “However, the materials are not easy to make so the concept may take a few years to realise.”

Microscope unravels the intricacies of protein folding

Proteins – the building blocks of life – consist of a long chain of molecules called amino acids folded into a 3D shape. An atomic force microscope (AFM) can be used to study this folding by attaching one end of a protein to a substrate and the other end to the AFM’s cantilever. As the protein is stretched, the cantilever is oscillated and the force restoring the protein and cantilever back into equilibrium is measured.

In theory, monitoring this non-equilibrium force should provide information about the many intermediate equilibrium energy states that the protein goes through on its way to being fully extended. In practice, however, interpreting the data has proved controversial, and until now researchers have only had a clear understanding of the equilibrium states at the beginning and end of the folding process.

Now Ching-Hwa Kiang and colleagues from Rice University have improved the AFM technique to determine the intermediate states. To do this, they built a computer program based on an equation formulated by University of Maryland physicist Chris Jarzynski a decade ago.

Although scientists had believed “Jarzynski’s equality” could be used to obtain equilibrium information from non-equilibrium measurements, none had been able to apply it successfully. “Through numerous discussions with Jarzynski, we had a thorough understanding of where and how the theory applies,” Kiang explained.

The Rice group proved their technique works using section of “titin”, the largest known protein and the one that constitutes elastic muscle in the heart, and managed to map eight individual energy states as they used an AFM to unfold it. This, they say, paves the way for investigating how environmental changes such as temperature affect protein folding.

Atoms swap spins

Optical lattices use criss-crossing laser beams to create a matrix of potential wells that can each trap one or more atoms. They are one of several experimental systems that could be used to create practical quantum computers, which exploit the ability of quantum systems to exist in two states at the same time. Rather than use bits, which are either 1 or 0, quantum computers use qubits, which can be in a superposition of both 1 and 0 simultaneously. The idea is that if a quantum computer has N such qubits, these can then be combined or “entangled” to represent 2N values at the same time. By processing each of these values simultaneously, a quantum computer could, in principle, operate exponentially faster than its classical counterpart.

A SWAP gate exchanges the state of two qubits — the spin state of two atoms in an optical lattice, for example. If one atom starts in spin state 1 and the other in 0 (1-0), they end up in 0 and 1 respectively (0-1). What is more interesting to those trying to build quantum computers is the half-SWAP gate, whereby the process is stopped halfway when the state of each individual atom is simultaneously 1 and 0 — and the atoms are entangled. Then, the two atoms could, in principle be physically separated while still entangled.

Now Trey Porto and colleagues at NIST and the University of Maryland in the US, have created at SWAP gate in an optical lattice. They began with two overlapping lattices that were offset slightly in space from one another. In both lattices, each well was occupied by one atom and a radio signal was used to set the spins of all the atoms in one lattice to 1 and all the atoms in the other lattice to 0.

The researchers then carefully adjusted the laser beams to merge the two lattices into one lattice in which two atoms occupy one well. When the two atoms are in the same well, quantum mechanics dictates that the overall quantum state of the two atoms must have a specific symmetry and this restriction causes the system to oscillate between two spin states: 0-1 and 1-0.

By switching off the trapping lasers and applying a magnetic field gradient to the ensemble, the researchers were able to measure the spin state of the atoms at different points in time and confirm that they were oscillating between the two spin states with a period of about 0.4 ms

The team also performed the experiment with both atoms in the same initial states (0-0 and 1-1) and saw no oscillations. Porto told physicsworld.com that this was particularly challenging to achieve because these states are more likely to be destroyed by noise than 0-1 and 1-0.

This is not the first time that a SWAP gate has been demonstrated – in 2005 physicists at Harvard University swapped spin states between electrons confined to two quantum dots. However, the Harvard experiment could only swap 0-1 and 1-0 states, and not 0-0 and 1-1. Although the latter two swaps seem trivial, any practical gate must be able to handle these states.

Porto accepts that the NIST team have also fallen short of performing a complete half swap, because they did not separate the atoms in the entangled state, something that the team are now working on.

Another key challenge facing Porto and others who are trying to build quantum computers based on optical lattices is how to read and write information from individual atoms and manipulate individual wells. This was not done in this experiment — instead the measurements were made on the entire ensemble of atoms and all the wells were controlled in unison.

Graphene oxide weaved into ‘paper’

First isolated in 2004, graphene is a one-atom-thick sheet of graphite that, aside from having unique electronic properties, is very strong. But as of yet there is no way of producing it in large quantities, which has limited its potential as a building block for new types of specialist materials.

Now, however, a group from Northwestern University in the US including Rodney Ruoff have discovered that large quantities of oxidized graphene can be weaved together to create a new type of “paper” that is stiffer and stronger than other thin materials.

“My dream has been to disassemble graphite into individual sheets, and then reassemble those sheets in different ways,” Ruoff told physicsworld.com. To do this his group begins by oxidizing graphite to make graphite oxide, which leaves roughly half the carbon atoms with an attached oxygen atom. When graphite oxide is mixed into water, these oxygen atoms repel water molecules, forcing the individual layers – graphene oxide – to disperse or “exfoliate”. The researchers filter this exfoliated mixture through a membrane, which collects the layers in such an arrangement to produce graphene oxide paper.

Normal graphite has a delicate structure, needing only a small lateral force to break apart its regularly-stacked layers. Conversely, the layers in graphene oxide paper interweave with one another and wrinkle on larger scales. This allows load to be distributed across the structure, making it stronger than graphite foil and “bucky paper”, which is made from carbon nanotubes. In fact, Ruoff claims, the only material stronger could be diamond.

Graphene oxide paper
Many layers

The interwoven structure also lets individual layers shift over each other, so that the collective layers become pliable. But most importantly the paper can be chemically tuned by altering the amount of oxygen on the layers. Reducing the oxygen content, for example, would take it from being an electrical insulator to a good conductor. Moreover, the paper could be infused with polymers, ceramics or metals, to make composite materials that outperform their pure counterparts.

This wide array of properties could mean applications as diverse as membranes with controlled permeability to supercapacitors for energy storage.

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