Page 1677 – Physics World

Home » Page 1677

Capacitance sheds light on complex flows

Flowing mixtures of liquids and gases play an important role in the transport and processing of materials as varied as carbonated drinks, pharmaceuticals and petroleum. The complex flow patterns that can occur in such mixtures must be monitored to ensure that, for example, a drug is mixed properly or that oil can flow up through an undersea pipeline.

However, current techniques for measuring such “multiphase” flows have limited efficacy. While a flow can be monitored using high-speed cameras, they provide little quantitative information about the mixture. Probes placed directly in the flow can provide more useful information, but this is limited to the area near to the probe, which is of little use when studying complex multiphase flows. More sophisticated techniques such as gamma-ray tomography can do better, but the added expense and radiation hazards are often not justified by the modest improvement in results.

Now, Uwe Hampel and colleagues at the Forschungszentrum Dresden-Rossendorf (FZD) in Dresden, Germany, have come up with a sensor that promises to change all this. The sensor consists of two planes of parallel wire electrodes stretched across a pipe flange. The planes are perpendicular to each other and separated slightly to form a mesh.

An oscillating voltage is applied to wires in one plane and the resulting electrical fields are detected at the electrodes in the second plane. This allows the sensor to measure the capacitance of materials in the regions where two wires cross.

Different materials have different capacitances, which allow the sensor to differentiate between different phases in the flow. According to Hampel, the measurement can be done very rapidly so that the flow in a typical pipe cross-section can be scanned up to 10,000 times per second at a spatial resolution of 0.5 mm.

“This catches even small bubbles moving at several metres per second through the sensor, giving highly accurate information on the flow structure,” said Hampel. The sensor is similar to an earlier prototype built by the team that measured the electrical conductivity between the wires and therefore could only be used with conducting liquids.

According to multiphase flow expert Barry Azzopardi at the UK’s University of Nottingham, the new sensor shows great promise because it can be used with organic fluids such as petroleum, which are non-conducting.

The sensor will probably never be deployed in a real oil pipeline because it would be destroyed by the solid “pigs” that are sent through the line to clean and inspect the pipe. Instead the sensors could be used on mock pipelines in the lab gain a better understanding of multiphase flow.

The team will be taking the sensor to Nottingham in November, where they will working with Azzopardi to study the flow of gas and oil mixtures in pipelines.

The team now plans to study various complex flows, such as oil-gas transport through pipelines in order to identify a range of applications for the device. “Currently, the sensor exists as a prototype but we hope to think further about commercialization once a broader range of applications have been identified”, said Hampel.

Work begins on Chinese-US neutrino experiment

Over the last decade or so, physicists have found convincing evidence that neutrinos can transform – or oscillate – from one “flavour” (electron, muon or tau) into another as they travel through space, a phenomenon that means neutrinos have mass. The Daya Bay Reactor Neutrino Experiment is designed to measure Θ₁₃, the only one of nine “mixing angles” that has not been accurately determined.

These mixing angles are needed together with other parameters to fully characterize neutrino oscillations, and better constraints on Θ₁₃ will improve our understanding of why the universe appears to contain matter but no antimatter.

Located about 55km north-east of Hong Kong, the experiment will consist of a set of eight detectors, each filled with 20 tonnes of gadolinium-doped liquid scintillator and placed in tunnels at 360 m, 500 m and 1800 m from the centre of the reactor complex. The idea is that each day the detectors will capture a few thousand electron antineutrinos emitted by the reactions taking place in the power station, in order to measure how the neutrino flux from the reactors varies as a function of distance.

The project – set to start collecting data in 2011 – was approved in 2006 by the Chinese Academy of Sciences, which is providing 50 million yuan (about $6m). Other major donors include China’s Ministry of Science and Technology and the US Department of Energy and National Science Foundation. Additional funding for the project is coming from Hong Kong, Taiwan, the Czech Republic and Russia.

An official groundbreaking ceremony took place on 13 October attended by government officials from China and the US Department of Energy. The first experimental hall is expected to be ready by the end of 2008 and the last detector should be installed by 2010.

‘The project is now progressing well [and we] are now ready for manufacturing and mass production of the equipment”, says Yifang Wang of China’s Institute of High Energy Physics.

Friends, not policies, win elections

In countries that employ proportional-representation elections with open lists, parties running for government give voters in each district a choice of candidates. On election day, citizens vote for both their preferred party and their preferred candidate from that party. Seats in each district are shared out to parties based on how well they did, and the parties then fill them with their most favoured candidates. The party with the most votes overall wins.

Santo Fortunato from the Institute for Scientific Interchange Foundation and Claudio Castellano from the University of Rome in Italy looked at election data sets from 1958, 1972 and 1987 in Italy, 2003 in Finland and 2005 in Poland, all of which used this election system. They began by plotting the distribution of the number of votes received by candidates, or v. They then assumed that, in addition to v, the distribution would be influenced by the number of candidates in each party and the total number of votes collected by each party. In actual fact, they found that the histogram was dependent on the ratio of the two quantities — in other words, the average number of votes received by a party’s candidates, or v₀.

This in itself is not surprising because it simply means that, for example, candidates from a more popular party with a higher v₀ would generally get more votes. After a closer inspection, however, they realized that the distribution was not dependant on both v and v₀ separately, but only on v/v₀ — a measure of the performance of a candidate compared with the other candidates in his or her party.

Remarkably, when they plotted the histogram of this single parameter for all five election data sets, they found that they all fell almost precisely onto the same curve. This suggests that it doesn’t matter, for example, how different polices are between parties, the distribution of votes will remain unchanged. “The elections considered span a period of 30 years, in which deep cultural, economic and social transformations have occurred,” say the researchers in their paper. “There is no hint of that in the data pattern.”

Talk the talk

In attempt to explain this universal distribution, Fortunato and Castellano supposed that a candidate gains popularity, and hence votes, by a word-of-mouth effect. They considered each candidate at the top of a tree-like structure, persuading a small number of close friends of to vote for them with a certain probability, who in turn would promote the candidate to more people, and so on. The number of contacts that each person would have to spread the word would have a distribution of its own, which the researchers assumed would follow a “power law” of a generic parameter, α.

Fortunato and Castellano simulated this structure for different values of the persuading probability, the minimum number of contacts a person could have and α. They discovered that their simulation could exactly replicate the curve underlying the election data sets. This, they claim, proves that different candidates in proportional elections gain votes by word of mouth.

The researchers are now looking to see if the word-of-mouth model fits other data, such as local elections for mayor. “Our preliminary results show that municipal elections in Rome, Sao Paolo and Rio de Janeiro give results fitting our curve,” Fortunato told physicsworld.com.

Laser reveals new type of magnetic ordering

The effect, which is called ferrotoroidicity, was spotted in the oxide material LiCoPO₄ by physicists Bas Van Aken and Manfred Fiebig from the Max-Born Institute in Berlin together with chemists from the University of Geneva.

The team discovered that the material contains regions (or domains) where all of these vortices were either clockwise or anticlockwise. Each vortex is not much bigger than the spacing of atoms in the crystal lattice and their extremely small size distinguishes them from the much larger (and unrelated) vortices that have been seen in disks of magnetic material.

The domains were detected using a technique called second harmonic generation in which a laser beam shone onto the sample is doubled in frequency as it passes through. When the beam hits a ferrotoroidic domain oriented in the opposite direction to an adjacent domain, part of the beam is phase shifted by 180°. Studying the light leaving the sample, the researchers found regions of constructive and destructive interference, indicating that such oppositely-aligned domains, and hence ferrotoroidicity, were present.

In theory, the direction of the vortices can be flipped using a combination of electric and magnetic fields. The team therefore believe that ferrotoroidics could be used in data storage devices with clockwise and anticlockwise domains representing the 0s and 1s of data bits. Because both electric and magnetic fields are needed to flip a vortex, such bits would be less prone to flipping by stray magnetic fields than conventional ferromagnetic bits.

Ferrotoroidicity is the latest “ferroic” ordering effect to be discovered. The other ferroics are ferromagnetism, which is the spontaneous ordering of magnetic moments in one direction; ferroelectricity, which is a spontaneous electric polarization of a material; and ferroelasticity, which is a spontaneous strain on a material.

Stern-Gerlach effect goes chiral

Chiral molecules exist in one of two structures that are mirror images of one another, usually described as “right-handed” and “left-handed” enantiomers. Chirality can be very important in medicine because a right-handed drug molecule may have the desired biological effect while the left handed molecule does not, for example. As a result, a great deal of effort has gone into developing ways to sort molecules according to their chirality.

An important feature of chiral molecules is that they can respond differently when exposed to light. Now Christoph Bruder and Yong Li at the University of Basel along with Chang-Pu Sun of the Chinese Academy of Sciences have worked out how to separate enantiomers using the “Rabi transitions” that can happen when a molecule is exposed to laser light. Such transitions occur when a laser beam causes a molecule to oscillate between two energy levels at a frequency corresponding to that of the laser light.

Their technique involves a chiral molecule with three possible Rabi transitions at three different laser frequencies. Two of the transitions would have to be identical for left- and right-handed molecules, but the third would have a phase difference of 180 degrees between the oscillations that occur in the two enantiomers.

According to the team’s calculations, if these molecules were all oriented in a similar way and then sent directly into the three laser beams, the phase difference would cause right-handed molecules to be deflected off course more than left-handed molecules. Moreover, the direction of deflection would depend on which of the two spin states — up or down — the molecules are in. The upshot is that the molecules would be sorted into four groups according to chirality and spin: right-up, right-down, left-up and left-down.

While some molecules could have appropriate three-level systems, Bruder told physicsworld.com that the experiment would be challenging for several reasons. For one thing, it would require three lasers operating at three very specific frequencies. Also, the molecules would have to be very cold for the effect to be noticeable and the molecules would have to be oriented beforehand in a specific way.

Although these difficulties mean that the technique will probably not lead to a practical way of separating real molecules, Bruder is hopeful that experimental physicists will work out a way to confirm the team’s calculations.

CERN boss quashes LHC delay rumours

The Large Hadron Collider (LHC) is a 27-km ring around which beams of protons are accelerated using hundreds of superconducting magnets, grouped into eight sectors and cooled with liquid helium. To prevent the intensity of the beam dropping, the beam has to induce a “mirror” current with little resistance in the walls, an ability that requires electrical continuity throughout. But because the sectors shrink by about 10 metres in total when cooled down to their 1.9 K operating temperature, the connections between components in the sectors must be provided by collections of sliding copper fingers or “plug in modules” (PIMs).

In the first week of August, however, PIMs in “sector 7-8” of the LHC did not expand properly when the sector was warmed up from the operating temperature — a procedure occasionally necessary in the long-term running of the accelerator. This caused the PIMs to buckle into the space reserved for the beam.

To see the extent of the problem, CERN technicians quickly devised tiny radio transmitters housed in shells that could be sent down the vacuum pipes containing the PIMs. If these transmitters, which were slightly smaller than ping-pong balls, encountered an obstruction, then they would fail to pass a signal to one of the beam position monitors located every 50 m in the pipes.

The rumours — which spread largely on internet blogs — started after LHC project leader Lyn Evans gave a colloquium on 13 September to CERN staff in which he reviewed the PIM problem and proposed the technicians’ solution to it. Various blogs claimed that the LHC could be substantially delayed and that the first data runs could be pushed into 2009.

But according to Aymar’s statement, which appeared on Monday, the problem is just one of many to be expected in the run up to launch. “So far there have been no show stoppers,” Aymar says. “We can all look forward to the LHC producing its first physics in 2008.”

Speaking to physicsworld.com, Evans says that the obstruction detection technique showed that only six out of 450 PIMs in sector 7-8 were damaged, and they are currently being fixed. “If all the PIMs were affected it would have been a serious problem,” he said. “Now that we know it’s only a small number, we’re all much more relaxed.”

Evans also said that another rumour, which suggested the LHC was having problems sourcing enough helium-4 for cooling, was completely unfounded. He explained that the LHC has two contracted parties to supply the helium, but that it also has another two in reserve in case there are any problems.

Nobel prize recognizes GMR pioneers

Giant magnetoresistance, or GMR, is the sudden change in electrical resistance that occurs when a material consisting of alternating ferromagnetic and non-magnetic metal layers is exposed to a sufficiently high magnetic field. In particular, the resistance becomes much lower if the magnetization in neighbouring layers is parallel and much higher if it is antiparallel. This change in resistance is due to “spin up” and “spin down” electrons scattering differently in the individual layers.

GMR has since been used to develop extremely small and sensitive read heads for magnetic hard-disk drives. These have allowed an individual data bit to be stored in a much smaller area on a disk, boosting the storage capacity greatly. The first commercial read heads based on GMR were launched by IBM in 1997 and GMR is now a standard technology found in nearly all computers worldwide and is also used in some digital cameras and MP3 players.

Albert Fert

In Grünberg’s original work, he and his team studied an iron/chromium/iron trilayer system that showed an decrease in resistance of 1.5%. Fert and colleagues, in contrast, studied an iron/chromium multilayer system in which the electrical resistance decreased by nearly 50%.

“These films started out as being very esoteric, but it turned out that they would have great practical importance,” says Tony Bland, a physicist from the University of Cambridge. “They paved the way for substantial information densities of commercial disk drives. It also paved the way for new physics, such as tunneling magnetoresistance (TMR), spintronics and new sensor technology, for example biosensors. The caveat is that GMR has already become old technology and people are now interested in TMR for future technology.”

TMR gives rise to a more pronounced resistance change in small applied fields than is found in GMR devices.

Albert Fert was born in 1938 in Carcassone, France, and received a PhD in physics in from Université Paris-Sud, Orsay in France. He is now also scientific director of CNRS/Thales Unité Mixte de Physique in Orsay. Peter Grünberg was born in 1939 in Pilsen (now in Czech Republic) and is a German citizen. He gained his PhD in physics from the Technische Universität Darmstadt, Germany.

Grünberg, who holds a patent on GMR, originally submitted his paper slightly before Fert, although Fert’s was published first. “But whereas Fert was able to describe all the underlying physics, Grünberg immediately saw the technological importance,” adds Bland.

Particle-physics detector warms to forest fires

Satellites can normally see fires covering areas of 30 acres or more, and smaller fires can often be spotted using smoke scanners, which look for light scattered by smoke particles in infrared beams. But on windy days when smoke is dispersed, or when fires are in their nascent stage, the only reliable methods are detectors that search for flames directly. To avoid fires being confused with sunlight these detectors must be sensitive to UV light with wavelengths shorter than 185 nm, which are absorbed in the ozone layer but are emitted by all flames.

Peskov and Zichichi’s device has its origins at CERN’s lepton asymmetry analyzer (LAA) project, which between 1988 and 1992 developed detectors for future high-energy hadron experiments such as ALICE at the Large Hadron Collider, due to start up next May. About the same length as a fountain pen and three times as wide, their device is a tube filled with the photosensitive vapour TMAE (trimethylaminoethyl) with a cathode at one end and a high-voltage anode wire running through the centre. When a UV photon coming from a flame strikes the cathode, it produces an electron that is subsequently accelerated towards the anode. On its way, the electron knocks other electrons off atoms in the vapour, and these electrons in turn ionize more atoms, ultimately causing an electron “avalanche”. This avalanche creates a pulse of current in the anode to be recorded by external electronics as a fire signal.

The highest European Union standard of UV detector is “class 1”, which requires a detector to be able to identify a 30 cm³ flame at a distance of 25 m. Peskov and Zichichi claim that their detector can detect the flame from a cigarette lighter at 30 m, and — based on the number of pulses every 10 seconds — is a thousand times more sensitive than commercially available UV fire detectors. In addition, its response time is just a few microseconds, making it suitable to detect, for example, sparks on oil-drilling platforms or lightning. The researchers say that the detectors would cost no more than €100 each, similar to the price of other commercial detectors, and could feasibly be mass produced and employed in tower networks to monitor forested regions.

Peskov and Zichichi have already demonstrated their device to fire-detector manufacturers Finsecur in Houilles, France and Maier in Milano, Italy. Christophe Bonazzi, the director of Finsecur, told physicsworld.com that he is not prepared to fund the development of the project at this time, but would be interested in implementing Peskov and Zichichi’s detector when they have refined it further.

Wrinkle researchers bag physics Ig Nobel

Mahadevan and Cerda Villablanca were honoured for a series of papers published in several prestigious journals. In their first paper (Nature 419 579), mathematician Mahadevan and physicist Cerda Villablanca considered the wrinkling effects of a stretched elastic sheet and were able to derive a set of scaling laws for the wrinkle wavelength and amplitude.

The pair then focussed on the wrinkling of thin elastic sheets over a range of length scales and geometries (Phys. Rev. Lett. 90 074302) and deduced a general theory of wrinkling occurring over a range of length scales. They also produced a set of simple scaling laws that could be the basis for characterizing the mechanical properties of thin solid films.

In a third paper (PNAS 101 1806) the pair considered what happens when a flat isotropic elastic sheet falls onto a 3D object. The scaling laws they derived were consistent with commonplace observations of drapes, and they argued that their results could lead to “qualitative guides to fashion design and virtual reality animation”.

The physics Ig Nobel was presented by two genuine Nobel laureates: Roy Glauber from Harvard University, and Robert Laughlin from Stanford University.

The Ig Nobel prizes were founded in 1991 and the 2007 awards were presented yesterday at a special ceremony at Harvard University. The theme of this year’s ceremony was “chicken”, which bizarrely involved keynote speaker Doug Zonker repeating the word “chicken” for two minutes, accompanied by technical diagrams.

The Nobel Prize in Physics will be announced next week.

Artificial-atom laser debuts

Oleg Astafiev and colleagues at NEC’s Nano Electronics Research Laboratories and Japan’s RIKEN national lab have built their maser by placing a nanometre-sized “island” of superconducting aluminium at one end of a microwave resonator that is several millimetres long. The electrons in the superconductor exist in “Cooper pairs”. However, if a voltage is applied to the island, a pair can be broken with one electron leaving the island immediately via a conducting lead and the other being promoted to a higher energy state.

After some time, the second electron also exits, leaving the island in an intermediate energy state. This state eventually decays by emitting a microwave photon into the resonator, where the photon is trapped as a standing wave. The trapped photon can then stimulate the emission of another photon from the island, rapidly filling the resonator with multiple coherent photons.

This amplification process is similar to that which occurs in a conventional maser – but instead of involving many identical atoms or molecules, the process involves only one artificial atom – the superconducting island. Astafiev told physicsworld.com that this gives the device several technological advantages over conventional masers. One is that the wavelength of the microwaves can be tuned very precisely by simply varying the voltages that are applied to the island and changing the resonator frequency. This is unlike conventional masers, which produce microwaves at wavelengths that are fixed by the energy levels of the constituent atoms or molecules.

The device only requires dc voltages to run — unlike a conventional maser, which requires an external radio-frequency power source. As a result the entire maser system can be very small and generate very little heat. This makes it relatively easy to operate at temperatures near 1 K, where aluminium is a superconductor.

According to Astafiev, the maser could be used to create tunable microwave sources and amplifiers that are integrated onto silicon chips. Such devices could be used to study molecular dynamics and other properties of matter. He also believes that single-atom masers could be used as a source of coherent photons in quantum computers that are based on supercomputing quantum bits (qubits). Indeed, the NEC– RIKEN team has been making qubits from tiny islands of superconductor since 1999.

Browse articles by content type