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Home » Page 1274

Getting to the bottom of foamy physics

Researchers in the US have created a new mathematical model to describe the complex evolution of foamy bubbles – something that has proved fiendishly difficult to model thanks to the hugely varying length and time scales involved. Their computed results closely match theoretical models as well as lab-based observations of foamy bubbles. The team hopes the underlying equations could have a variety of applications, including helping to make better metal and plastic foams, developing lightweight crash-absorbent materials and also to model a number of biological processes such as the growth of cell clusters.

Heady maths

Foams are all around us: from the froth on a cappuccino or beer to the soapy suds in a bubble bath. However, scientists have found it difficult to describe exactly how such clusters of bubbles coalesce, grow and change shape over time – before they ultimately go pop. An early attempt at understanding the structure of soapy foams is encapsulated in “Plateau’s laws” – formulated by 19th-century Belgian physicist Joseph Plateau. Then Lord Kelvin developed his theory of an “ideal foam” of equal-sized bubbles in 1887, an accurate version of which was finally made in the lab in 2012 by a team at Trinity College, Dublin. But a more general set of equations describing bubbles on varying length and time scales remained elusive, until now. The challenge is to create mathematical models that describe how interfaces between bubbles move and how they “meet” in complicated phases.

Key phases

Now, James Sethian and Robert Saye of the University of California, Berkeley have separated the various processes that determine a foam’s evolution according to the different length and time scales at which they occur – and have created a model for bulk foam dynamics. The researchers say that the model accurately describes how fluid moves within a bubble and how the individual cells form and how their junctions (or borders) are rearranged as individual bubbles within the foam burst.

To do this, Sethian and Saye identified three distinct regimes or phases of foam evolution. “We identified and separated the three phases – the drainage of liquid from a bubble’s membrane, the rupture of the drained bubble and the macroscopic rearrangement of the bubbles within the foam – to simulate the system,” explains Sethian.

The first set of equations describes how the liquid drains from a bubble wall, thanks to gravity, so that the wall eventually becomes so thin that it ruptures. The next set of equations explains the liquid flow at the junctions between bubble membranes; while the third set considers how the entire foam rearranges to move closer to equilibrium, a motion that happens on a macroscopic scale.

Beach bubbles

Sethian and Saye tested their formulae on bubble clusters of different sizes and found that they could accurately predict the interactions of gases and liquids in these foamy materials. They also developed a fourth set of equations that allowed them to simulate a movie that shows how light would reflect off a small foam sample as its bubbles rearrange. The researchers picked a beach scene for the simulation, so that they could “visualize and see how well the model captures what you would see in real life, while still accurately showing how the light would reflect”, as Sethian explains.

These processes are all influenced by a variety of factors, including viscosity, surface tension, gravity and other terms of fluid dynamics. Some of these factors can be modified in the current model, but others, such as evaporation, that are currently not included can be added quite easily, according to the researchers.

Sethian points out that it took the team five days to solve the full set of equations of motion using a supercomputer to get the most refined solution of the algorithms. He says that the entire mathematical formulation and codes will be available to anyone who is interested in running similar simulations at whatever scales they wish, for any applications, including industrial ones.

While a large part of the aim of this work was to develop a fundamental model, the researchers claim that it could have other applications. When it comes to biological modelling, Sethian says the equations could help to understand highly complex systems, such as cell cluster growth, that may go from being organized to unorganized systems. According to him, the models might help “to better understand how cells group together and aggregate…and to study the kind of physical forces involved – such as adhesion between cell boundaries, fluid dynamics, etc – as well as the mechanisms involved in how cell cluster grow from clusters of 5 to 10 cells to those of hundreds to thousands of cells”.

Take a look at the video below of a collapsing soap-bubble cluster, shown with thin-film interference and computed using Sethian and Saye’s multiscale model.

The research is published in Science.

Research galore in Singapore

By James Dacey

The Helix Bridge in Singapore. (Courtesy: iStockphoto/TommL)

Despite its modest size, the city-state of Singapore is clearly an ambitious nation, boasting a leading financial centre and one of the world’s busiest ports. During a recent visit to Boston I met a man called Lim Tze Min who works for a government agency called Contact Singapore, which exists to try and attract skilled people to live and work in Singapore. I wanted to know why a physicist might consider relocating to the country. Listen to our conversation here.

Tze Min talks about research facilities including the Centre for Quantum Technologies (CQT), the founding director of which is the Polish-born physicist Artur Ekert, who is also affiliated with the University of Oxford in the UK.  According to Tze Min, one of the major bonuses of being a researcher in Singapore is the small amount of bureaucracy invovled, which allows scientists to get on with just doing the science. Give it a listen and decide for yourself whether it sounds like a place where you could imagine yourself working.

Hawking’s academic boycott divides opinion

By James Dacey

Do you agree with the principle of academic boycotts?

Yes
No

Have your say by taking part in this week’s Facebook poll. As always, please feel free to explain your answer by posting a comment on Facebook or below this article.

This question has arisen after it was revealed yesterday that Stephen Hawking will be boycotting a prominent conference in Jerusalem in protest against the policies of the Israeli government. The British cosmologist and science communicator had been set to talk at the Israeli Presidential Conference: Facing Tomorrow, which will take place in June and which will feature a string of high-profile speakers, including Bill Clinton and Tony Blair. But the University of Cambridge has confirmed that Hawking, who is director of research at its Centre for Theoretical Cosmology, has pulled out of the conference for political reasons. Physics World has spoken to a university spokesperson who confirmed that Hawking has sent a letter to the conference organizers to explain his decision not to take part. The UK newspaper the Guardian has today published what it says is the full text of this letter, dated 3 May.

“I accepted the invitation to the Presidential Conference with the intention that this would not only allow me to express my opinion on the prospects for a peace settlement but also because it would allow me to lecture on the West Bank. However, I have received a number of e-mails from Palestinian academics. They are unanimous that I should respect the boycott. In view of this, I must withdraw from the conference. Had I attended, I would have stated my opinion that the policy of the present Israeli government is likely to lead to disaster.”

(more…)

Cold-atom random laser simulates stellar clouds

Physicists in France are the first to create a random laser in a cloud of cold atoms under laboratory conditions. The effect was first seen decades ago in stellar clouds and the team believes that its set-up could provide basic insights into the conditions necessary for random lasing. This could improve our understanding of astrophysics and even lead to practical applications of the phenomenon here on Earth.

A conventional laser usually comprises a gain medium (solid, liquid or gas) that is sandwiched between two mirrors. Light bounces back and forth many times in this optical cavity, stimulating the emission of more light and creating a coherent field of light. In a random laser there are no mirrors and the light simply bounces around between particles located at random positions in the gain medium. This light can stimulate the emission of light from the medium just as in a conventional laser. However, because of the random paths taken by the light, a laser beam is not produced. Instead, coherent light is emitted in all directions.

Random lasing was first proposed in the 1960s to explain why certain specific emission lines in some clouds of stellar gas are more intense than theoretically predicted. On Earth, random lasers have been made using liquid suspensions and solid powders. In these lasers, the light-scattering particles are classical objects such as grains of zinc oxide – whereas astronomers believe that atoms do the scattering in stellar-gas lasers.

Forbidden transition

Robin Kaiser and colleagues at the CNRS Non-linear Institute of Nice made their laser with a cloud of cooled rubidium-85 atoms confined to a magneto-optical trap. They used a pump laser to create a population inversion between two hyperfine levels of the same electronic orbital and a second, tunable laser to stimulate emission back to the lower level. The key to creating a random laser is to ensure that some of this emitted light will be scattered by the atoms, rather than being reabsorbed by them. This was done by adjusting the wavelength of the tunable laser such that the emitted light corresponded to that of a forbidden transition in the atom. The researchers found that, when the laser light had exactly the same frequency as the forbidden transition, there was a bump in the intensity of the laser output – a sign that it was being boosted by random lasing.

This is the first demonstration of a random laser in the laboratory in which photons are scattered by atoms, as happens in stellar gases. Whereas the effect of radiation pressure on a grain of zinc oxide is insignificant, the scattering of a photon will cause measurable recoil in an atom. Whether or not this and other effects of random lasing have macroscopic effects in astrophysics remains an open question, however. “It’s possible in principle that, if you put more stimulated emission into the radiation, the sign of the radiation pressure could be inverted,” Kaiser explains, “so we could have attractive components of the radiation pressure.” He points to the team’s observation of an oscillating cloud of cold atoms bears similarities to Cepheid variables, an oscillating equilibrium between gravity and radiation pressure. So signs of negative radiation pressure should be accessible to experimental observations.

Kaiser hopes that such ideas might be testable in the laboratory with a gaseous random laser. He points out, however, that significant differences between the team’s laboratory device and a stellar-gas laser remain. The atoms in their magneto-optical trap are cooled to about 50 µK, whereas the atoms in a stellar gas are hot. The team pumped the random laser with another laser, while a stellar gas is pumped by broad spectrum light from a star. Kaiser hopes to optimize the team’s system to resemble more closely the conditions in a stellar environment. “I want to interact more with people from astrophysics to find out which schemes are more or less realistic,” he says.

Quantum effects

Diederik Wiersma, an expert on disordered photonics at the University of Florence, is impressed by the work. He believes that, in addition to radiation pressure, it may be possible to study other phenomena, such as the effect of photon scattering from several atoms in an entangled state and quantum interference between atoms and photons. “You might have access to knowledge about which path the photon took,” he explains, “and if you have that, quantum mechanics tells you it will not behave as a wave anymore.”

Further into the future, Wiersma suggests that the system may aid understanding of quantum computing. “There have been proposals about a possible quantum Internet,” he says, “where you could use optical connections to connect matter at different locations in a single quantum state.”

The research is published in Nature Physics.

What is quantum gravity?

In less than 100 seconds, Leron Borsten explains that general relativity and quantum mechanics are very successful in their own domains, but the jury is still out on how to unify these two great theories of physics.

Watch more from our 100 Second Science video series.

Nuclear physics goes pear-shaped

An international team of physicists has found the best evidence yet that some heavy nuclei are not spherical or ellipsoidal – but “pear-shaped”. The researchers found clear signs of this lopsidedness in two particular nuclei – radon-220 and radium-224 – that were created by smashing protons into a uranium carbide target at the REX-ISOLDE facility at CERN. As well as providing new information about the forces that bind protons and neutrons together in nuclei, the discovery could also help shed light on physics beyond the Standard Model of particle physics.

Physicists have known for decades that nuclei can occur in different shapes beyond simple spheres. In most cases, these non-spherical nuclei look either like a rugby or American football, or like a discus – both these shapes having axial and reflection symmetries. Any departure from a sphere is usually described by a “quadrupole deformation”, driven by underlying “quadrupole” interactions between the nucleons (protons and neutrons) in the nucleus. However, physicists have also suspected that there are even more subtle “octupole” interactions between nucleons, which would be expected to cause some nuclei to be pear-shaped – or even resemble a pyramid.

Although there was some limited evidence for pear-shaped nuclei in experiments carried out on radium-226 and neodynium-148 in the 1990s, neither study was conclusive. What Peter Butler of the University of Liverpool and colleagues in Belgium, Finland, Germany, Poland, Spain, Switzerland, the UK and the US have now done is to find strong evidence for octupole transitions in radon-220 and radium-224. These transitions are a sign that the nuclei are lopsided and appear in the spectrum of gamma rays these nuclei emit as they decay from an excited state.

Smoking gun

After the radon and radium nuclei were created at REX-ISOLDE’s uranium-carbide target, they were then accelerated and passed through thin targets of nickel, tin and cadmium. As they travel through these targets, some radon and radium nuclei come close to a target nucleus such that its intense electric field excites the radon or radium – an experimental technique called “coulomb excitation”. The nuclei were then fired through CERN’s MINIBALL detector, which can detect gamma rays emitted in all directions from the nuclei.

A representation of the pear-shaped radium-224 nucleus
Pear-shaped nucleus: a representation of the radium-224 nucleus in the x, z plane, with the colours as the y-axis scale. (Courtesy: L P Gaffney)

By looking at the energy and spatial distributions of the gamma rays, the team could conclude that both nuclei are pear-shaped. In particular, the team found that a certain octupole transition was much more common than expected if the nuclei were not pear-shaped – indeed, in the case of radium it was about 30 times stronger than expected. “This ‘enhanced’ [transition] could be considered a smoking gun for a static octupole deformation of the nuclear matter in such nuclei,” says Paddy Regan of the University of Surrey, who was not involved in the research.

Enhanced electric dipoles

According to Butler, the new results provide some insight into the efficacy of several models that attempt to describe the structure of large nuclei such as radon and radium – something that continues to be very hard to achieve. But beyond nuclear physics, the study of pear-shaped nuclei could also shed light on why there is far more matter than antimatter in the universe. This is because atoms that have a pear-shaped nucleus are expected to be a good place to look for violation of time (T) and charge–parity (CP) symmetries beyond that allowed by the Standard Model.

These symmetries forbid an atom from having a permanent electric-dipole moment (EDM). So far, physicists have not found evidence for an atom with an EDM but Butler reckons that the experimental signature of EDM would be enhanced by a factor of about 1000 in an atom with a pear-shaped nucleus. Measuring CP and T violation beyond the Standard Model could explain why most of the antimatter created in the Big Bang has since vanished – and why the universe is dominated by matter.

Once the accelerators are up and running again at CERN in 2015, the team plans to study other nuclei that are expected to be pear-shaped.

The study is described in Nature.

What is cosmic inflation?

In less than 100 seconds, Andrew Jaffe explains why cosmologists believe that the universe underwent a period of vast and rapid growth when it was just fractions of a second old.

Watch more from our 100 Second Science video series.

Stephen Hawking boycotts high-profile Israeli conference

By James Dacey

Stephen Hawking has decided to pull out of the fifth Israeli Presidential Conference: Facing Tomorrow 2013, which is taking place in June. The world-famous British cosmologist and science communicator was due to deliver a keynote speech at the conference in Jerusalem, which boasts other presenters including Tony Blair, Bill Clinton and Mikhail Gorbachev. But it appears that Hawking has made a late U-turn. That is according to a statement published by the British Committee for Universities for Palestine – an organization of UK-based academics, set up in response to the Palestinian call for an academic boycott of Israel.

We understand that Professor Stephen Hawking has declined his invitation to attend the Israeli Presidential Conference Facing Tomorrow 2013, due to take place in Jerusalem on 18–20 June. This is his independent decision to respect the boycott, based upon his knowledge of Palestine, and on the unanimous advice of his own academic contacts there.

(more…)

New insights into what triggers lightning

Cosmic rays interacting with water droplets within thunderclouds could play an important role in initiating lightning strikes. That is the claim of researchers in Russia, who have studied the radio signals emitted during thousands of lightning strikes. The work could provide new insights into how and why lightning occurs in the first place.

Although most people have witnessed a flash of lightning during a thunderstorm at some point in their lives, scientists still do not completely understand what triggers the discharge in the first place. Lightning has been studied for hundreds of years, yet while many possibilities for observation are available – there are about 40 to 50 lightning strikes per second across the globe – predicting the onset of a strike is difficult.

There are three basic types of lightning: lightning that occurs within a single cloud; lightning that occurs between two clouds; and lightning that occurs between a cloud and the Earth’s surface. In a typical cloud-to-ground lightning strike, scientists know that an electrically-conducting plasma channel forms between the cloud and the ground, which allows the discharge to occur. However, the factors that cause the initial charging of the cloud and its subsequent discharge are not clearly understood.

Cosmic ray kick-off

Now, Aleksandr Gurevich of the Lebedev Physical Institute in Moscow and Anatoly Karashtin of the Radiophysical Research Institute in Nizhny Novgorod have suggested a new model that includes two crucial factors that could help explain the process: the behaviour of water or ice particles inside clouds, dubbed “hydrometeors”; and showers of ionized electrons that might be created by cosmic rays.

The theory that cosmic rays may cause the ionized showers that initiate lightning was first put forward by Gurevich more than 20 years ago. Known as “runaway breakdown”, Gurevich suggested that the ionized particles create free electrons within thunderclouds that are then accelerated to extremely high energies by electric fields within the clouds. These electrons collide with other atoms in the air to cause an “avalanche” of high-energy particles within the cloud – and this provides the seed for the onset of lightning. While the theory was widely discussed, Gurevich was not able to find proof that cosmic rays do indeed trigger the avalanche.

In a bid to gather more evidence, Gurevich and Karashtin have now done a new analysis using a radio interferometer of radio pulses emitted at the onset of 3800 lightning strikes across Russia and Kazakhstan. A long series of these short yet strong pulses is emitted just before lightning strikes and, according the researchers, the pulse data match Gurevich’s model of electrical breakdown.

Pulses of information

The researchers also point out that the amplitude of a pulse is proportional to the number of secondary electrons, and so also to the energy of the initial cosmic ray that generates the shower. But when they calculated the cosmic-ray energy, Gurevich and Karashtin found it to be about 1017 eV – a surprising figure as cosmic rays of this energy are too rare to explain what was measured.

To explain why such high energies were observed, the researchers suggest that the hydrometeors they used become electrically polarized as the strong electric field inside the cloud builds up and that a further “micro-discharge” occurs at the hydrometeor as the field reaches its threshold, thereby effectively amplifying the cosmic-ray-initiated breakdown. When this is taken into consideration, then much more common cosmic-ray particles with energies of about 1012–1013 eV are sufficient to initialize a discharge.

Physicist and lightning expert Joseph Dwyer of the Florida Institute of Technology, who was not involved in the current research, says that the new model is “an interesting idea, but much more work is still needed, for example experiments to measure radio pulses and air showers at the same time”, which is something that Dwyer and his colleagues are currently working on themselves.

Gurevich and Karashtin say that their observations show that the radio emissions are generated by the specific discharges in thunderclouds, which are different from the conventional electric discharges expected and that the “runaway breakdown” plays a significant role too. Further observations will be necessary to finally crack the mystery of the atmospheric crackle.

The research is published in Physical Review Letters.

What can scientists gain from blogging?

In less than 100 seconds, science blogger Andrew Jaffe shares his thoughts on what scientists can gain from sharing their insights and experiences with the wider scientific community.

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