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Home » Page 1509

Dungeons and Dragons dice pack densely

From the apples at your local grocery store to the pills in your medicine cabinet, packing products in an efficient manner is an important consideration in many industries. In new research, a group of physicists in the US has investigated the packing properties of a less familiar object, though it may be recognizable to players of the game Dungeons and Dragons – the tetrahedral die. They find that these shapes pack incredibly densely, despite taking on a highly disordered configuration.

Tetrahedra are regular convex shapes possessing four triangular faces. To date very little research has been carried out on how these shapes pack together. But a better understanding of this process could be of interest to geological industries such as oil companies when choosing where to drill their wells. This is because granular matter is more similar to tetrahedra than spheres, which is how it is depicted in basic geological models.

Dense packing

In the past year or so the applied mathematics community has taken up the challenge to investigate tetrahedra, and it has become clear that these shapes could pack much more densely than spheres, at least in theory. In the extensive research on spheres over the years, they have never filled more than 64% of a container, despite a conjecture by Kepler that they could pack to a fundamental limit of 74.05%. In contrast, some recent numerical models have shown that tetrahedra can pack to fractions of more than 85%.

With this latest research, Alexander Jaoshvili at New York University in the US, working with colleagues, has taken a closer look at how tetrahedra pack together in the real world. In a fairly straightforward experiment, the researchers assembled a large number of identical tetrahedral-shaped dice and began adding these to different-shaped containers, shaking and adding more dice until no more could be added. Packing fractions were then determined by injecting a well known filling fluid until the containers were full and subtracting these volumes of fluid from the total volumes of the containers. For one of the large radius containers, a packing density of 0.76 was recorded, which compared with 0.64 for spheres added to the same container.

To probe a little deeper and examine the packing structure, Jaoshvili’s team then placed the packed containers in an MRI scanner. This enabled the researchers to locate the centres of particles and to resolve the kinds of configurations that the dice were taking on. What they saw is that, despite their ability to pack so tightly, the dice are in fact highly disordered within the containers. This finding adds weight to recent theoretical work that suggests that the tetrahedra are aligning themselves into a form of quasicrystal structure upon compression.

All shook up

Jaoshvili and his team were slightly surprised by the disorder. “One would expect that if particles are highly packed they would be highly ordered as well, but with tetrahedrons we find that they are packed with high density and are highly disordered,” says Jaoshvili.

This surprise is shared by Daan Frenkel, a theoretical chemist at the University of Cambridge, who believes that, at the moment, the result can only be explained qualitatively, by comparing tetrahedra with other shapes. “With cubes, the gap-less packing can be continued indefinitely – they can pack 100% of the space. Tetrahedra cannot “tile” space – but they are better at it than spheres.”

Since Jaoshvili submitted his paper there has been a flurry of activity regarding tetrahedral packing and he expects further light to be shed on the quasicrystal structure of the packing in the near future.

This research is published in Physical Review Letters.

Earth’s magnetic field gathers momentum

Physicists in France have linked subtle variations in the length of day with conditions in the Earth’s core – where the Earth’s magnetic field originates. The finding could improve our poor understanding of how the field is generated and why it changes in response to conditions deep within the Earth’s interior.

Molten iron flowing in the outer core generates the Earth’s geodynamo, leading to a planetary-scale magnetic field. Beyond this, though, geophysicists know very little for certain about the field, such as its strength in the core or why its orientation fluctuates regularly. Researchers do suspect, however, that field variations are strongly linked with changing conditions within the molten core.

As we cannot access the Earth’s core directly, researchers look to clues at the Earth’s surface. One intriguing suggestion is that changing conditions at the core could have an impact on angular momentum throughout the whole Earth system. The implication is that variation to the flow patterns in the core could have an impact on the Earth’s rotation, which could lead to slight variations in the length of a day.

New wave

Nicolas Gillet and colleagues at the Université Joseph Fourier claim to have the strongest evidence yet that this is indeed happening. By reconstructing flow within the Earth’s core using an established model of the geodynamo, the researchers see a type of wave – called an Alfven wave – emerge from within the core. They believe that this wave, not seen before in simulations, is transferring angular momentum through the core towards the overlying mantle.

Closer inspection of the simulations revealed that these Alfven waves are dragged by the magnetic field and they recur just once every six years. The key result is that this periodicity corresponds with a six-year signal in the variation to the length of day, leading the researchers to link the two phenomena. They argue that the Alfven waves play a role in balancing angular momentum throughout the Earth. “When the core rotates faster, the rotation of the mantle must be slower in order to compensate, which in turn increases the length of day,” explains Nicolas Gillet.

Having established this link, Gillet’s team focused their attention on the Alfven wave as it propagates through the core. Realizing that the wave takes approximately four years to reach the mantle, they were able to calculate the strength of the Earth’s magnetic field within the core – approximately 4 mT. This value is the most reliable yet for the magnetic field in the core, claim the researchers.

Good value

Ulrich Christensen, a geophysicist at the Max Planck Institute for Solar System Research is impressed by the unified approach taken by Gillet’s team. “I like the value derived from this analysis as it is in line with what I would expect from the recent geodynamo simulations,” he says.

Previous estimates of the magnetic field within the core had come directly from numerical simulations, or from interpreting geomagnetic data gathered at the surface. “Our study revisits the estimate from geophysical data, and reconciles it with geodynamo simulations,” says Gillet.

And the full significance of this research may not be realized yet. The researchers believe that they can go on to develop a more complete model of the geodynamo and the way angular momentum is transferred through the core. “It is important in order to understand how the geodynamo works and how this is linked with the thermal history of the planet,” says Gillet.

This research is published in Nature.

Ultracold dipoles are under control

Physicists in the US have created an ultracold gas of molecules with “adjustable” dipole moments. The experiment, which is the first to study the effect of long-range dipole interactions in an ultracold gas, could lead to new ways of using trapped molecules to simulate quantum effects that occur in solids

Ultracold gases make ideal “quantum simulators” because some of the interactions between the component atoms or molecules can be “tuned” by adjusting the applied magnetic and laser fields that keep the particles in place. While physicists have been successful at dialling up short-range interactions between atoms and molecules, simulating long-range interactions – such as those between charged particles – has proven more difficult.

Earlier this year Jun Ye and colleagues at the National Institute of Standards and Technology (NIST) in Colorado and Maryland cooled potassium-rubidium (KRb) molecules to see how they react chemically to form Rb2 and K2. By doing so, they were able to observe how the initial quantum states of the molecules affect reaction rates – something that cannot be observed at room temperature.

Now, the same NIST team has turned its attention to the long-range electric-dipole interaction between the molecules – and how it affects the reaction rate. As in their previous experiment, the team created ultracold KRb molecules by cooling a mixture of potassium and rubidium atoms to just a few hundred nanokelvin and then exposing them to a magnetic field gradient. This binds the atoms together, with the bond being further strengthened by exposing the atoms to laser light.

Direction matters

If there is no applied electric field, the molecules have no electric dipole moment. A pair of molecules will therefore only react if they can tunnel through an energy barrier that arises because the particles are fermions, having non-integer spin. The result is a relatively low reaction rate.

But if a small electric field is applied to the gas, the molecules acquire electric dipole moments that all point in the same direction along the field. So when two molecules collide, they feel a dipole-dipole interaction that depends on the relative orientation of the collision and the electric field.

If the collision occurs along the direction of field, the positive end of one dipole collides with the negative end of the other (a head-to-tail collision) and the force is attractive. However, if the collision is perpendicular to the field, the force is repulsive.

The attractive force lowers the height of the tunnelling barrier, making it more likely that the molecules react. Even though the tunnelling barrier gets bigger for perpendicular collisions, the overall effect is to make the reaction go faster, which should lead to the gas warming up.

The temperature of the gas can be measured by switching off the magnetic trap and determining the rate at which the gas expands – the faster the rate the higher the temperature. By measuring the expansion rate in different directions, the team found that collisions are more likely to occur in a certain direction.

Expanding gas

Ye and colleagues studied the dipole interaction by repeating the expansion measurements for a series of samples that were exposed to different electric fields of different strengths. They found that the reaction rate did not change significantly as the dipole moment was increased – until it reached a specific level, above which the reaction rate increased rapidly.

The results could help physicists to gain a better understanding of how to create long-lived ultracold dipolar gases. According to Ye, the importance of the head-to-tail interactions suggests that the lifetime of such gases could be boosted if they are confined to a 2D “pancake” so that head-to-tail collisions cannot occur. He said the team have already managed to suppress losses due to the dipole-dipole interaction.

The laser at 50

By Matin Durrani

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Regular users of this site will be well aware that we are currently celebrating the 50th anniversary of the invention of the laser.

It was on 16 May 1960 that Theodore Maiman – then a 32-year-old engineer-turned-physicist at Hughes Research Laboratories in the US – eked out the first pulses of light from a pink-ruby crystal, since which the laser has become a workhorse of physics and ingrained in everyday life.

To celebrate the laser anniversary, we’re offering a free PDF download of the May issue of Physics World (right), which you can get by following this link.

Packed with great laser features, we relive the race to build the world’s first working laser – a story still laced with controversy. Find out about the technological impact of lasers in fibre optics and the quest for green-wavelength laser diodes that could let mobile phones project images onto any surface.

Basic research gets a look-in, too – in terms of both ultrahigh power lasers to promote fusion as well as ultrafast lasers that can probe the motions of atoms and molecules. And don’t miss our special, colour-coded timeline of laser history.

And if that’s not enough, don’t forget you can also view a series of great video interviews with leading laser experts via the physicsworld.com multimedia channel.

If I can recommend just one of the videos, it’s the one with Tom Baer, former president of the Optical Society of America, in which he overviews 50 years of laser physics, and makes some predictions about the next 50. Watch it here.

Of course, we’re not the only ones to be marking the laser anniversary. Thanks to the efforts of my colleague Joe Winters at the Institute of Physics press office, today’s edition of the Sun – the UK’s best-selling newspaper – has a great article marking the laser anniversary. Check it out via this link.

But don’t spend too long at the Sun – for the real deal on lasers, you really mustn’t miss the May issue of Physics World.

Herschel sheds light on star and galaxy formation

  • Herschel has found a giant star lurking in the cloud RCW 120
  • Previously unseen distant galaxies
  • Part of the Milky Way where hundreds of stars are born
  • An image of the constellation Vulpecula
  • An infrared image of the Rosette molecular cloud

« previous | next »

The first scientific results from the Herschel infrared space observatory have been unveiled by the European Space Agency (ESA). Some images reveal billowing clouds of gas and dust that astronomers believe will go on to form stars and planets. Others provide new views of the early universe, showing distant galaxies that are invisible to the likes of the Hubble Space Telescope.

Launched in May 2009, Herschel is a far-infrared and submillimetre telescope that probes the universe’s coolest objects, from the era when the first stars and galaxies were formed to the present day. It started taking data in July last year and the new images include those of the star-forming cloud RCW 120, which Herschel has discovered contains an embryonic star that could evolve into one of the largest in the Milky Way.

The star is already 8–10 solar masses and is surrounded by about 2000 solar masses of dust that it could suck in. “This star can only grow bigger,” says Annie Zavagno of the Laboratoire d’Astrophysique de Marseille, adding that its discovery could help astronomers improve current theories of star formation, which limit star size to about eight solar masses.

Meanwhile, at the farthest reaches of the universe, Herschel has so far discovered more than 1000 distant galaxies. These galaxies emit large amounts of infrared radiation, and the Herschel images show that these objects are responsible for more than half of the cosmic infrared background radiation originating from that part of the universe.

Unlike the Milky Way, which creates about three new stars per year, some of these ancient galaxies are forming thousands of stars per year. “We can use these results to study what controls star formation in these distant galaxies, and how galaxies like the Milky Way formed,” says Dieter Lutz of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Piercing the veil

These star-forming regions are hard to see because they are usually shrouded in gas and dust that blocks visible light. But as infrared radiation pierces this veil, Herschel has the resolution to reveal the details of how clouds of cool atoms and molecules coalesce into stars. Furthermore, the clouds themselves emit mostly infrared radiation and therefore Herschel should shed light on both the origins of these clouds and how they evolve into stars.

Herschel operates in Earth orbit because water vapour in the atmosphere absorbs much of the infrared radiation from space – and because the Earth itself emits vast quantities of infrared radiation that can swamp ground-based telescopes. Sensitive to light with wavelengths of 55–670 µm, its mirror is 3.5 m across – the largest ever deployed in space. Infrared radiation is detected using instruments that must be cooled to temperatures of near absolute zero using liquid helium. The mission will end when the coolant runs out, which is likely to be some time in 2012.

All images courtesy of the European Space Agency.

iPhone goes nuclear

iphone.jpg

By Hamish Johnston

There really is an iPhone app for everything…

Researchers at the University of Utah’s Nuclear Engineering department have used an iPhone visualization application – or app – to display simulations of nuclear reactor cores (see right).

Called ImageVis3D Mobile, the app was first developed by the university’s Scientific Computing and Imaging (SCI) Institute to look at medical CT or MRI scans.

Now it seems that Tatjana Jevremovic and colleagues have come up with a way to use the app to visualize the results of reactor simulation software named AGENT (Arbitrary Geometry Neutron Transport).

Jevremovic’s ultimate goal is to develop a secure way for nuclear engineers in academic settings to share simulation data with those at commercial power plants.

Although the ImageVis3D can be downloaded for free from the Apple App Store, don’t expect to be simulating reactor cores anytime soon – to do that you’ll need access to the university’s computers.

Dark matter ‘no result’ comes under fire

A war of words has broken out in the dark-matter community over a report posted on the arXiv preprint server earlier this week. The preprint from the XENON100 collaboration poured cold water on claims that dark matter has been detected by two other experiments – but now the report itself has been attacked by other researchers in the field.

On Monday the XENON100 collaboration published an analysis of the first experimental results from its dark-matter detector. It reported no evidence of dark matter, the substance thought to constitute over 80% of mass in the universe. The experiment covered a similar parameter range as dark-matter searches DAMA and CoGeNT, which have previously claimed possible evidence for dark matter. As a result, the XENON100 team concluded that both the DAMA and CoGeNT evidence could be excluded.

But now the DAMA and CoGeNT collaborations claim that the XENON100 researchers’ analysis is flawed, and that their original evidence for dark matter should remain intact. Indeed, the CoGeNT collaboration is even requesting that the XENON100 collaboration retract its preprint. “These results cannot be defended,” CoGeNT spokesperson Juan Collar of the University of Chicago told physicsworld.com.

Flash evidence

The past couple of years have seen several experiments turn up evidence for dark matter. In 2008 the DAMA collaboration, based at the Gran Sasso laboratory in Italy, found what seemed to be the tell-tale flashes of dark-matter particles – known as weakly interacting massive particles, or WIMPs – colliding with sodium-iodide nuclei buried in huge underground detectors. The flashes were less frequent in winter than summer, suggesting that the Earth’s orbit was periodically taking our planet with and against our galaxy’s prevailing “wind” of dark matter.

Earlier this year, the CoGeNT collaboration based in the Soudan underground laboratory in Minnesota, US, reported more tantalizing evidence. It had recorded hundreds of charge bursts inside a germanium detector, consistent with WIMPs striking the germanium nuclei.

The XENON100 experiment – also in Gran Sasso – seemed to have the potential to uphold or overturn such results. Using liquid xenon as a detector material, which is much heavier than sodium-iodide and therefore more susceptible to WIMP collisions, the researchers needed only take data over a matter of days. Moreover, they claim that it has the lowest background noise of any dark-matter experiment.

In their preprint, the XENON100 researchers explained how over an 11 day period last year they found no WIMP collisions. In particular, they described an upper limit on the possible WIMP mass as a function of the probability or “cross section” of the WIMP interaction. This upper limit excludes the fairly low-mass WIMPs of between 7 and 20 GeV, as seen by DAMA and CoGeNT.

Sagging efficiency?

But the CoGeNT and DAMA collaborations believe that the XENON100 results are unreliable at such low energies. They say that the efficiency of the XENON100 detectors to record dark-matter interactions, which the XENON100 collaboration calculates to be constant, in fact drops with decreasing energy. If true, this would mean that the xenon detectors are unable to rule out DAMA and CoGeNT’s evidence for low-mass WIMPs.

We invite the XENON100 researchers to reconsider their claims Dan McKinsey, Yale University

“The onus of unequivocally demonstrating the existence of [loss-mass WIMP interactions] is on the XENON100 collaboration,” wrote CoGeNT’s Collar together with Dan McKinsey, a dark-matter physicist at Yale University, in a comment uploaded to the arXiv server yesterday. “Attempts to substitute this with a biased analysis represent a lack of consideration for the many efforts made by other dark-matter researchers working towards similar ends. We invite the XENON100 researchers to reconsider their claims.”

Rita Bernabei, spokesperson for the DAMA collaboration, also rejects the XENON collaboration’s conclusions. “[Its] result has no impact on the [DAMA] evidence for dark-matter particles in the galactic halo,” she says. “In fact, no direct model-independent comparison is possible among experiments that use different target materials and approaches, and have different sensitivities to different dark-matter candidates and scenarios.”

For me the best answer is not in words but in measurements Elena Aprile, XENON100 collaboration

But Elena Aprile, spokesperson for XENON100, thinks Collar’s claims about efficiency are wrong. “For me the best answer is not in words but in measurements,” she explains. “I understand that he feels not too good about our results, [and] in the end he can say all he wants, but there is simply no sign of hypothetical 7 GeV WIMPs in our data, and we state that very clearly. It is not about [the detector efficiency], on which they dwell for most of their response with some misleading and arbitrary statements. There is absolutely no reason why we should consider McKinsey data more than our own.”

The real test of the XENON100 collaboration’s analysis will be its peer review in Physical Review Letters, where they have submitted it for publication. In the meantime, they are preparing a response to the criticisms which they will soon upload to arXiv.

The XENON100 preprint can be read at arXiv: 1005.0380 and the comment at arXiv: 1005.0838.

Saudi Arabia to create renewable energy ‘city’

Researchers have welcomed a plan by Saudi Arabia to build a new renewable-energy “city” as a sign of the oil-rich nation’s commitment to developing alternative fuel sources. The King Abdullah City for Atomic and Renewable Energy (KACARE) will be based in the nation’s capital Riyadh after the Saudi ruler, King Abdullah, issued a royal decree in April to order its creation. It will serve as a centre for renewables research and for co-coordinating national and international energy policy.

While it is not yet clear when KACARE will be opened, the King has appointed a president for the city – Hashim bin Abdullah Yamani, a former minister for commerce and trade. In a statement to the Saudi Press Agency, Yamani said, “Establishment of the city will contribute to achieving sustainable development in the kingdom through exploiting the science, research and industry of atomic and renewable energy for peaceful purposes.”

The announcement to create the new city comes just six months after the official opening of the King Abdullah University of Science and Technology (KAUST), a multi-billion dollar research centre with energy and environment amongst its core research activities. Both these projects have received the financial backing of the King Abdullah. Since coming to the throne in 2005, the King has been aware that while the country’s oil and gas reserves are deep they are not infinite – and that Saudi Arabia must use its current wealth to prepare for a future with dwindling fossil fuels. His vision for KAUST is to provide a world-class university that can develop, among other things, more sustainable technologies.

More focused approach

It makes a whole lot of sense to diversify [Saudi Arabia’s] energy future, particularly by seeking to utilize solar energy – KSA sits in a region of high solar intensity and minimal cloud cover Tony Eastham, director of labs, KAUST

With KACARE, King Abdullah wants to create a more specialized centre that focuses on harnessing the nation’s other natural resources. Given its desert climate, Saudi Arabia is keen to develop a solar energy infrastructure, and it is also looking to develop nuclear energy, an approach mirrored by the other member states of the Gulf Cooperation Council – the United Arab Emirates, Quatar, Bahrain, Kuwait and Oman. KACARE will also be given the responsibility of drafting a national policy for nuclear power, as well as supervising the use of atomic energy and nuclear waste. It will represent Saudi Arabia at the International Atomic Energy Agency (IAEA).

The creation of KACARE is welcomed by Tony Eastham, the director of labs at KAUST who agrees that Saudi Arabia is right to realize that its oil reserves will not last forever. “It makes a whole lot of sense to diversify its energy future, particularly by seeking to utilize solar energy – KSA sits in a region of high solar intensity and minimal cloud cover,” he says.

Eastham is keen to build links between KAUST and KACARE from the outset. “We and KACARE need to develop collaborations with the best in the region and the best in the world. No-one has an exclusive on great ideas – we need to listen, be aware of what is happening worldwide, and form partnerships to stay at the leading edge of energy science and technology,” he says.

Wider cultural change?

KACARE will be more specialized than KAUST [above], which opened in September
KACARE will be more specialized than KAUST [above]

Chukwumerije Okereke of the Smith School of Enterprise and Environment at the University of Oxford thinks that the creation of KACARE could be symbolic of a wider cultural change among the oil-producing nations of the Middle East. He believes that Saudi Arabia and the other countries belonging to OPEC (Organization of the Petroleum Exporting Countries) are coming to realize that sustainability and climate change are now key national issues.

Okereke says that the OPEC countries have previously been somewhat “retrogressive” in international climate negotiations and he blames this on a lack of effective policy-making in these countries. “In so many cases, climate change was framed as a win-lose situation – if you act on climate change, by investing in new technologies, you lose out on your economy.” But he believes there are signs to show that the situation is changing. “There have been various announcements and ‘noises’ over the past five years from countries including Kuwait and the United Arab Emirates to suggest that they are willing to consider renewables now.”

Once KACARE is inaugurated by King Abdullah, it will have an independent annual budget, but will be able to draft budgets for programmes lasting for more than a year if required. Financing the centre will be made through allocations of the state budget, and revenues achieved by the KACARE in addition to grants and endowments if accepted by the city council.

“The city will first attempt to specify priorities and national policies in the field of atomic and renewable energy to build a strong scientific and technologic base in the fields of power and desalinated water, in addition to medical, industrial, agricultural and mineral fields,” says Hashim bin Abdullah Yamani, the future KACARE president.

The cool universe

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It’s dusty out there: image taken by NASA’s Wide-field Infrared Survey Explorer

By Hamish Johnston

This morning the BBC’s Melvyn Bragg gathered three of the UK’s top astronomers for a chat about the “cool universe” – the vast amounts of matter between the stars that is invisible to optical telescopes.

This dust and gas is best studied using instruments sensitive to infrared radiation – technology that only really got going in the 1960s and works best in space, away from Earth’s infrared glow.

Bragg was joined by Carolin Crawford of Cambridge University, Paul Murdin of Liverpool John Moores University and Imperial College’s Michael Rowan-Robinson, who explained what these infrared telescopes have revealed.

They described a “dynamic universe” in which gas and dust created by the death of past stars is recycled to create stars – and planets – of the future.

“As a result of the new research, we are now beginning to see first-hand the way our planet was formed when the solar system was born,” says Bragg.

You can listen to the broadcast here.

Bacteria band together to beat the system

Physicists in the US are the first to show that E. coli bacteria can work together to swim through a tiny ratchet that would normally block individual organisms. The research, which combines experiments on live bacteria with theoretical modelling, could help scientists to gain a better understanding of how micro-organisms move in confined spaces such as the human gut.

E. coli are rod-shaped bacteria about 2 µm long that can be found in the large intestines of warm-blooded animals. Like many other micro-organisms, E. coli can move using whip-like structures called flagella. This movement often occurs in response to changes in the local concentration of certain chemicals – a process called chemotaxis.

Now, however, Robert Austin, Guillaume Lambert and David Liao at Princeton University in the US have shown that groups of E. coli make collective use of chemotaxis in order to navigate through tiny barriers. The team came to this conclusion by watching how the bacteria move through a microchannel that is 100 µm wide and about 13 mm long.

The microchannel is divided into 85 succesive chambers by walls, which act like microscopic ratchets. If a bacterium swims in one direction through the ratchet, it is guided by funnel-shaped structures into the next chamber. But if a bacterium swims in the opposite direction – against the bias – it is likely to be blocked from entering the next chamber.

Bacterial bands

In their experiment, Austin and his team introduced bacteria into the chamber at one end of the microchannel, in which most of the ratchets were arranged to prevent the micro-organisms from reaching the other end. When fewer than about 200 bacteria were in the first chamber, none of the micro-organisms could swim to the opposite end of the microchannel.

But at higher concentrations, the bacteria formed “travelling bands” that worked together to find their way through the ratchets. For example, when 1000 bacteria were present, the band was able to travel the entire length of the microchannel in about two hours.

The team believes that this collective behaviour is related to how the bacteria modify their environment by consuming nutrients. As the micro-organisms consume the local food supply, a nutrient gradient builds up that directs the bacteria towards regions of plenty – and ultimately through a reverse-bias ratchet and into the next chamber.

To understand how the bacteria were beating the ratchets the team used a model based on the “Keller–Segel equations”, which were formulated 40 years ago to explain why chemotaxis causes some micro-organisms travel in bands through capillary tubes. At low bacterial densities, the equations predict an exponential decline in the number of bacteria as a function of distance along the microchannel, which is exactly what the researchers saw in their experiments. Above a certain concentration, however, the Keller–Segel equations predict that the chemotaxis will cause bacteria to make their way through the microchannel in a pulse – which is exactly what the team saw.

Collective benefits

Lambert and Liao told physicsworld.com that they believe that the ability to beat ratchets could have important implications for our understanding the behaviour of some micro-organisms. For example, the ability to remain trapped until their numbers reach a critical level could benefit those micro-organisms whose survival hinges on collective behaviour.

“In other words, we could ask whether the trapped state is ‘a failure to escape’ or instead a ‘success’ in remaining sheltered,” say the researchers.

According to Lambert and Liao, natural environments such as tissue could include a variety of asymmetric barriers and therefore trapped and travelling configurations of micro-organisms could occur in biological systems.

The discovery could also have important implications for the design of “cancer traps” – ratchets that could someday be placed in the body to prevent the migration of cancer cells. “Our results with bacteria lead us to ask whether cancer cells might achieve densities sufficient for escaping the traps,” say Lambert and Liao.

Breeding better escapers

The team is now planning to find out if it can breed a strain of bacterium that is better at escaping the ratchets than the wild-type strain used in previous experiments. “Our system provides a relatively simple selection pressure, which could allow us to relatively easily select only for cells which can escape the funnels,” the researchers say.

Jane Hill, an environmental microbiologist at the University of Vermont in the US, describes the work as “interesting” and suggests that it could be relevant to the large intestine, where bacteria may be trapped between structures called microvilli. However, she points out that microvilli are larger than the ratchets in the Princeton experiment.

The work is described in Phys. Rev. Lett. 104 168102.

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