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Physicists hit the rippled road

Unpaved roads around the world are plagued by surface ripples — called washboards — that are several centimetres high and formed under the rolling wheels of cars, buses and lorries. Unfortunately, numerous attempts by road engineers to find out how washboards could be avoided have met with little success. Indeed, the only practical way of dealing with these ripples is using heavy machinery to smooth-out road surfaces on a regular basis, which can be a very costly exercise in the remote regions where unpaved roads are usually found.

Now Jim McElwaine of Cambridge University, Nicolas Taberlet of the École Normale Supérieure de Lyon and Stephen Morris of the University of Toronto have devised a very simple experiment to study ripple formation. The researchers placed a flat bed of sand on a round table, which could be rotated at a constant rate of about 0.6 revolutions per second. A rubber wheel could be lowered onto the moving sand, where it could move up and down according to the level of the sand (see Ripples in the sand).

The researchers found that the wheel typically caused a small single ripple to form at one location in the sand after about ten or so rotations of the table. New ripples then grew rapidly from the single ripple and spread until the entire path of the wheel was covered in ripples.

The experiment was repeated using a different type of sand and again using long-grain rice. To their surprise, the researchers discovered that changing materials had little effect on ripple formation – suggesting that washboards cannot be avoided by using a specific type of material in road construction.

Indeed, the experiments and related computer simulations revealed that ripple formation is governed only by the speed of the wheel, its weight per unit width and the density of the granular material. Ripples were not seen when the wheel was kept below a critical speed – about 8 km/h for a car – leading the team to conclude that at higher vehicular speeds a flat road is unstable and will quickly become rippled. The study also suggests that heavier wheels will produce smaller ripples because their greater mass inhibits the vertical motion required to make a washboard.

According to McElwaine, ripple formation begins because no road surface is perfectly flat. When a wheel encounters a random hump in the road, it rises up and falls back down, pushing a bit of material out of its way to create a trough and a second hump. Subsequent wheels continue this process and a pattern of regular humps and troughs emerges.

McElwaine believes that the emergence of ripples could be delayed by making make the road as smooth as possible. Indeed, in their experiments, it took hours for ripples to emerge if the sand bed started off being extremely smooth.

Another way of avoiding ripples, according to McElwaine, is the development of active vehicle suspension systems that prevent the wheels from going up and down at frequencies that correspond to ripple creation – something that the team are currently investigating.

Several movies of the experiment can be viewed online.

‘Cosmic train wreck’ stumps dark-matter physicists

Most physicists think dark matter exists because large structures in the universe appear to be held together by the gravitational attraction of much more mass than we can see through telescopes. One way to test theories of dark matter is to study cluster mergers, which are collisions between galaxy clusters after they have steadily gravitated towards each other. Cluster mergers are also a testing ground for alternative theories of gravitation, such as modified Newtonian dynamics (MOND), that eschew the possibility of dark matter altogether.

Observations of the Abell 520 cluster by Andisheh Mahdavi and colleagues at the University of Victoria, together with Peter Capak from the California Institute of Technology, however, seem to be inexplicable using either dark-matter or alternative-gravity theories.

The researchers used data taken from the Canada-France-Hawaii telescope and the Subaru telescope in Hawaii, along with data from the Chandra X-ray telescope, to see how gravity in the Abell 520 cluster acted as a lens to bend light passing through it on the light’s journey to Earth. Using this “gravitational lensing” technique, they mapped the distribution of the three components of the cluster: galaxies, prevalent hot gas and dark matter.

Mahdavi and colleagues discovered a core of dark matter and hot gas, with a bound group of galaxies separated to one side. This goes against accepted “collisionless” dark-matter theories because both the galaxies and the dark matter should have remained unimpeded in the collision – in other words, they should be in the same place. Although the observations could be explained by using a “collisional” dark matter theory, this would not simultaneously be able to explain other cluster mergers, such as the Bullet Cluster, that are already described well by the collisionless theories.

The researchers also say that they could not account for the observations using MOND. However, Hong-Sheng Zhao – a physicist from St Andrews University in Scotland who was part of a group that explained the dynamics of the Bullet Cluster using a relativistic alternative-gravity theory called TeVeS – told physicsworld.com that this might be because current simulations of MOND tend to ignore a subtle time-dependent effect of the gravity field. By including this effect in future simulations, he says, both the Bullet Cluster and the Abell 520 cluster could have the chance to be explained with an alternative-gravity theory. “Right now it is very curious,” he said.

Helices swirl in space-dust simulations

Vadim Tsytovich and colleagues at the Russian Academy of Science along with researchers at the Max Planck Institute for Extraterrestrial Physics and the University of Sydney simulated the behaviour of mixtures of inorganic interstellar dust in a plasma, which is an extremely hot gas of charged particles. Such plasmas are common in space and can even occur naturally on Earth at the point of a lightning strike.

Dust particles in a plasma are themselves charged, which leads to electrostatic interactions between the particles. The researchers assumed that — at certain separations — two dust particles would be attracted to one another, while at other separations the particles would repel. A computer program was then used to simulate how a large number of dust particles would interact within a plasma.

The simulations suggested that under conditions commonly found in space, the dust particles first form a cylindrical structure that sometimes evolved into helical structures. Along some spirals, the radius of the helix was seen to change abruptly from one value to another and then back again, providing a mechanism for storing information in terms of the length and radius of a section of a spiral.

In some simulations, a spiral would divide into two, effectively reproducing itself. In other simulations, two spirals induced structural changes in each other, and some spirals even appeared to evolve with time into more robust structures.

While many scientists would balk at calling such structures life – if they indeed exist in the first place — Tsytovich has no doubt. “These complex, self-organized plasma structures exhibit all the necessary properties to qualify as candidates for inorganic living matter,” he said. “They are autonomous, they reproduce and they evolve”. The team has also suggested that such inorganic life could have been a precursor to organic life here on Earth.

While these specific simulations have not been verified in the laboratory, experiments by other researchers on other dust-plasma systems have revealed the emergence of simple helical structures.

As for finding inorganic life in dust clouds surrounding nearby stars, the researchers say this could be done by looking for changes in infrared light from distant astronomical objects as it passes though a cloud of spirals – a measurement that could in principle be done with NASA’s Spitzer space telescope.

Opaque lens focuses light

Optical microscopy and spectroscopy both rely on the controlled transmission of light through a sample. But if a sample is covered by an opaque layer – or indeed if a sample itself is opaque – the amount of light randomly scattered can render these techniques next to useless.

Allard Mosk and Ivo Vellekoop from the University of Twente, however, claim that they can not only circumvent the problems of an opaque medium but can actually exploit its properties to focus light to a point over a thousand times brighter than it would be when it is scattered normally.

The researchers start by expanding the diameter of the beam from a laser using a lens, and then split the cross-section into a number of segments by passing the beam through the pixels on a liquid crystal display (LCD). After focusing this expanded beam back to its normal diameter, they shine it through an opaque sample onto a digital camera.

The crucial part of Mosk and Vellekoop’s technique is their computer program, which reads the intensity of the light hitting the camera and makes corrections to the LCD’s pattern to make this intensity as large as possible. For example, if one beam segment scatters through the sample in such a way that it interferes destructively with the rest of the beam when it arrives at the camera, the program adjusts that segment’s propagation before it reaches the sample using a phase modulator. When every segment’s phase has been optimized to interfere constructively, the brightest possible image is obtained (See Opaque lens).

Mosk and Vellekoop tested their technique for several opaque samples, some of which turned out to better at focusing than others. A fresh flower petal, they found, focused the beam to an intensity about 60 times greater than the normal scattered beam. On the other hand, titanium dioxide – a white pigment and one of the most strongly scattering materials known – could intensify the beam more than a thousand times over.

In practice, such an intense beam could be scanned over a biological sample to image it in a similar manner to a scanning electron microscope by using any opaque layers of tissue covering it as the “lens”. However, the technique would still require a camera or other detector behind the sample to read the intensity for optimization. “We are starting to work on optimization using local nanoscale probes that can be put inside tissue,” Mosk told physicsworld.com.

Nanotubes guide phonons with ease

Heat can be transported through a solid by phonons, which are quantized sound waves. Some physicists believe that phonons could be used to transmit information along a fibre if a suitable material could be found. However, any fibre capable of carrying phonons would have to be just tens of nanometres in diameter. While it is very difficult to make such fibres from most materials, carbon and boron nitride nanotubes can be made at such thicknesses.

Now, Alex Zettl and colleagues at the University of California at Berkeley have shown that such nanotubes are exceptionally good at transporting phonons – even when the nanotubes are severely deformed. The researchers fixed individual carbon and boron nitride nanotubes between a suspended heat source and sink that created a temperature gradient across the nanotube (see Phonon waveguide). The individual nanotubes had diameters ranging from 10 to 40 nm and were several micrometres in length.

Bent nanotubes

This method, pioneered by Berkeley’s Arunava Majumdar, allowed the researchers to measure how much heat was transferred through a tube. The apparatus was also attached to an electron microscope so that the inner and outer diameters of the nanotube could be measured while it was being bent using a piezo-electric manipulator.

Previous work showed that deforming a multiwalled nanotube produces ripple-like structures about 10 nm in size on the nanotube’s inner radius. The researchers had expected these ripples to scatter phonons and so degrade the nanotube’s thermal conductivity. To their surprise, they found that the thermal conductivity does not change at all even when the nanotubes are drastically bent (see Bent nanotubes).

Team member Chih-Wei Chang told physicsworld.com that the result could be important for overcoming the problems of heat dissipation in microelectronic devices. “Our findings would have immediate applications for heat management since nanotubes exhibit high thermal conductivity (around one order of magnitude higher than that of silicon) and are robust against mechanical deformations too,” he said.

More importantly, the work may lead to the use of phonons as information carriers. The team has already made solid-state thermal rectifiers, tuneable thermal links and phonon waveguides using nanotubes. It is now building more “phononic” devices that are analogous to optical devices used in electronics and photonics.

Interferometry images living cells in 3D

Tiny biological samples must normally be prepared before they can be viewed in 3D. Cells, for example, often have their inner components highlighted with fluorescent dyes. But such modifications can disrupt a cell’s normal functions, limiting the possibilities for analysis.

Feld and colleagues have done away with such preparations, and instead use the optical properties of the cell in its natural state to generate a 3D image. First, a laser beam is split into two: one beam goes through the sample while the other bypasses it. The beams are then recombined and shone onto a digital camera where they produce an interference pattern.

From this pattern the US team deduce the phase difference between the two beams, which changes according to the refractive index of the material that the sample beam passes through. By mapping this refractive index, a 2D image of the cell’s interior is generated.

To get a 3D image the researchers must place a mirror in front of the sample and rotate it incrementally with a galvanometer – a device that converts a small current into a mechanical motion. For each rotation, which alters the angle of the laser beam through the sample, they record an interference pattern.

Feld and colleagues demonstrated their technique, called tomographic phase microscopy, by imaging a cervical cancer cell (See Inside a cell). For the first time, an unaltered cell’s detailed 3D structure with elements such as the nucleus can be seen.

“Accomplishing this has been my dream, and a goal of our laboratory, for several years,” said Feld. “For the first time the functional activities of living cells can be studied in their native state.”

The resolution currently stands at about 0.5 µm, but the group says it should be able to improve it to 0.15 µm or less. They expect that it will complement electron microscopy, which can probe as small as 10 nm but requires samples to be either frozen or coated in a layer of conductive material.

Physicists minimize ‘sticky friction’ in tiny machines

Stiction is a problem in nano- and micro-electromechanical systems (NEMS and MEMS) whereby the tiny components stick together, often greatly reducing the reliability and long-term durability of these devices. It occurs when capillary, van der Waals and electrostatic forces between surfaces overpower the built-in restoring forces of the overall structure. In smaller systems the effect is more pronounced because of a larger surface-area-to-volume ratio.

Practical methods to eliminate stiction-related failures involve designing devices with high mechanical restoring forces, or by using “passivants” – special treatments for reducing surface energy. Some researchers have employed time consuming and costly molecular-dynamics simulations to see how roughness affects stiction, but so far these have not given any useful insights. Liu and co-workers, however, have performed experiments that demonstrate a correlation between surface roughness and stiction – a result that they hope could be used to minimize stiction in MEMS, and possibly NEMS, components.

The US team started with a series of silicon wafers, each with a different average roughness – that is, with different sized lumps or “asperities” on the surface. They then brought the cantilevers of an atomic force microscope with various tip radii into contact with a single asperity on the surface and measured the size of the adhesive force.

The researchers found that the adhesive force falls quickly as the average roughness is increased, but reaches a minimum beyond which it steadily rises again – in other words, there exists an optimal roughness. This value increases with the radius of the cantilever tip.

According to Liu and co-workers, this is because a tip resting on a completely smooth surface is strongly attracted by the majority of the surface’s adhesive forces. A small asperity on a slightly roughened surface acts to distance the tip from the surface, so these forces are less pronounced. But too large an asperity on a very rough surface will have its own strong adhesive forces which cancel the distancing effect.

“Our work suggests a promising way to minimize adhesion between two surfaces by tuning asperity height to feature-size in MEMS devices,” Liu told physicsworld.com. “We didn’t quantify by how much stiction could be reduced, but our model can provide a useful predictor of the behaviour of adhesive contacts down to the nano scale.”

X-ray holography breaks the femtosecond barrier

In their technique, Chapman and co-workers start by firing a coherent pulse of light from the X-ray free-electron laser at the DESY lab in Hamburg through a small hole in a “detector” mirror. This pulse then encounters a thin, translucent membrane that has been covered with a sample material – in this case 140-nm-diameter polystyrene balls – that lies just in front of a second, “backing” mirror (see Quick as a flash).

If the pulse hits one of the balls, it strips the polystyrene chains of their electrons, causing the material to explode under the repulsion of the remaining positive charge. The X-rays then scatter off the ball before travelling to the backing mirror, which reflects it.

Because the pulse has a finite width, however, not all of the X-rays interact with the ball – some of them bypass it and continue in their original direction before bouncing off the backing mirror. These X-rays can then scatter off the ball, albeit a fraction of a second after the initial scattering event, by which time the ball has got bigger as a result of the explosion.

Both parts of the now-scattered pulse – known as the “reference” and “object” beams – then travel back to the detector mirror, which reflects them onto a digital camera whereupon they interfere with each other to produce a “holographic” pattern. The researchers then analyze this pattern to reveal the structure of the sample’s material and how it evolved during the explosion.

Holographic pattern

Although other X-ray holography techniques have been used as far back as the 1970s, Chapman and co-workers’ technique is much faster, having a temporal resolution of just one femtosecond. This is the timescale of atomic motion, meaning that different samples could be imaged to see the stages in, for example, chemical reactions. “This is certainly the fastest hologram ever recorded,” Chapman told physicsworld.com.

Since a relatively long wavelength of 32.5 nm was used in this experiment, the spatial resolution of the technique currently stands at 50 nm, but Chapman explains that with future, shorter-wavelength free-electron lasers it should be possible to resolve features as small as 1 nm.

The researchers say they were inspired to make holographic patterns in this way by Isaac Newton, who noticed that sunlight produced “strange and surprising” light and dark bands on a screen after he had bounced it off a mirror speckled with dust particles.

Dirac medal honours charm-quark physicists

The charm quark was predicted in 1970 by Iliopoulos and Maiani when, with future Nobel laureate Sheldon Glashow, they formulated the now-famous “GIM mechanism” in an attempt to understand the weak interaction. This quark – the fourth predicted to exist – is now known to have a positive charge of two-thirds of that of an electron. “The GIM mechanism was a seminal contribution to the developing theory of the electroweak interaction,” David Gross, a member of the Dirac medal selection committee, told physicsworld.com.

John Iliopoulos

Their theory was confirmed in November 1974 with the discovery of the J/Ψ particle – a bound state of a charm quark and a charm antiquark – at both the Brookhaven National Laboratory and the Stanford Linear Accelerator Centre in the US. The discovery persuaded many physicists for the first time to realize that quarks exist.

Maiani says he is extremely honoured to win the medal. “Dirac has been my hero since the beginning of [my] physics studies,” he said. “I will never forget the impression made upon me by the hole theory of the positron, and reading his book – together with [Richard] Feynman’s – is the way I learned quantum mechanics.”

The Dirac Medal is awarded to scientists previously unrecognized by the Nobel prize, Fields medal or Wolf Foundation prize who “have made significant contributions to physics.”

Phoenix blasts off to Mars

The $420 million Phoenix mission, the result of an international collaboration led by the University of Arizona, US, is the first project in NASA’s Mars Scout mission. It started life in 2003 as an attempt to revive the 2001 Mars Surveyor Lander, which was cancelled after the Mars Climate Orbiter and Mars Polar Lander failed in 1999. “We have worked for four years to get to this point, so we are all very excited,” said project manager Barry Goldstein at NASA’s Jet Propulsion Laboratory.

Phoenix will land using descent engines on a site in the northern hemisphere at 68.35° north latitude and 233.0° east longitude. Although these engines have not been used successfully since 1976 – NASA has recently favoured airbag systems – they enable the craft to carry the weight of seven different instruments, some of which were those mothballed from the Mars Surveyor Lander.

First, Phoenix will use a 3.35-metre-long robotic arm to dig into the surface and reach the icy layer residing a few centimetres beneath. Mounted on the end of the arm is a visible-light camera, which will provide high-resolution colour images of the soil and ice.

Samples delivered to the lander by the arm will be heated by a “thermal and evolved gas analyzer” to see how much water vapour, carbon dioxide and volatile organic compounds are contained in the soil. The samples will also be distributed to optical and atomic-force microscopes, which will examine mineral grains, while electrochemistry cells will be able to measure properties such as acidity or alkalinity, and a conductivity probe can check thermal and electrical properties.

Phoenix rises

Other instruments will look at the wider environment. During descent, a camera will record the geography around the landing site, and after Phoenix has landed a stereo camera will observe the local terrain in 3D. Finally, meteorological equipment will monitor changes in water abundance, dust, temperature and other variables.

“[Saturday’s] launch is the first step in the long journey to the surface of Mars,” said principal investigator Peter Smith of the University of Arizona. “We certainly are excited about launching, but we are still concerned about our actual landing, the most difficult step of this mission.”

NASA is still reviewing proposals for the second Mars Scout mission, due to fly in 2011.

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