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Going with the flow

When two fluids that don’t mix – or are ‘immiscible’ – are placed in a vessel, an interface develops between them. Prins and co-workers based their device on the behaviour of the interface – or meniscus – between a conducting fluid and an insulating fluid inside a cylindrical channel with a diameter of just 0.35 millimetres. They found that the tension in the meniscus and the tension in the interface between the conducting fluid and the channel wall – which has a water-repellent coating – compete with each other. The tensions can be adjusted electrically relative to one another to modify the behaviour of the fluids.

Electrodes embedded in the walls of the channel create a potential difference between the wall and the conducting fluid. The charges in the channel wall attract the conducting fluid, reducing the tension of the interface between them. The conducting fluid therefore flows along the inner wall of the channel, reaching speeds of several centimetres per second. The channel’s effective diameter falls and this ‘electrocapillary’ effect causes the insulating fluid to flow along the centre of the tube.

The new device is made up of thousands of these tiny cylindrical channels, which can be selectively operated. It is unaffected by gravity and meets many other criteria for a commercially viable device – it is electrically controlled, reversible, quick to respond, and uses little power.

Existing fluid control techniques have limited the miniaturization of inkjet printers. “Electrocapillary technology will allow more nozzles to be integrated into a single printer head, increasing resolution and printing speeds”, Prins told PhysicsWeb. The device can also act as an optical filter because it is easy to selectively fill or empty the channels. “It could switch optical signals in telecommunications systems or spatially filter x-rays for improved image quality and a lower radiation dose in medical x-ray imaging”, says Prins.

Superconductivity: it’s in the genes

Following on from the discovery that carbon nanotubes can act as electrical wires, Kasumov showed two years ago that these rolled up sheets of graphite atoms lose their resistance when connected to superconductors. Now Kasumov has shown that this is also true for DNA by connecting double-stranded DNA molecules to rhenium and carbon superconducting electrodes 0.5 µm apart. By cooling the electrodes to below their superconducting transition temperatures, the researchers observed so-called ‘proximity induced’ superconductivity in the DNA.

Evidence for electrical conductivity in DNA molecules has been inconclusive until now. Optical experiments have shown that a transfer of charge may be possible in such molecules. But the message from transport measurements has been mixed: some have indicated that DNA could be a conductor while others suggested that DNA is an insulator. Kasumov and colleagues have found that above 1 K, the resistance per molecule is less than 100 kilo-ohm, a figure that varies weakly with temperature and is an order of magnitude lower than previous measurements. Even at very low temperatures, the researchers found that DNA molecules can conduct ohmically over distances of a few hundred nanometres.

However, the physical mechanism responsible for conduction in DNA remains unclear and it is possible that the contacts act as strong dopants of electrons or holes. The researchers add that conductivity measurements could in turn help biologists to look for particular sequences of base pairs within DNA molecules.

New light on dark clouds

Traditional methods of studying dark clouds study the emission spectra of the molecules that make up the clouds. However, the signal of the main constituent of the clouds, hydrogen molecules, are very weak so astronomers must rely on the signals from other – much more scarce – molecules in the clouds. But these measurements have proved to be inadequate because astronomers do not know enough about the temperature or distribution of the dust inside the cloud. Instead, the new method uses sensitive infrared cameras to measure how the clouds block light from background stars. “The understanding of the physical structure of a cold dark cloud has always been the missing link in our current picture of star formation”, Alves told PhysicsWeb.

Alves and colleagues chose a cloud known as Barnard 68 because they believed that its dense background of stars – which are mostly giant stars with well-known spectral ranges – would provide plenty of data on how the light is blocked out. Most importantly, the cloud currently shows no signs of star birth – although a third of similar clouds contain young stars – making it likely that the conditions inside it could give rise to new stars in the future.

The stars behind Barnard 68 cannot be detected at visible wavelengths, but the cloud becomes more transparent at longer wavelengths. Alves’ team compared the light from these stars, which had been ‘reddened’ by the cloud, with the light from a group of nearby ‘control’ stars. They combined their existing knowledge of the cloud with established astrophysical laws to conclude that Barnard 68 is in equilibrium – in other words, its outward thermal pressure exactly counteracts its inward gravitational pull – and that it probably has a steady magnetic field. But the astronomers believe that over time various factors – such as cooling, dissipation of the magnetic field and any increase in external pressure – could destabilize the cloud and lead to the birth of low-mass Sun-like stars.

“These measurements constitute a major breakthrough in the understanding of dark clouds”, says Alves. “For the first time, the internal structure of a dark cloud has been specified with a detail approaching our knowledge of stellar interiors”.

Photonics and particles win 2001 King Faisal prize

John is awarded the prize for his pioneering research into optical communications and photonic crystals. He proposed a new method of processing and transmitting information in communications devices and computers that uses light instead of electrons. Real devices based on John’s theory could revolutionize the telecommunications industry.

Yang is recognized for his lifelong contribution to theoretical particle physics. Yang proposed the framework for the properties of matter at very small scales and very high energies, on which modern particle physics is based. Yang shared the 1954 Nobel Prize for Physics with T-D Lee for his work on so-called gauge theories – now known as Yang-Mills theories – which demonstrate that particles with opposite spins behave differently in weak nuclear interactions.

The Milky Way’s last meal

The early hints that a small galaxy merged with the Milky Way are based on data gathered for 1500 stars as part of the Anglo-Australian Old Stellar Populations Survey (AAOSPS), carried out at the Anglo-Australian Observatory in New South Wales, Australia. Current theories of galaxy formation propose that many small galaxies came together to produce the Milky Way. But until now astronomers had found no evidence for the absorbed galaxies.

The team hopes that the study of a further 10 000 stars in the so-called thick disk of the Milky Way will strengthen its evidence. The thick disk is deeper than the well-known thin disk, which is the main feature of our galaxy. “This puffing up effect is most likely to be the result of the entry of a fairly massive satellite galaxy into the Milky Way”, says Wyse.

It is thought that the thick disk formed as the Milky Way absorbed the orbital energy of the incoming galaxy. The creation of the thick disk is thought to be the last significant change in the Milky Way’s structure. Wyse’s earlier work suggests that the thick disk formed 10 billion years ago – when the Milky Way was only a third of its current age. This places important limitations on theories of galaxy formation.

Current cosmological theories suggest that around 70% of the matter in the universe is ‘dark matter’ – material that cannot be seen, but which influences the motion of stars and galaxies. According to this view, small galaxies cluster together to form large galaxies. But this model also predicts that many small galaxies would escape the process – leaving more small galaxies than astronomers currently observe. The discovery by Wyse and colleagues of a possible galactic remnant within the Milky Way will certainly add to the debate.

How to build your own rocket

When I was first asked to review A Tribble’s Guide To Space, I thought – wrongly as it turned out – that the book was a send-up. Star Trek enthusiasts may recall that in one episode, entitled The Trouble with Tribbles, a Trader called Cyrano Jones brought a small furry animal aboard the Starship Enterprise. It proceeded to reproduce exponentially and Captain Kirk only prevented the ship from being swamped with Tribbles by using the transporter room to move them all to a rival Klingon ship – a selfish move that was not appreciated on the departing Klingon vessel. However, A Tribble’s Guide to Space really is by someone called Alan Tribble, a rocket scientist who has designed dozens of spacecraft for various missions. The title is, I suspect, merely tongue-in-cheek.

Ever since Sputnik 1 heralded the dawn of the space age less than 45 years ago, space travel has progressed at an incredible rate. Man has landed on the Moon and there have been unmanned visits to all of the planets with the exception of Pluto. Those of us who have been privileged enough to witness – or even be involved in – these missions are perhaps used to the many technical accomplishments. We accept the incredible discoveries as routine.

This book is a serious attempt to fill the gap between those people who have a mild interest in rocket science and those wanting to study the field in depth. We may forget that a generation (or more) has grown up who are perhaps more aware of the events and technology in Star Trek than they are of our own modest efforts. In A Tribble’s Guide to Space, the author has skilfully condensed the history of man’s achievements in space flight into a readable and chronological account that does not require the reader to have any specialist knowledge of the field.

Along the way he uses many comparisons to relate the science of space flight to experiences in everyday life. For example, he uses the zero or negative g momentarily felt on a rollercoaster to describe the sensation of weightlessness, and the pitching of a baseball to explain the velocity needed to send a rocket into space. He often draws parallels with the improbable technology demanded by Star Trek – before gently, but gracefully, pointing out where our knowledge of real physics prevents such technology from being built at the present time. Other chapters deal with virtually every aspect of the scientific, technological and commercial aspects of space flight.

We are told how the ancient astronomers made significant progress in understanding the motion of the planets, and it is interesting to note how well they grasped the immense distances involved. This knowledge was, however, virtually lost until the close of the Middle Ages. It was the subsequent invention of the telescope and the inspiration of mathematicians that rekindled mankind’s interest in the cosmos, as the author points out. Tribble also explains how the simple concepts of circular and elliptical orbits match those previously discovered by the early astronomers.

The book goes on to describe the technical innovations needed to secure a foothold in space. We move carefully though the logic of rocket performance and are told of the necessary advantages of multi-stage rockets, which need throttleable engines so that they do not accelerate too fast as the fuel mass depletes.

Many writers dream of a future in which people can take holidays in space. Tribble does not shirk from giving the cost of a single Space Shuttle mission, which he says is close to a billion dollars. Just imagine converting a shuttle to carry 100 fare-paying passengers – they would each have to pay $10m for the privilege of a few days of space sickness.

I have to admire any author who bravely tries to discuss the differences between the physics of Newton and Einstein without recourse to advanced mathematics. Tribble not only does this but also embraces time and length dilation – the fact that time passes more slowly and you get shorter as your speed increases – as well as the concept of gaining infinite mass as the speed of light is attained. Star ships may not be for us, which is bad news for Star Trek fans. The Enterprise would shrink as it speeds up and would need a fuel tank the size of the universe! (All of our hopes must rest with the unique properties of dilithium crystals.)

Space is not, however, an empty place. The environment is potentially lethal, and the author discusses the effects of radiation on both humans and electronics as well as the effects of tenuous gases, such as ozone, on materials. The author’s treatment is, however, realistic. He notes how most missions have succeeded thanks to proper precautionary designs. He also describes the serious biological effects of living in a zero-gravity environment for extended periods of time. Indeed, astronauts arriving at Mars might have problems exploring the planet due to their weakened state – assuming that they actually landed safely. Once again, Star Trek has the advantage: the crew on the Enterprise benefit from artificial gravity, without the complications of having to spin the craft to obtain weight via the effects of centrifugal force.

Despite its attempt to be overly politically correct by always referring to astronauts in the female gender, I found this book easy to digest and hard to put down. The author has also resisted the temptation to swamp the reader with references for further reading, which are confined instead to a well commended shortlist that will be recognizable to those in the field.

In conclusion, Tribble has drawn on his extensive experience in applied physics to create a concise book that deals effortlessly with a wide range of otherwise tricky concepts in space science and technology. It will provide a general background for those who are curious about the science and technology of space flight, and serve as an introduction for specialists seeking information about parallel fields to their own.

On the road to two-proton radioactivity

Radioactivity is the transformation of an atomic nucleus by one of several mechanisms. Henry Becquerel observed alpha particles: helium-4 clusters consisting of two protons and two neutrons ejected from heavy nuclei. Meanwhile, in beta decay, a neutron transforms into a proton by emitting an electron and an antineutrino. Gamma radioactivity rearranges the structure of the nucleus by ejecting a photon. Nuclear fission – in which a nucleus splits into two roughly equal chunks – can be regarded as the fourth mode of decay. In 1981 a fifth decay mechanism known as “proton radioactivity” was observed in which proton-rich nuclei with an odd number of protons eject the “unpaired” proton.

In the January issue of Physics World, Bertram Blank of the CEN Bordeaux-Gradignan, France, describes the recent studies by Alfredo Galindo-Uribarri at Oak Ridge National Laboratory in the US of another method of decay – two-proton radioactivity (J Gomez del Campo et al. 2001 Phys. Rev. Lett. at press).

Organic magnetism finally demystified

In the January issue of Physics World, Kazuyoshi Tanaka of the Department of Molecular Engineering, Kyoto University, Japan, describes how Bakhyt Narymbetov of the Institute for Molecular Science in Okazaki, Japan, and co-workers have succeeded in differentiating two distinct magnetic phases in a carbon-60 compound (B Narymbetov et al. 2000 Nature 407 883).

Sensors reveal the secrets of car tyres

The best place for these sensors is in the tyres themselves because this is where all the external forces on the car, with the exception of wind resistance and gravity, are felt.

In the January issue of Physics World, Franz Dollinger of Siemens Corporate Technology in Munich, Germany, explains how his team collaborated with the Technical University of Darmstadt to develop just such a system – and it is expected on our roads in a few years’ time.

Sound waves deliver a faster pint

With millions of litres of beer and lager sold every day, brewing is big business. One of the main aims of the industry is to increase production by reducing the time needed to turn the barley, hops, yeast and water into a refreshing pint. Now a team of physicists and engineers at EA Technology in Chester and UMIST in Manchester, both in the UK, has teamed up with a major brewer to investigate how ultrasound could slash days off the “de-gassing” process – the most time-consuming part of the brewing cycle.

Currently, it takes anything from five to seven days to produce the perfect pint from scratch. For most of that time, the beer is stored in large vessels while much of the carbon dioxide produced during fermentation is removed. To do this, brewers typically pass tiny bubbles of nitrogen gas through the liquid, which displaces the heavier carbon dioxide – a process that can take several days. Now Chris Ellwood at EA Technology together with Alexandra Clark and Peter Payne of UMIST are developing an ultrasonic “whistle” that will help beer manufacturers to de-gas their ales and lagers almost instantly.

The stainless steel whistle is 30 cm long and has a nozzle through which carbonated liquid is forced. The whistle also has a reed that vibrates to produce sound waves at ultrasonic frequencies. As the sound wave compresses and rarefies, gas that is dissolved in the water diffuses into the micron-sized carbon-dioxide bubbles, causing them to expand. These large bubbles then simply float to the top of the liquid.

Wet your whistle

It sounds simple, but the device must be carefully designed so that the vortices that are shed by the fluid as it flows through the nozzle excite the reed at ultrasonic frequencies. Moreover, for the gas to diffuse into the bubbles, the intensity of the resulting sound wave must be above a certain threshold that depends on the bubble size and the frequency. If the intensity is too high, then a process called transient cavitation – best known for causing damage to the propellers of ships – can occur. In this case, the sudden growth and collapse of bubbles when they reach regions of high pressure leads to extreme pressures that could corrode the metal surfaces of the whistle and the vessel holding the liquid.

In preliminary tests with carbonated water, Clark has shown that the whistle can remove up to 40% of the carbon dioxide as 110 litres of liquid rushes through the whistle every minute. Further improvements are expected when the design of the nozzle and reed is optimized. The results were presented at the Institute of Physics Physical Acoustics Group conference at the end of October last year.

The whistle developed by EA Technology is also attracting the attention of soft-drinks manufacturers. Currently, the amount of carbon dioxide in fizzy drinks can be controlled by cooling the pipes and other components in the processing stream. Ellwood predicts that the ultrasonic whistle could eliminate the need for costly refrigeration equipment that is also expensive to run.

Crystal growth

A more sober application of ultrasound is in the pharmaceutical industry, where sound waves are improving the quality of crystals produced from solution. Crystallization is a complex affair with few ways of controlling the process on an industrial scale. However, Peter Cains and co-workers at AEA Technology in Harwell, UK, have developed an ultrasonic technique that can speed up crystallization and allow manufacturers to tailor the size of their crystals (see www.aeat.co.uk/sono/index.html).

The size and shape of crystals affects the rate at which they dissolve – an important factor in the pharmaceutical industry. In general, crystals are made by dissolving increasing amounts of a solid compound in a liquid until the solution is saturated. The solution is then cooled slowly until the first few atoms arrange themselves in an orderly crystalline pattern. The crystals grow as more atoms gather at these so-called nucleation sites. With a quick and powerful blast of ultrasound, however, the first crystals can form at higher temperatures, thereby speeding up the process.

Alternatively, manufacturers can forgo speed and initiate crystal growth in much less concentrated solutions using ultrasound. In this case, Cains and co-workers have demonstrated that smaller numbers of larger and purer crystals can be produced (see figure). They have also found that additional ultrasound pulses generate secondary nucleation, which leads to smaller, more uniform crystals that are ideal for certain pharmaceuticals that are later dissolved in water.

The ultrasound causes bubbles a few tens of microns wide to form in the liquid, which then expand and collapse violently. These collapsing bubbles act as sites where new crystals can form and grow. But no one really understands what makes such cavitation initiate crystal growth. It could be the pressure changes inside the liquid or the extremely high temperatures that are reached inside the bubble when it is compressed (see Physics World May 1998 pp38-42).

Cains and co-workers are now investigating compounds that are difficult to crystallize by conventional methods, such as sugars and proteins, and the food industry starting to take an interest in the work. Chocolate manufacturers, for example, have previously used sound waves to control the size and type of crystals formed as the molten confection solidifies.

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