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Bubble bursts for ‘sono-fusion’

Bubbles trapped in a liquid can be forced to expand and contract by firing acoustic pulses into the liquid. When a bubble expands, molecules from the surrounding liquid evaporate into it. This vapour is then compressed when the bubble contracts, and can reach temperatures and pressures that are high enough to kick-start chemical reactions and spark the emission of light – a phenomenon known as sonoluminescence.

In March, physicists in the US caused a stir when they claimed to have seen deuterium nuclei fuse in bubbles in ‘deuterated’ acetone. The team led by Rusi Taleyarkhan of Oak Ridge National Laboratory calculated that the temperature inside the bubbles must have reached tens of millions of degrees for the reaction to proceed. But many researchers working in the field dismissed these claims.

Now Didenko and Suslick have shed some light on the controversy by studying how the acoustic energy is distributed between chemical reactions, light emission and bubble collapse during sonoluminescence. To do this, they created a bubble – which was 30 µm across – in a water-filled cell, and made it oscillate using an acoustic signal with a frequency of 52 kHz.

To monitor the production of hydroxyl ions, nitrous oxide ions and photons in the bubble – which contained air and water vapour – the pair used fluorescence techniques and spectroscopy. Measurements were made at both 3 and 22 °C.

At its largest, the bubble had a potential energy of several MeV, and the researchers found that most of this energy is dissipated as shock waves and motion in the surrounding liquid. They also discovered that less than a millionth of this energy is converted into light, but about a thousandth of the energy is used to ionize the molecules of water vapour in the bubble.

According to Didenko and Suslick, this suggests that chemical reactions would soak up too much of the energy for nuclear fusion to take place, especially for bubbles in volatile liquids like acetone. The molecules of vapour in such bubbles are complex, and would absorb much more energy than the water vapour that they studied. But Suslick does concede that “the possibility of fusion occurring in low-volatility fluids – such as liquid metals and molten salts – cannot be ruled out at this time.”

There have been previous studies of the chemical reactions and light production associated with sonoluminescence, but these have focused on clouds of bubbles rather than single ones. These experiments have been difficult to interpret because it is hard to tell how many of the bubbles in the cloud are active at any one time.

US maps fusion landscape

The US was a founder member of the ITER project – along with Europe, Japan and Russia –but pulled out in 1998 when the huge cost of the scheme became clear. However, the US recently expressed an interest in rejoining the project following the design of a new reactor by the existing partners that is less ambitious – and cheaper – than the original.

At the Snowmass meeting the scientific and technical merits of ITER were compared with those of two more modest proposals – FIRE, being developed by researchers in the US, and IGNITOR, an Italian project. Physicists at the meeting concluded that all of the projects could be used to study burning plasmas – plasmas that can maintain their high temperatures largely through the self-heating of fusion reactions – and agreed that none of the projects poses serious engineering problems.

Delegates decided that IGNITOR would provide the earliest opportunity to study a burning plasma, but also noted that this project would produce such a plasma only fleetingly. FIRE and ITER would enable investigations of longer-lived plasmas, they said, with ITER aiming to demonstrate a sustained output of fusion energy.

“There are no show-stoppers for any of these devices, although there are still issues to be overcome in each case,” says Ned Sauthoff of Princeton University, one of the co-chairs of the meeting. “I’m confident that we would learn things from each of them.”

The conclusions from Snowmass will be used by an advisory committee of the US Department of Energy to decide which project or projects the US should back. Sauthoff estimates that FIRE would cost about $1.2bn and that the total cost of ITER will be around $5bn. IGNITOR, he says, would have a price tag of a few hundred million dollars.

Delegates at the Snowmass meeting also discussed inertial confinement fusion and agreed that good progress has been made in developing a number of relevant technologies: lasers, heavy ion beams, Z-pinch systems, chamber and target technology, and fast ignition. Those present noted that the US National Ignition Facility is also expected to produce a burning plasma.

DNA goes spintronic

Conventional electronic devices only exploit the charge of electrons. But much more powerful devices could be built if the ‘spin’ of electrons – which can be either +1/2 or -1/2 – could also be controlled. When the spins of electrons are aligned in one direction – or ‘polarized’ – by a magnetic field, the resistance they experience when they travel through a conductor is different to that experienced by electrons polarized in the opposite direction.

This effect is known as magnetoresistance and can be studied by sandwiching a conductor between two ferromagnetic electrodes – a device known as a spin valve. When these electrodes are placed in a magnetic field, the spins of the electrons that they contain become polarized. If a voltage is then applied across the electrodes, the polarized electrons can flow through the conductor, from one electrode to the other.

To see how polarized currents would flow through DNA molecules, Zwolak and Di Ventra considered two spin valves – one with iron electrodes and one with nickel electrodes – in which DNA was the conductor. They calculated the current that would flow through the DNA molecules when the electrodes in each spin valve were magnetized first in the same direction – or ‘parallel’ – and then in opposite directions – or ‘anti-parallel’.

The researchers found that the current flowing through the DNA in a nickel-based spin valve would increase by up to 26% when the magnetization of the electrodes was switched from anti-parallel to parallel. In an iron-based spin valve, the current would increase by up to 16% for a similar switch. This shows that electrons would be able to travel through DNA more easily when both the source and drain electrodes were magnetized in the same direction.

Zwolak and Di Ventra believe that it should be possible to observe these effects experimentally, and hope that their results will stimulate further studies into the electrical properties of DNA and their potential use in molecular electronics devices of the future.

Mobile NMR leaves the field behind

In conventional nuclear magnetic resonance – or NMR – a detector fires magnetic field pulses at a material to align the spins of certain nuclei in it. When the magnetic field is switched off between pulses, these nuclei emit ‘echoes’ that show how abundant they are in the sample. These detectors use an array of magnets to create a strong, uniform magnetic field over a significant volume into which the sample is placed.

But mobile NMR detectors cannot generate such uniform magnetic fields, which means that these devices have smaller sensitive regions and are influenced by the Earth’s magnetic field. These effects make it harder to interpret the echoes and can reduce the accuracy of the measurements.

A more promising technique for mobile devices is based on rapid reversals of the magnetic field applied to the sample. This technique enlarges the detection area of the device, but the echoes it produces only last a few milliseconds because the Earth’s magnetic field quickly misaligns the nuclear spins. This has hindered attempts to develop the technique into a practical tool.

To combat this problem, Brill and co-workers have now developed a technique that eliminates the effect of the Earth’s magnetic field by keeping the nuclei aligned for longer. Using two sets of coils set at right angles to each other, the team slowly rotated the orientation of the magnetic field and – at the same time – rapidly switched its direction.

They found that the slowly rotating field cancelled out the misalignment introduced by the Earth’s magnetic field each time the applied magnetic field switched direction. Since this technique does not depend on the ‘Larmor frequency’ of the nuclei under study, the researchers say it could also be used to detect different nuclei in a single sample.

Ultrashort pulses help gene therapy

Tirlapur and König pierced a cell membrane with a Ti:sapphire laser, allowing DNA delivery through the resulting hole – a process known as transfection. This hole quickly closed up, and the implanted cell and the new DNA appeared to be undamaged by the process.

The researchers say that this is a better method than conventional cell-perforation techniques. For instance, electroperforation cannot target individual cells, while mechanical or chemical techniques can damage either the cell or the foreign DNA.

Nanosecond laser pulses from a frequency-tripled Nd:YAG source have also been tried, but according to the researchers, this technique damages the cells beyond repair.

The femtosecond pulses were focused at the cell membrane using a high-numerical-aperture objective lens. The target cells were exposed to the 50-100 mW average power beam for 16 ms during transfection.

Having implanted the foreign DNA into the target cells, Tirlapur and König used the same set-up to study the success of the transfer. To do this, they tagged the foreign genes with green fluorescent protein. The Ti:sapphire source then generated two-photon fluorescence images of the cell expressing this protein.

The researchers say that irrespective of the type of cell implanted with foreign DNA, the femtosecond laser method worked each time. They add that the high level of selectivity prevented transfection into any neighbouring cells, and claim that there are “no detrimental effects on growth and division, and virtually no cell death”.

In gene therapy, a cell which lacks a certain gene is fixed by implanting foreign DNA that can express the missing gene. Though potentially revolutionary, the technique is also highly controversial.

Earthshine sheds light on extrasolar planets

To date, astronomers have discovered over 100 extrasolar-planets – that is, planets beyond our own solar system. Most of these planets were detected from the effects of their gravity on the motion of their parent stars, but at least one extrasolar planet has been spotted by detecting the starlight reflected from its surface.

This technique is tricky because the light reflected from such a planet is usually obscured by the glare of the parent star. But as observation techniques continue to improve, more extrasolar planets are likely to be detected in this way. Now Woolf’s team has paved the way for interpreting the spectra of visible light gathered from such planets.

Using the Steward Observatory 2.3-metre telescope at Kitt Peak in Arizona, the researchers analysed moonlight and ‘earthshine’. This is sunlight that is reflected from the Earth onto the dim portion of the Moon’s disk, and then reflected back towards Earth.

By combining the spectra of the earthshine and the sunlight reflected directly by the Moon, Woolf and colleagues produced a spectrum similar to the one that would be seen by a distant observer who could not resolve the Earth and the Moon. This is equivalent to how we would see an extrasolar planet.

The strongest signals in this spectrum arose from the ozone, water vapour and molecular oxygen present in clouds and the atmosphere. But the spectrum also revealed the existence of oceans, the phytoplankton that they contain, and land covered with vegetation. This means that an observer in a nearby stellar system could detect evidence of life from the sunlight reflected from Earth.

The researchers hope that their discovery will help astronomers to establish the properties of extrasolar planets from the visible light they reflect. They say that their study could be refined by observations of earthshine from space – which would not need to subtract the effects of the atmosphere – and longer-term studies that would account for fluctuations in the spectra.

Entanglement passes screen test

It is well known that photons can be pass through such gratings by converting into ‘surface plasmons’ – surface excitations that involve billions of electrons. Now Woerdman and colleagues believe that their experiment has revealed the quantum nature of surface plasmons – which are macroscopic objects – for the first time.

Entanglement is a feature of quantum mechanics that allows particles to share a much closer relationship than classical physics allows. A measurement on one particle in an entangled system reveals the properties of the other part, even if they are widely separated. A pair of entangled photons can be produced by ‘down-converting’ an ultraviolet photon into two infrared photons in a crystal with nonlinear optical properties. If the polarization of the first photon is horizontal, the polarization of the second photon will be vertical, and vice versa.

Woerdman and colleagues wanted to see if entangled photons with a wavelength of 813 nm could survive transmission through a gold film containing an array of holes that measured just 200 nm across. When photons hit such a subwavelength grating, they are converted into surface plasmons that can tunnel through the film and be re-emitted on the other side as photons.

When the researchers compared the counts registered by two single-photon detectors on the far side of the grating, they found that most of the photons were still entangled – despite their temporary conversion into excitations containing around 10¹⁰ electrons.

According to Woerdman and colleagues, the demonstration of quantum properties on a macroscopic scale is very significant. “In general, it is very difficult to maintain entanglement in a system that contains a great number of particles,” team member Erwin Altewischer told PhysicsWeb.

The researchers cannot yet fully explain their observations, but they hope that by combining the fields of quantum information and nanostructured metal optics, they will prompt further studies of entanglement in condensed matter systems.

Small systems defy second law

The second law of thermodynamics says that the entropy – or disorder – of an isolated system undergoing a cyclic process will increase or remain the same. But this law only applies to large systems over significant periods of time. To explain the behaviour of smaller systems, Evans and colleagues devised their ‘fluctuation theorem’, which calculates the probability that entropy will be consumed at any point in the cycle. They found that it predicted that measurable violations of the second law would take place in small systems over short time-scales.

To test the idea, the researchers put about 100 latex beads – each 6.3 µm across – into a water-filled cell, which was placed on the stage of a microscope. The researchers focused a laser onto one of the beads, which induced a dipole moment in the bead and drew it towards the most intense region of the electric field in the laser beam. The force acting on the particle near the laser focus was harmonic.

With the bead trapped, the researchers moved the microscope stage backwards and forwards repeatedly, dragging the bead in and out of the laser focus. The stage moved through 540 such cycles in ten seconds, and the team measured the position of the bead a thousand times every second. Combining these measurements with the laser power and fluid drag, Evans’ team was able to calculate the forces acting on the bead – and its entropy production – as it moved.

Evans and co-workers found that – during some trajectories – entropy was consumed rather than generated. This effect was seen when the researchers looked at the bead’s behaviour over periods of about a tenth of a second. Over periods approaching two seconds, the proportion of entropy-consuming trajectories fell, and above two seconds, none were observed. The team also found that their results closely fitted a computer simulation of the fluctuation theorem.

Evans and colleagues say that their discovery could be important in the design of nanomachines. They also point out that as thermodynamic systems become smaller, the probability that they will run ‘in reverse’ increases, and this could improve our understanding of how many small biological systems – such as ‘protein motors’ – work.

UK science nets extra cash

The review also takes into account the recommendations made by Sir Gareth Roberts in his recent review of the supply of science and engineering skills in the UK. The budget for resources that support the transfer of knowledge from the science base will be almost doubled to £114m for 2005/2006.

The Wellcome Trust – the world’s largest biomedical research charity – will also contribute an additional £280m over the next five years to support new research programmes and better training for science teachers.

Alun Jones, chief executive of the Institute of Physics, welcomes the extra cash. “The additional funds for science and education represent a great opportunity for physics and for the continued strength of the UK’s science base in the future,” he said.

The UK government conducts the review every two years to allocate the £400bn of government funds.

Silicon atoms play a bit part

“We were looking to reach the ultimate density limit of bit-wise storage in a solid, which obviously is coding the information of one bit in one atom,” says team member says Roland Bennewitz. “The atoms must have a certain distance between them so that they do not interact and thereby obscure the information. Using the presence or absence of an atom on a self-organized pattern provides these requirements in an elegant way.”

The scientists created the memory by depositing 0.4 monolayers of gold onto a Si(111) surface at 700°C. Then an annealing treatment at 850°C created the well-known Si(111)5×2-Au structure.

“The memory consists of self-organized gold wires that support the silicon atoms at regular distances,” explains Bennewitz.

The memory’s self-assembled tracks are five rows of atoms – or 1.7 nm – wide. Each bit is encoded by the presence or absence of a silicon atom inside a two-dimensional unit cell of 5 x 4 atoms. The other 19 atoms in the unit cell stop adjacent bits interacting with each other.

Data can be read from or written to the memory using a scanning tunnelling microscope, or STM. The scientists preformatted the memory with ones by the controlled deposition of silicon onto vacant sites. To write zeros, they used the STM tip to remove silicon atoms from the surface.

“It is interesting to note that the system we propose meets the ultimate density predicted by the mastermind of nanotechnology, Richard Feynman,” added Bennewitz. “In his legendary talk he came up with 5 x 5 x 5 atoms to store one bit – in our system we use 5 x 4 atoms.”

Bennewitz says that the system is stable at room temperature, which is a big step forward compared with earlier low-temperature atom-manipulation experiments. What’s more, the atoms are positioned at well-defined distances along tracks, which allows the use of systematic read-out procedures well known from magnetic hard disks.

However, there are two drawbacks: the memory needs to be prepared and preserved in a vacuum, and the writing and reading speeds are relatively slow.

“When comparing storage density and read-out speed of our atom memory with the information density and replication speed in DNA, we end up with comparable numbers,” said Bennewitz.

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