Archive for the ‘Mean machines’ Category

The waves beneath the sea

Tuesday, October 14th, 2008

dead-water.jpg

Dead water is the curious phenomenon when ships become sluggish and difficult to control in stratified waters in which a fresh layer sits on top of salty water. Such conditions often occur in arctic regions where water run off from melting glaciers or ice flows can float on top of denser salty water.

The effect was first noted by the Norwegian explorer Fridtjof Nansen in 1893 who noted that while his boat, Fram, could cruise easily at 7 knots in ordinary seas, in dead water she was unable to make 1.5 knots. “When caught in dead water Fram appeared to be held back, as if by some mysterious force,” he wrote.

Now Romain Vasseur and pals from the University of Lyon in France show how the effect is even more pronounced when three layers of water are involved: a fresh layer sitting on a salty layer sitting an even saltier layer.

They have even made a rather beautiful video showing how a toy boat is dramatically slowed by the effect.

The explanation is that movement of the boat causes a wave to form beneath the surface at the interface between the fresh and salty waters. This wave eventually catches up with the boat and breaks, dragging the boat to a halt.

What’ s fascinating is that while all this is going on beneath the water, the surface remains absolutely flat.

Presumably these guys have posted this paper in anticipation of the Gallery of Fluid Motion 2008 at the upcoming meeting of the APS Division of Fluid Dynamics in San Antonio in November.

Ref: arxiv.org/abs/0810.1702: Dead Waters: Large Amplitude Interfacial Waves Generated by a Boat in a Stratified Fluid

 

The first printed plastic magnetic field sensors

Thursday, October 2nd, 2008

 plastic-magnetoresistance.jpg

Conducting polymers just keep getting better. This week, Sayani Majumdar at Åbo Akademi University in Finland and pals say they’ve used using an inkjet printer to print a plastic circuit onto a plastic substrate that clearly shows magnetoresistance at room temperature.

That means they can print plastic microchips capable of sensing magnetic fields.  Cool, huh?

Ref: arxiv.org/abs/0809.3864: Towards Printed Magnetic Sensors Based on Organic Diodes

Forget black holes, could the LHC trigger a “Bose supernova”?

Monday, September 29th, 2008

lhc-higgs

The fellas at CERN have gone to great lengths to reassure us all that they won’t destroy the planet (who says physicists are cold hearted?).

The worry was that the collision of particles at the LHC’s high energies could create a black hole that would swallow the planet. We appear to be safe on that score but it turns out there’s another way in which some people think the LHC could cause a major explosion.

The worry this time is about Bose Einstein Condensates, lumps of matter so cold that their constituents occupy the lowest possible quantum state.

Physicists have been fiddling with BECs since the early 1990s and have become quite good at manipulating them with magnetic fields.

One thing they’ve found is that it is possible to switch the force between atoms in certain kinds of BECs from positive to negative and back using a magnetic field, a phenomenon known as a Feschbach resonance.

But get this: in 2001, Elizabeth Donley and buddies at JILA in Boulder, Colorado, caused a BEC to explode by switching the forces like. These explosions have since become known as Bose supernovas.

Nobody is exactly sure how these explosions proceed which is a tad worrying for the following reason: some clever clogs has pointed out that superfluid helium is a BEC and that the LHC is swimming in 700,000 litres of the stuff. Not only that but the entire thing is bathed in some of the most powerful magnetic fields on the planet.

So is the LHC a timebomb waiting to go off? Not according to Malcolm Fairbairn and Bob McElrath at CERN who have filled the back of a few envelopes in calculating that we’re still safe. To be doubly sure, they also checked that no other superfluid helium facilities have mysteriously blown themselves to kingdom come.

“We conclude that that there is no physics whatsoever which suggests that Helium could undergo
any kind of unforeseen catastrophic explosion,” they say.

That’s comforting and impressive. Ruling out foreseen catastrophies is certainly useful but the ability to rule out unforeseen ones is truly amazing.

Ref: arxiv.org/abs/0809.4004: There is no Explosion Risk Associated with Superfluid Helium in the LHC Cooling System

The fine line between the visible and invisible

Monday, September 22nd, 2008

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The man who built the world’s first invisibility cloak is back and this time he’s got an even better idea.

His first design was a triumph for headline writers and Harry Potter fans alike, although most glossed over the fact that this first cloak worked only for microwave-sensitive eyes and even then only at a single specific frequency. Oh, and only in two dimensions. And it wasn’t really a cloak at all, more of an invisibility canister.

Nevertheless, nothing should be taken away from the technical achievements of David Smith and colleagues at Duke University in North Carolina. They’ve done a job almost as spectacular as their PR team.

The new idea gets around one of the most pressing problems associated with invisibility cloaks which is that they are impossible to construct well. Invisibility cloaks work by distorting the permeability and permittivity of the cloaking material in way that forces light to bend around an internal cavity. This makes the cavity invisible to an observer.

But the technique requires the permeability and permittivity to take infinite values at certain points, particularly on the boundary between the cavity and and the cloaking material . And this just isn’t possible.
Various ideas have been proposed to get around this problem but all have their own weaknesses.

So Smith and colleagues say they might as well accept that an invisibility cloak cannot be perfect and use it to their advantage. Instead of attempting to hide the internal cavity completely or crushing it to a point as others have done, the new idea is to make it appear as a single line. Such an invisibility cloak wouldn’t hide an object entirely but instead make it look like a thin line, like a defect in the structure of the cloak.

That’s clever because it dramatically relaxes the constraints placed on the types of metamaterials you can use for cloaking. Smith and his buddies say that such a cloak would be “very easy to realize” using known techniques.

Smith is known for publishing theoretical predictions just ahead of the practical realisation.

So if his past form is anything to go by, we can expect to see a working invisibility cloak that employs this technique in the coming weeks or months.

Ref: arxiv.org/abs/0809.2317/: Invisible Cloak With Easily-Realizable Metamaterials

The incredible climbing droplets

Wednesday, September 17th, 2008

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Here’s a curious finding from the University of Bristol in the UK.

Place a droplet onto an inclined plexiglass sheet and shake it up and down. I know what you’re thinking: even without the shaking the drop should dribble down the plate due to gravity unless it is pinned in place by surface tension. Vertical shaking should loosen the drop’s “grip” on the surface and so enhance the speed at which it dribbles. Right?

Actually, exactly the opposite, say Philippe Brunet and pals.  The drop starts to climb up the plate.

Brunet and friends think the shaking introduces a non-linear effect in the frictional force between the drip and plate because of the way the surface area of the drop in contact with the sheet changes with time. So there is less resistance to its upwards motion than its downward progress.

Interesting, no? And useful too, they say.  Engineers need to know how to move droplets around inside microfluidic devices and this could give them an extra tool to play with.

Ref: arxiv.org/abs/0809.1962: Vibration-induced climbing of drops

Nanotube springboard is world’s most sensitive weighing scales

Tuesday, September 16th, 2008

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Vibrating springboards have long been the darlings of nanomechanics wanting to measure the mass of small things.

Their thinking goes like this: a springboard vibrates at a specific resonant frequency that depends on its stiffness and mass. So you can work out the mass of anything that becomes stuck to the springboard by measuring any change in its resonant frequency.

Various groups have used this idea to detect all kinds of organic and inorganic molecules using springboards carved out of silicon.

But improving the sensitivity even further means reducing the mass of these springboards. The question is how.

The answer is provided today by Alex Zettl and his team at the University of California, Berkeley, who have created a springboard out of a single carbon nanotube. And their machine is one helluvan elegant device.

For starters, they exploit nanotubes’ unusual ability to act as radio transmitters to determine how fast it is vibrating. They zap the nanotube with radio waves and listen out for the radio signals it emits in return. This signal tells them how fast the nanotube is vibrating.

And because nanotubes are four orders of magnitude lighter than silicon cantilevers, they are four orders of magnitude more sensitive to mass.

All that adds up to device that is able to measure the mass of individual gold atoms as they settle on to its surface. “The sensitivity of our device is 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator,” say Zettl and buddies.

And if that doesn’t impress you, how about this: their measurements were made at room temperature rather than in cryogenic conditions.

The team points out that their device works as a unique kind of mass spectrometer: it is compact, does not require powerful magnets and can easily be built into a chip. Expect to see more of them.

Ref: arxiv.org/abs/0809.2126: An Atomic-Resolution Nanomechanical Mass Sensor

First printed graphene circuits

Monday, September 15th, 2008

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The world of solid state electronics is in awe of graphene. This single layer of carbon chickenwire has the potential to revolutionise electronics (and much else) because it has enviable electronic, mechanical and thermal properties that no other material can match.

The news today is that Ellen Williams and buddies at the University of Maryland at College Park have worked out how to use graphene in printed circuits. The technique is conceptually straightforward: simply stamp the graphene onto a plastic circuit board in the required shape and hope it sticks. Then stamp other components on top of it. And voila! A printed graphene circuit.

Williams and her team have wasted no time demonstrating the technique, by printing a field effect graphene transistor onto a plastic substrate. “This represents the ultimate extension of printing technology to a single atomic layer,” they say.

Engineers are going to be fascinated by the properties of these transistors, not least because they are almost perfectly transparent at optical frequencies.  Anybody think why that might be useful?

Of course, there is much work to be done, not least in making graphene itself. At the moment, Williams (and almost everyone else) can only get graphene by chipping away at a block of graphite and hunting for graphene in the debris.  That’s a technique even Fred Flintstones would recognise. Any advance is going to be hugely welcomed.

Ref:  arxiv.org/abs/0809.1634: Printed Graphene Circuits

The LHC: let the lead fly

Wednesday, September 10th, 2008

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There’s no doubt that protons will be the stars of the show when the LHC switches on this morning. But in all the fuss it’s easy to forget that the machine is designed to carry other particles too.

So Paolo Giubellino at the National Institute of Nuclear Physics in Turin, Italy, outlines what to expect next year when the machine starts smashing together nuclei of lead. The idea is to reproduce the conditions that existed in the first three minutes after the Big Bang.

Giubellino is unusually optimistic about the results. He says that every new generation of heavy ion collider as immediately produced  revolutionary  data and, in a strange application of logic, says we should therefore expect the same from the LHC

The first results obtainable with Heavy-Ion beams at the LHC will qualify it as a discovery machine, capable to provide fundamental new insight to our knowledge of high-density QCD matter

Let’s hope he’s not tempting fate.

Ref: arxiv.org/abs/0809.1062: Heavy Ion Physics at the LHC

The amazing powers of silicon carbide

Monday, September 8th, 2008

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Silicon carbide is one of those workhorse materials that can do almost anything. Because it has a high melting point, it is used in high performance brake discs, as a the matrix for particulate filters in engines, and because it is a semiconductor in high temperature and high voltage applications.  Come to think of it, silicon carbide is also used as an abrasive in sand paper.

Today you can add terahertz generator to the list. Jared Straight and pals from Cornell University bombarded silicon carbide with infrared light pulses and found that it in response, it emitted coherent radiation with a frequency of between 1 and 6 Thz.

That’s handy because the current best way to make terahertz radiation in this way is to bombard galium arsenide or zinc tellurium with infrared light. But both of these material are relatively fragile.

Silicon carbide, on the other hand, is unusually tough and so could lead to terahertz devices that are much more robust.

Ref: arxiv.org/abs/0809.0756: Emission of Terahertz Radiation from SiC

Why aluminum should replace cesium as the standard of time

Monday, August 25th, 2008

micromagic-clock

The second is defined as 9,192,631,770 vibrations of a cesium atom and measured in a device known as a fountain clock. These work by cooling a tiny cloud of cesium atoms to a temperature close to zero, tossing it up in the air and zapping it with microwaves as it falls.

Then you watch the cloud to see if it fluoresces. This fluorescence is maximised when the microwave frequency matches a hyperfine transition between two electronic states in the atoms, at exactly 9,192,631,770 Hz.

Various labs around the world use this method to run clocks with an accuracy of around 0.1 nanoseconds per day. That’s impressive but not perfect. Fountain clocks have one drawback: the clouds of cesium tend to disperse quickly and that limits how accurately you can take data.

Now there’s a new kid on the block which looks as if it’s going to be better at keeping time.

Today some chaps from the the University of Nevada in Reno and the University of New South Wales in Sydney outline a new clock that relies on an effect called the Stark shift in which a spectral line is split by an electric field (this is the electric analogue of the Zeeman effect in which spectral lines are split with a magnetic field).

This is a complex phenomenon but the key thing is that the same electric field can influence the split in different ways. In fact, a couple of groups have recently discovered that in certain circumstances these can cancel out each other at specific “magic” frequencies of an electric field. When that happens, the line splitting vanishes.

This should be pretty straightforward to measure. The electric field is supplied by trapping the atoms in a standing electromagnetic wave, otherwise known as a standard optical lattice. Then change the laser frequency while looking at the atomic spectra. When the line splitting vanishes, you’ve hit the magic frequency.

The big advantage of this method is that you can trap millions of atoms easily in an optical lattice and that should make such a clock much more robust than a fountain, while achieving at least the same kind of accuracy.

So what kind of atom should we choose to sit at the heart of these “micromagic clocks”? The Ozzie-American group says that, contrary to previous reports, cesium does not have a magic frequency and so can’t be used in this technique. Aluminum, on the other hand, should be perfect.

The second is dead, long live the second.

Ref: arxiv.org/abs/0808.2821: Micromagic Clock: Microwave Clock Based on Atoms in an Engineered Optical Lattice