Archive for January, 2008

Why silos burst

Thursday, January 31st, 2008

Force chain

Believe it or not, grain silos are interesting structures. They’ve been known to explode without warning, which is hard to explain since they are filled with, well, grain.

But grain turns out to be kinda interesting too. In recent years, researchers have begun to get a handle on some of the strange and counterintuitive ways in which grain behaves as it flows and as it is placed under pressure.

One of the most interesting developments has been the discovery of “force chains”, networks of particles that form as the force is passed from one grain to the next (see picture). In this way, forces of many orders of magnitude greater than expected can be transmitted through the medium.

John Wambaugh and colleagues at Duke University in Durham have been studying the force networks that are set up within a two-dimensional silo and how these can make the forces behave in an extraordinary, non-linear way.

When grain is added to the top of the silo, the pressure in the medium increases but goes on increasing in a non-linear way even after the addition of material has stopped before decaying, a so-called “giant overshoot” effect.

How to explain this? Usually, force chains break and reform as the pressure changes in a granular medium and this helps to spread the forces evenly within it.

But Wambaugh thinks the non-linear behaviour suggests that something else is going on. He says that in certain circumstances, the force chains become locked in place and so that the additional pressure spreads much further and deeper than usual, creating the giant overshoot.

It might also explain why silos sometimes burst unexpectedly.

Ref: arxiv.org/abs/0801.3387: Force Networks and Elasticity in Granular Silos

Australians make interstellar hologram

Wednesday, January 30th, 2008

Interstellar hologram

Measure the beam from a pulsar for an hour or so and you’ll see all kinds of interference fringes in amongst the noise.  This intereference is caused by light scattered from the interstellar medium, probably in the form of whisps of  gas and dust although nobody knows for sure.

There’s all kinds of infromation locked in this data. In fact, this interference pattern can be thought of as a kind of  hologram of the interstellar medium. But to unlock it, to reconstrcut the image from the interference  data, you’ve gotta know the structure of the light field before it was distorted and this depends on thousands of unknown parameters.

A few astrophysicists have tried to reconstruct these holograms by guessing what these parameters should be but their results have been predictably poor.

Now Mark Walker at the University of Sydney and a few cobbers have made a dramatic improvement by developing an algorithm that can simultaneously optimise the eight thousand coefficients that describe the electric field.

This has allowed them to reconstruct the field and create an image of the interstellar medium. It’s impressive work.
Having done that, Walker and co say the information can be used to correct other observations of the  such as timing measurements on the pulsar beams. And it might also give an idea what’s doing all this scattering and how it changes over very short time scales.
Cool stuff.

Ref:  arxiv.org/abs/0801.4183: Interstellar Holography

How to reduce extremism? Travel!

Tuesday, January 29th, 2008

Extremism

Andre Martins studies agent-based computer models of extremism at the University of Sao Paulo in Brazil.

We’ve heard from him before following his claim that extremism is an emergent phenomenon in our society.

Now he’s back with the results of a study on how to reduce extremism.

Martins creates a network model in which agents can hold any position on a continuous scale of opinion. Each agent updates its opinion using a simple calculation after observing the opinions of others nearby.

Martins defines extremism as “an agent who supports one choice fervently, even when a large group
believes a different idea to be a better choice”. One of the impressive aspects of Martins’ model is that extreme behaviour emerges naturally, just as it does in real societies.

Now he has studied ways in which extremism can be reduced. He offers tantalising evidence that extremism is linked to the structure of a society because different types of networks produce different levels of extremism.

But his most interesting conlcusion is that the mobility of agents within a network is crucial:

“The extremism problem can become far less important in societies where the mobility of its agents is above a certain threshold. Therefore, efforts to reduce such a mobility can have important negative impacts in the diminishing of extremism.”

So get moving.

Ref: arxiv.org/abs/0801.2411: Mobility and Social Network Effects on Extremist Opinions

Extreme ice and the blues

Monday, January 28th, 2008

Ice

There are 15 different types of ice known to science and I’m not talkin’ Baskin Robbins here. These are materials with different structures that form when water freezes at various temperatures and pressures. Types XIII and XIV were only discovered in 2006

Most ice we come across naturally is type I, which forms at ambient pressures and we understand many (but not all) of its properties pretty well. But other phases of ice, which although they form at higher pressures, can be stable at ambient pressures.

So what of the properties of these ices? Renjun Xua and colleagues from the National Laboratory of Superhard Materials at Jilin University in China, have calcuated the optical properties of ices X, XI and (the still theoretical) XV.

They say that the optical properties of these materials are significantly blue shifted and that the range of frequncies at which they absorb and reflect light become broader.

The group hints that these qualities should be taken into account by climatologists trying to understand the effects of ice on our climate.

This is a disingenuous attempt to jump on the climatology bandwagon. Although ice covers 5 per cent of the surface of Earth and has covered much more during the various ice ages in the past, there is no indication that this ice is anything but phase I.

Why bother making this link when a much better one would be to the study of other planets and moons where ice forms in much more extreme conditions?

Ref: arxiv.org/abs/0801.0187: Ab Initio Investigation of Optical Properties of High-Pressure Phases of Ice

In case ya missed ‘em…

Sunday, January 27th, 2008

…this week’s posts

The mysterious volume of a black hole

Fractal fingers and zero surface tension

Extragalactic meteor spotted over Russia

Worm tracking: never lose another nematode

Soliton attacks and freak waves

Shades ‘n’ shadows

Saturday, January 26th, 2008

The best of the rest from the the physics arXiv:

Extinction Risk and Structure of a Food Web Model

The Price of Anarchy in Transportation Networks

The Earth System Grid: Supporting the Next Generation of Climate Modeling Research

Structural Motifs of Biomolecules

Robustness of the European Power Grids under Intentional Attack

Soliton attacks and freak waves

Friday, January 25th, 2008

Soliton attack

I was observing the motion of a boat which was rapidly drawn along a narrow channel by a pair of horses, when the boat suddenly stopped – not so the mass of water in the channel which it had put in the motion; it accumulated round the prow of the vessel in a state of violent agitation, then suddenly leaving it behind, rolled forward with great velocity assuming the form of a large solitary elevation, a rounded, smooth and well defined heap of water which continued its course apparently without change of form or diminution of speed. I followed it on horseback, and overtook it still rolling at a rate of some eight to nine miles per hour, preserving its original figure some thirty feet long and a foot to a foot and a half in height.
John Russell

Report of the 14th Meeting of the British Association for the Advancement of Science, 1844.

This is Russell’s famous description of the formation of a soliton, a phenomenon that has been well studied since then.

But what of Russell’s contention that, as the soliton formed, the boat suddenly stopped? An attempt in 1995 to recreate this effect failed but there are numerous anecdotal accounts of boats appearing to hit non-existent barriers.

Now Stanyslav Zakharov and Alexey Kryukov from the Lebedev Physical Institute at the Russian Academy of Sciences in Moscow have put the phenomenon on firmer theoretical footing and added a little intrigue of their own.

They say that there are two solutions to this problem that depend very sensitively on the width and depth of the reservoir in which the ship is moving. In one case an ordinary soliton is formed without the ship experiencing a hydrodynamic shock. In the other much rarer case, a different kind of soliton forms along with the hydrodynamic barrier that decelerates the boat. Zakharov and Kryukov call this a “soliton attack”.

They suggest that the phenomenon can occur in open sea when the water near a ship becomes unexpectedly shallow and might explain the formation of freak waves that are sometimes reported.

More intriguingly, Zakharov and Kryukov suggest that a similar phenomenon may occur when particles interact with electromagntic waves in specific circumstances.

So if you’ve  seen an electron stopped dead in its tracks, let us know. You may have been the first to witness an electromagnetic soliton attack.

Ref: arxiv.org/abs/0801.2295: Ship-induced Solitons as a Manifestation of Critical Phenomena

Worm tracking: never lose another nematode

Thursday, January 24th, 2008

C. elegans

The nematode Caenorhabditis elegans is a much loved workhorse in many biology labs. This worm may only be 1mm long and move at snails pace but it is one of the most heavily studied organisms on the planet.

C. elegans was the first metazoan to have its genome sequenced. We know that a fully grown adult consists of 959 cells, 302 neurons, 5000 chemical synapses, 600 gap junctions and 2000 neuromuscular junctions. We even have a full wiring diagram.

And despite it’s simiplicity, C. elegans demonstrates a rich variety of behaviours, says Nektarios Tavernarakis,  a biobod at the Institute of Molecular Biology and Biotechnology in Crete and his mate George.  These include elaborate responses to all kinds of stimuli such as heat, light, touch and chemicals.

But to properly study C. elegans, researchers have to monitor and characterise the organisms’ movements.

So Nek and George have developed a worm tracking algorithm that spots nematode worms in an image and then follows them as they race across the field of view. They call their system Nemo (from NEmatode MOtion, geddit?).

This automates the tedious taks of characterising  nematode locomotion. You can almost hear grad students all over the planet sinking to their knees to give thanks.

Ref: arxiv.org/abs/0801.1012: Nemo: a Computational Tool for Analyzing Nematode Locomotion

Extragalactic meteor spotted over Russia

Wednesday, January 23rd, 2008

Intergalactic meteor

On 28 July 2006, Victor Afanasiev from the Russian Academy of Sciences observed the spectrum of a faint meteor as it burned up in the Earth’s atmosphere. He recorded the event using a 6 metre telescope in the remote Zelenchuksky region of Russia near the border with Georgia.

It soon became clear to Afanasiev that this was no ordinary meteor. It hit the atmosphere at 300 kilometres per second, an order of magnitude faster than most other particles.

That’s puzzling. The Earth moves around the galactic center at about 220 km/s and so the meteor’s origin cannot easily be explained by reference to the Milky Way.

So where did it come from? Afanisiev and a few pals worked out that it appeared to come from the direction in which the Earth and the Milky Way is travelling towards the centre of our local group of galaxies. “This fact leads us to conclude that we observed an intergalactic particle, which is at rest with respect to the mass centroid of the Local Group and hich was “hit” by the Earth,” they say.

That’s an extraordinary claim. We can see alotta stuff out there beyond the galaxy but actually interacting with it ain’t common.

This meteor had other interesting properties which raise important questions. The team calculated that it must have been several centimetres in size, an order of magnitude bigger than ordinary meteors. Why so big?

The spectra shows the particle was made of iron, magnesium, oxygen, iodine and nitrogen. This kind of stuff, the metals in particular, form inside stars. How could it have ended up in intergalactic space?

Is this kinda dust evenly spread or in clumps? If it is clumpy, can we spot it? Would it, for example, show up in the WMAP images of the cosmic microwave background? That would be a wasp in the picnic basket.

What we need is more data. Anybody else seeing any intergalactic meteors out there?

Ref: arxiv.org/abs/0712.1571: Detection of an Intergalactic Meteor Particle with the 6-m Telescope

Fractal fingers and zero surface tension

Tuesday, January 22nd, 2008

Zero surface tension

Ah always thought a fingering instability was what happened after a misunderstanding on a first date.

But apparently it’s also a hydrodynamic phenomenon, when one fluid displaces another.

This kinda displacement is a complex process; so complex that in most cases it is mathematically intractable. Ya just gotta try it and see.

However, one of the few mathematically tractable examples is the 2-dimensional case of a less viscous fluid displacing a more viscous neighbor with zero surface tension.

The math predicts that fractal-like fingers of the less viscous fluid should penetrate the more viscous one.

And that’s exactly what happens. And here’s the proof, the picture above taken by Sid Nagel and colleagues at the University of Chicago. The grey area is tiny glass beads that have been sandwiched in a thin layer between two flat plates. The black area is air injected into the plates.

The glass beads behave like a liquid but because there are no forces between them, the surface tension is zero. And the air indeed displaces its more viscous neighbor forming a fractal pattern, just as the math predicts.

A neat experiment. Next up, those awkward Prom moments.

Ref: arxiv.org/abs/0712.2019: Toward the Zero Surface Tension Limit: The Granular Fingering Instability