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100 years after Einstein’s landmark work on Brownian motion, physicists have discovered a new concept of temperature that could be the key to explaining how ice and snow particles flow during an avalanche, and could also lead to a better way of handling tablets in the pharmaceutical industry. This research is reported today in a special Einstein Year issue of the New Journal of Physics published jointly by the Institute of Physics and the German Physical Society (Deutsche Physikalische Gesellschaft).
Everything from powdery snow to desert sands, from salt to corn flakes are granular materials. Physicists have known for many years that granular materials have many perplexing properties that make them behave at times like solids, liquids, and even gases. This new research reveals for the first time how to measure a concept called “granular temperature” – that could be the key to explaining how they behave.
“Take the solid snow covering a ski slope, for instance”, suggests lead author of the paper Patrick Mayor of the EPFL in Lausanne, Switzerland. “While it stays still it is a solid, but as soon as it starts flowing downhill as happens during an avalanche the flowing material is behaving more like a liquid. Similarly, during a desert storm, sand grains are whipped up and behave like molecules in a gas, rather than as a solid”.
"Whereas most materials are usually described as solid, liquid or gases, granular systems do not seem to fall into any of these categories and are often considered a separate state of matter of their own," says Mayor, "The diverse behaviour of granular materials makes it extremely difficult to establish a general theory that accounts for the observed phenomena."
Mayor and his colleagues, Gianfranco D'Anna, Alain Barrat, Vittorio Loreto, have shown that shaken granular matter behaves in a way related to Einstein's theory of Brownian motion, first published in 1905.
The temperature of an object reflects the random motion of its constituent parts. For instance, the faster the molecules in a gas or liquid are moving around the higher the temperature of the material.
Temperature also measures the degree of agitation of molecules in a liquid or a gas. Mayor and his colleagues have now devised a thermometer that can measure the temperature of a granular material based on the degree of agitation of its component particles. The researchers also discovered that, unlike usual liquids, temperature varies depending on which way and how far they insert the "thermometer" into the granular material.
Being able to measure "granular temperature" might allow researchers to better understand the peculiar properties of a granular material, which is of crucial importance to industries that handle powders and particulate materials from pharmaceutical pills and food powders to sand and cement in the construction industry.
The paper was published on Monday 31st January 2005 in New Journal of Physics as part of a celebratory focus issue on "Brownian Motion and Diffusion in the 21st Century" ( http://stacks.iop.org/1367-2630/7/i=1/a=E01 ). The paper can be downloaded free of charge from 31st January at http://stacks.iop.org/1367-2630/7/28 . Reference: P Mayor et al. New J. Phys. 7 (2005) 28.
Scientists close in on “superbrakes” for cars (press release researched and written by David Bradley
for IOP)
A theoretical study of friction between solids that looks at the process just one molecule at a time could soon lead to a more effective way to stop cars in an emergency than simply slamming on the brakes or using ABS. This research is reported today in a special Einstein Year issue of the New Journal of Physics.
Scientists and engineers have assimilated an enormous amount of empirical information on the processes taking place when two surfaces rub against each other producing friction. They have even devised numerous physical rules and laws to explain these forces. These laws are adequate for most practical purposes, but according to Peter Reimann and colleagues, our understanding of how friction is traced back to the behaviour of solids at the molecular level where surfaces meet is still far from complete. He and his colleagues hope to improve our fundamental understanding of the microscopic laws governing materials in contact.
"In our work, we consider theoretically a somewhat simplified setup," explains Reimann, "This consists of a single, very small point, which is pulled over an atomically flat surface." This, Reimann explains further, is an exceptionally simple and well controlled "minimal'" system that allows he and his team to study the forces between the point and the surface. Experimentalists studying friction use a similar setup to measure the actual forces involved.
Reimann's team begins with a mathematical description of the system that takes into account the forces between the point and the surface at the microscopic level as the point is drawn across the surface. They found that their model could explain previous experimental findings confirming its validity. However, they have also drawn a surprising conclusion. The model suggests that the frictional force increase as the point begins to move, then reaches a maximum as it speeds up, and then falls if the point continues to be accelerated across the surface.
"We find this prediction quite surprising and experimentalists have already signalled their excitement to test it in their labs," says Reimann. If similar behaviour were seen with the friction between car tires and the road, then there are important implications for road safety. The findings suggest that neither locking the wheels nor the usual ABS-system is the most effective method of stopping a car in the shortest possible stopping distance, explains Reimann. He says that a compromise between the two approaches to braking could be much more effective.
100 years after Einstein’s landmark paper, optical tweezer technology could confirm the theory of classical Brownian motion in details that Einstein missed when he first proposed it a century ago. This research is reported today in a special Einstein Year issue of the New Journal of Physics.
“Optical tweezers” use a focused laser beam to trap and study microscopic objects, such as the individual bio-molecules that power muscle cells and propel sperm, and those that read the genetic code. The device is disturbed, however, by a subtle effect in Brownian motion known as the back-flow effect.
100 years ago in 1905, Einstein published a landmark paper on Brownian motion. He theorised that it is the constant buffeting of microscopic particles that goes on in any fluid as the fluid molecules randomly knock those particles around. He missed the subtle "back-flow effect" in which the very movement of a particle disturbs the water which ultimately bounces back to nudge the particle in return. "It's like a boat that tries to stop, and then is pushed by its stern wave when that wave catches up with the boat," explains Henrik Flyvbjerg of Risř National Laboratory in Denmark. "Optical tweezers sense the back-flow effect," adds Flyvbjerg, "but that also means it can be studied with them."
Einstein described Brownian motion as arising from the "white" noise of random molecular motion due to heat. But, the back-flow effect makes higher frequencies slightly more likely, making the white noise "bluey white". Flyvbjerg and his colleagues demonstrate that optical tweezers technology is now at the point where this colour shift can be measured directly. He is collaborating with Stanford University's Steve Block to push the technology to do it. If successful, they will confirm Brownian motion's last unobserved trait, 100 years after Einstein's initial theory for it.
The paper was published on Monday 31st January 2005 in New Journal of Physics as part of a celebratory focus issue on "Brownian Motion and Diffusion in the 21st Century" (http://stacks.iop.org/1367-2630/7/i=1/a=E01).
Noise is usually nothing more than a disturbance, but sometimes it can be useful. Researchers have discovered that noise could bring order to chaotic systems, protect and maintain entire marine ecosystems, and even make the chemical industry greener. This research is reported today in a special Einstein Year issue of the New Journal of Physics.
Changsong Zhou and a group of physicists at the University of Potsdam, Germany, are studying chaotic systems, known as excitable media. The firing of neurons in the brain is an example of such a system, as is the growth and receding of blooms of plankton in the sea. Such systems do not become excited by small signals but if they are stimulated above a threshold amount, then they give it their all: neurons fire and plankton blooms.
“Similarly, excitable non-linear behaviour is also found in chemical reactions”, explains Zhou, “where an external pressure or light can push a reaction down one route instead of another.”
Zhou and his colleagues have found that the key to this sort of excitation is chaotic mixing and noise. The researchers demonstrated how a non-linear system can be controlled to become synchronized even when its stimulus is below the threshold by the addition of noise to the system.
The results based on their model study imply that oscillatory behaviour in many natural systems, rather than being disturbed by noise, is thus sustained by it. For instance, the "noise" in a marine ecosystem due to temperature changes, ocean currents, wind-driven waves, fluctuations in nutrient levels, the movement of schools of fish, and wind-driven waves affect how plankton blooms grow and recede. If the conditions are below an optimum the plankton do not grow, but they can be forced into action by noise, and once they are stimulated the whole system is activated and a marine landscape is quickly blanketed by the bloom.
Zhou's results suggest that without noise such blooms might be physically unable to flourish in some areas or might not follow the usual seasonal cycles. "Noise might be essential to maintaining the stability and the persistence of marine ecosystems," Zhou says. This research might therefore help environmental scientists predict or even prevent toxic plankton blooms by observing the natural noise that affects them.
Zhou and his colleagues also suggest that noise might usefully be used to control chemical reactions. They explain that random disturbances in industrial mixing tanks could be promoted to make a reaction proceed more efficiently and so reduce chemical waste, making the chemical industry a little more environmentally friendly.
The paper will be published on Monday 31st January 2005 in New Journal of
Physics as part of a
celebratory focus issue on "Brownian Motion and Diffusion in the 21st Century" (
http://stacks.iop.org/1367-2630/7/i=1/a=E01 ). The paper can be downloaded
free of charge from 31st January at
http://stacks.iop.org/1367-2630/7/18 . Reference: C Zhou et al. New J. Phys.
7 (2005) 18.
Noise explains how Paddlefish see in the dark
Christiaan Huygens, the Dutch scholar who invented the pendulum clock,
observed 340 years ago that two clocks in the same cabinet moved in "sympathy"
so they eventually swing with the same frequency. Michael Schindler and his
colleagues at the University of Augsberg in Germany, have developed a theory
based on noise that will help experimentalists understand similar phenomena in
systems as diverse as animal reflexes, laser technology, and even the emergence
of ice ages. This research is reported today in a special Einstein Year issue of
the New Journal of Physics.
Scientists originally explained such synchronization effects in terms of non-random deterministic effects linking the two systems through so-called weak coupling. It turns out that such systems are robust against small amounts of irregular perturbations, but in general, too much is detrimental to synchronization.
Recently, however, certain synchronisation effects have been observed that only work well if the system is subject to random perturbations, noise in other words. Michael Schindler and his colleagues now provide a new approach to understanding one such phenomenon known as stochastic resonance in which random noise paradoxically amplifies a signal.
Stochastic resonance benefits from noise and explains how animals such as crayfish and paddlefish can detect faint mechanical or electrical signals in a murky marine environment. Moreover, explains Schindler stochastic resonance has found various technical applications, in laser technology, for instance. Researchers have also applied the principle to improving their understanding of ice ages and climate change.
Though there is some apparent similarity between stochastic resonance and synchronisation, the decisive role of the noise in stochastic resonance does not fit the original classical deterministic mechanism. The trouble with studying these systems is that they were difficult to control experimentally.
Now, Schindler and his colleagues have produced a proper phase definition for stochastic systems that exist in either of two states, which could help the experimentalists. Their careful analysis shows that stochastic resonance leads to synchronization phenomena and offers a new way to measure stochastic resonance simply by counting the "flips" between two states; warm or frozen in the case of ice ages, frequency locked or out of tune in the case of a radio. "Our study makes it easier for experimentalists to judge whether a system will show stochastic resonance and under which conditions" says Schindler.
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