Researchers at the UCLA Henry Samueli School of Engineering and Applied Science have come up with a theoretical model to predict when granular materials become jammed. This advancement not only broadens fundamental knowledge, it also provides new avenues to a number of practical areas that ranges from materials innovation to medicine. The study, currently available on the Nature Physics Web site, will be published in the journal’s print edition on May 1.

“We started this research by looking at the behavior of dry powders as solid lubricants as well as the behavior of a powdered rock in fault zones called gouge during an earthquake. What we found led us to a model that can accurately predict the behavior of dense granular flows. What we realized soon after was that the granular particles interact similarly to that of molecules in materials that jam, such as colloids and foam” said study’s author Pirouz Kavehpour, an assistant professor of mechanical and aerospace engineering and director of the Complex Fluids & Interfacial Physics Laboratory at UCLA. “From there, we were able to find a universal law that can predict the jamming behavior for the first time.”

According to Emily Brodsky, associate professor of earth and planetary sciences at UC Santa Cruz and also an author of the study, “We understand how water flows. We understand how honey flows. We even understand how elastic bands deform. But granular flows are complicated and hard to understand. If you’re pouring sand down a hill or in an hour glass, there was never a good formula for the strain or the strain rate as a function of stress. This formula is definitely new and unique.”

Kevin Lu, UCLA graduate student and lead author of the study, showed that the formula also quantified glass-transition. “Glass is a solid that flows. But structurally, it’s a liquid. The molecules in a glass are jammed and unable to flow past each other so the material actually flows sluggishly. One evidence of this can be found in the window panes of old churches in Europe. Studies have shown that the bottom of the windows are consistently thicker than the top. Glassy liquids flow very much in the same manner as granular media.” said Lu.

This new theoretical framework, the authors believe, can be applied to many different areas. Pharmaceutical companies can use the new equation to decide the size and quantities of pills that may or may not fit through a shoot that fills containers. Also, from knowing the fundamentals of jamming, scientists can now engineer materials that are both durable and strong. Instead of working with composites or alloys, the jamming theory provides a roadmap to tune material properties from pure substances.

“It can also help us to better understand certain diseases in medicine. In sickle cell anemia, for example, the abnormal blood cells are long and skinny, resulting in the obstruction of blood flow to various organs. Now we can do more to reduce the likeliness of death-threatening implications to benefit the medical community,” said Lu.

As a geologist who studies fault zones and earthquakes, Brodsky is particularly interested in the granular flow of gouge found in fault zones and having a formula to figure out when the rock is jammed and when it’s free flowing can be significant.

“Knowing how things flow and the granular behavior in a fault zone is one of the very important steps in trying to figure out how exactly faults slip,” said Brodsky.

The study was partially funded by the Air Force Office of Scientific Research.

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Researchers at The University of Manchester and Chernogolovka, Russia have discovered the world’s first single-atom-thick fabric, which reveals the existence of a new class of materials and may lead to computers made from a single molecule. The research is to be published in Science on 22 October.

The team led by Professor Andre Geim at The University of Manchester, has succeeded in extracting individual planes of carbon atoms from graphite crystals, which has resulted in the production of the thinnest possible fabric - graphene. The resulting atomic sheet is stable, highly flexible and strong and remarkably conductive. The nanofabric belongs to the family of fullerene molecules, which were discovered during the last two decades, but is the first two-dimensional fullerene.

The researchers concentrate on the electronic properties of carbon nanofabric. By employing the standard microfabrication techniques used, for instance, in manufacturing of computer chips, the team has demonstrated an ambipolar field-effect transistor, which works under ambient conditions. They found that the nanofabric exhibits a remarkable quality such that electrons can travel without any scattering over submicron distances, which is important for making very-fast-switching transistors.

In the quest to make the computer chip more powerful and fast, engineers strive to produce smaller transistors, shortening the paths electrons have to travel to switch the devices on and off. Ultimately, scientists envisage transistors made from a single molecule, and this work brings that vision ever nearer.

In terms of applications, the sort of quality demonstrated by graphene can only be compared with that demonstrated by some nanotubes. Professor Geim commented: “As carbon nanotubes are basically made from rolled-up narrow stripes of graphene, any of the thousands of applications currently considered for nanotubes renowned for their unique properties can also apply to graphene itself.”

Although the researchers are currently dealing with patches of graphene that are about ten microns across Professor Geim commented: “Computer engineers will need graphene wafers a few inches in size, before considering graphene as |”the next big thing”. However, all the omens are good, as there are no fundamental limitations on the lateral size of carbon nanofabric.” Dr Novoselov added: “Only ten years ago carbon nanotubes were less than a micron long. Now, scientists can make nanotubes several centimetres long, and similar progress can reasonably be expected for carbon nanofabric too”.

David Glover from University of Manchester Intellectual Property Ltd commented: “This is clearly an exciting breakthrough with huge potential, and with development graphene could soon compete in many niche markets where low energy consumption and high electron mobility are paramount requirements”.

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Scientists at Brookhaven Lab have discovered a state of two-dimensional (2D) fluctuating superconductivity in a high-temperature superconductor with a particular arrangement of electrical charges known as “stripes.”

The finding was uncovered during studies of directional dependence in the material’s electron-transport and magnetic properties. In the 2D plane, the material acts as a superconductor - conducts electricity with no resistance - at a significantly higher temperature than in the 3D state.

“The results provide many insights into the interplay between the stripe order and superconductivity, which may shed light on the mechanism underlying high-temperature superconductivity,” said Brookhaven physicist Qiang Li.

Understanding the mechanism of high-temperature superconductivity is one of the outstanding scientific issues in condensed matter physics, Li said. Understanding this mechanism could lead to new strategies for increasing the superconducting transition temperature of other superconductors to make them more practical for applications such as electrical transmission lines.

“As electricity demand increases, the challenge to the national electricity grid to provide reliable power will soon grow to crisis levels,” Li said. “Superconductors offer powerful opportunities for restoring the reliability of the power grid and increasing its capacity and efficiency by providing reactive power reserves against blackouts, and by generating and transmitting electricity.”

This research was presented at The March 2008 American Physical Society Meeting in New Orleans, La., March 10 -14.

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