Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Duke physicists reveal new insights into stresses between sliding grains

27.02.2003


Densely packed granular particles that inch past each other under tension interact in ways more complex and surprising than previously believed, two Duke University physicists have discovered.



Their observations, described in the Thursday, February 27, 2003, issue of the research journal Nature, could provide new insight into such geophysical processes as the behavior of a slowly moving glacier or an active earthquake fault, said Robert Behringer, a Duke physics professor who is one of the Nature article’s authors. The physicists’ findings could also have implications for industrial problems, such as how the contents of a hopper holding granular materials such as grain or coal flow, he added.

By using plastic beads made of a material that affects light differently when under stress, Behringer and graduate student Robert Hartley have for the first time shown what happens to grains in a granular network subjected to frictional or "shear" forces that may build slowly.


Their work, supported by the National Science Foundation and NASA, constitutes a new scientific view of phenomena that are difficult to visualize or measure, and for which there is no established theory, Behringer said. Granular materials interest scientists particularly because the materials behave in some ways like solids and in others like liquids.

According to the scientists, tensions in granular materials obviously increase when densely packed grains in close contact with each other seek to move in opposite directions. But nothing apparent happens to these grain collections until forces build enough to cause the grains to begin slipping past each other, the authors noted in their Nature report.

Physicists from the 1770s to recent times had believed that after sliding begins, frictional forces between the grains remain constant even as sliding speeds slowly increase. "This is still routinely taught in introductory physics courses," Behringer said in an interview.

Various experiments, particularly since the early 1980s, however, have suggested that frictional forces between the grains do not remain the same but instead appear to decrease as sliding speeds inch upwards, Behringer added. Such a weakening is no surprise, according to Behringer. "With increasing speed, the contacts between individual grains should be weakening," he said.

The surprises came when Hartley and Behringer explored in unprecedented detail what is happening within such systems. Their article illustrated how they were able to "zoom in uniquely" on individual beads, which served as laboratory surrogates for granular particles in nature.

In their experiments, they studied how the plastic beads interacted in a confined space between an outer ring and an inner rotating wheel. Using that apparatus, they demonstrated that built-up frictional stresses are actually transferred into jagged networks of "force chains" that some contacting grains develop within the tight networks.

So, although forces between individual grains drop overall as sliding speeds grow, the scientists found that the force chains reorganize and proliferate at the same time to sop up what are increasing stresses to the system. "The force network increases in strength with increasing speed," Behringer said.

In these experiments, speeds are relatively slow -- the fastest being one revolution of the wheel every 30 seconds.

Another surprise occurred when the researchers halted the grain interaction by stopping the rotating wheel. With the tightly packed grains no longer sliding past one another but instead remaining in constant contact, forces between individual grains should freeze in place, Behringer said.

Instead, the researchers found that stresses within the force chains began dropping, rapidly initially and slowly thereafter over periods of many hours. "Those contacts should remain exactly as they are over time. But nature does something different," he said.

These surprises "indicate that newly found and possibly subtle processes are at work that make collections of grains behave in a way that appears to be the reverse of what would be expected," he added.

"Our observations have important implications for modeling the internal stress states of geophysical systems," Behringer said. Examples might include California’s notorious San Andreas Fault, in which two rock faces move in opposite directions deep underground. In some places, the rock faces slowly creep past each other. In others, the rock faces bind, causing pressures to build up until the faces suddenly snap apart in an earthquake that is sometimes catastrophic.

There may also be lessons for the design of industrial devices, such as hoppers, he said. In the flow of coal or of wheat in hoppers, "there could be a large range of velocities in different parts of the hopper, from very fast near its outlet to very slow at the top," he said. "The stresses are likely to behave very differently for these different flow speeds."

Hoppers have been known to self-destruct because of hard to decipher changes inside.

Monte Basgall | EurekAlert!
Further information:
http://www.duke.edu/

More articles from Physics and Astronomy:

nachricht From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison

nachricht Prediction: More gas-giants will be found orbiting Sun-like stars
22.02.2017 | Carnegie Institution for Science

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Stingless bees have their nests protected by soldiers

24.02.2017 | Life Sciences

New risk factors for anxiety disorders

24.02.2017 | Life Sciences

MWC 2017: 5G Capital Berlin

24.02.2017 | Trade Fair News

VideoLinks
B2B-VideoLinks
More VideoLinks >>>