Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Researchers explain granular material properties

15.12.2011
Discovery may be boon to engineers, manufacturers

A stroll on the beach can mean sinking your toes into smooth sand or walking firm-footed on a surface that appears almost solid. While both properties are commonplace, exactly what it is that makes granular materials change from a flowing state to a “jammed,” or solid, state? Whether it’s sand on a beach or rice grains in a hopper, being able to predict the behavior of granular matter can help engineers and manufacturers of a wide range of products.

In a study out this week in the Journal Nature, researchers at Brandeis in collaboration with Duke University explain how granular materials are transformed from a loose state to a solid state when force is applied at a particular angle, in a process known as shearing.

“Traditionally people thought of shearing as a mechanism for breaking up materials,” says Dapeng Bi, a graduate student in the Martin Fisher School of Physics. “In this case, we find shear actually drives solidification.”

Bulbul Chakraborty, the Enid and Nate Ancell Professor of Physics, and Bi, analyzed an experiment performed at Duke which used photo-elastic discs of two different sizes to represent granular materials such as rice or sand. The discs were placed into a plastic box whose shape could be precisely manipulated and measured. The box was illuminated from the bottom, forcing light through the discs. A polarized lens placed on top of the box revealed the photo-elastic discs creating colorful patterns — called force chains — caused by the pressure they received when the sides of the box were moved to create a rectangle. Using a computer program the Duke researchers were able to determine the amount of force that was exerted by the discs on each other.

“The polarized light changes the index of refraction of the materials and makes the patterns non-uniform,” says Bi. “We then use those numbers to calculate the forces and the geometry of the contact ​network that the discs formed.”

The researchers found that when the shape of the box changed due to shear, the discs exhibited a solid state even without the density changing. This, Chakraborty says, is remarkable because usually it is an increase in density that transforms loose material to a solid.

“For theorists like us, these experiments are wonderful because we can see exactly what this system is doing,” says Chakraborty. “How these patterns change as the discs are pushed and altered gives us information such as how many contacts each grain makes, and the force at every contact.”

Chakraborty says that using this data she and Bi constructed a theory that explains how the solid is being formed.

“It’s possible that if there was no friction between the discs that they would have been able to slide past each other and not get jammed,” says Chakraborty. “We now are performing computer simulations to see if shear jamming will occur without friction.”

In an abstract written in 2008 in Jamming of Granular Matter, Chakraborty and Robert P. Behringer of Duke University explained that jamming is the extension of the concept of freezing to the transition from a fluid state to a jammed state. Understanding jamming in granular systems, they say, is important from a technological, environmental, and basic science perspective. A jamming of grains in silos can cause catastrophic failures. Avalanches are examples of unjamming, which need to be understood in order to prevent and control, such as the avalanche that killed pro skier Jamie Pierre on November 13, 2011.

Shearing is a major force in nature, explains Chakraborty. When wind blows over the earth, shearing occurs in the sand. Understanding what shear does, she says, is very important.

“We have a very good theoretical framework as to how water behaves, or ice or air,” says Chakraborty. “We don’t have any fundamental theoretical framework to predict how sand behaves when the wind is blowing fast or slow.”

This information could potentially be used to further understand​ things like avalanches and earthquakes and erosion.

“Those are effects of shearing of granular materials,” says Chakraborty. “What we’re trying to do is get at a basic understanding of how sand responds to shear. Most natural forces are shearing forces.”

The behavior seen here is similar to “shear thickening,” which has been used when manufacturing bulletproof vests that present as a soft material when worn, but hardens upon impact of a bullet.

“The research shows that friction can fundamentally change the nature of granular materials in intriguing ways,” says Daryl Hess, program director for condensed matter and materials theory at the National Science Foundation. “Friction and shear reveal the richness of possible states of granular matter, pointing us down a road paved with new discoveries. These may expose deeper connections between jamming and seemingly unrelated phenomena spanning from earthquakes to transformations occurring in other kinds of matter, like water to ice.”

In industries where hoppers are used, like loading rice grains onto a truck for example, jamming can be a problem. One possible solution, says Chakraborty, is to change the traditional shape in order to both prevent and break up jams.

“We need these sort of laboratory-based experiments to construct and test theories,” says Chakraborty. “Once you get into an industrial situation things are not controlled enough to understand.”

Susan Chaityn Lebovits | EurekAlert!
Further information:
http://www.brandies.edu

More articles from Materials Sciences:

nachricht Glass's off-kilter harmonies
18.01.2017 | University of Texas at Austin, Texas Advanced Computing Center

nachricht Explaining how 2-D materials break at the atomic level
18.01.2017 | Institute for Basic Science

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

Im Focus: How to inflate a hardened concrete shell with a weight of 80 t

At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).

Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

Nothing will happen without batteries making it happen!

05.01.2017 | Event News

 
Latest News

A big nano boost for solar cells

18.01.2017 | Power and Electrical Engineering

Glass's off-kilter harmonies

18.01.2017 | Materials Sciences

Toward a 'smart' patch that automatically delivers insulin when needed

18.01.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>