Researchers Find Link Between Disorder And Metastability

Using photo-elastic disks, researchers have shown that it is possible to detect and quantify metastability induced by disorder in a granular material.

AsianScientist (May 24, 2016) – Researchers in Japan have shown for the first time that it is possible to detect and quantify metastability induced by disorder. They published their findings in Physical Review Letters.

Metastability is a concept linked with the level of energy in a physics system. When a physics system is stable, it has only one state of low energy. Its energy is like a bucket in a well: when it goes down, it will always end up in the same pool of water.

But there are physics systems that can be equated to strange wells that have many pools of water at the bottom. When the bucket goes down in one of these wells, it is not possible to predict with certainty in which pool it will end up.

Furthermore, if the bucket is lifted to any height in the well and then released, it may end up in a different pool. These physics systems are metastable: the lowest state of energy for these systems is neither unique nor predictable.

Professor Mahesh Bandi, lead researcher from the Okinawa Institute of Science and Technology Graduate University and colleagues, Mr. Naoki Iikawa and Professor Hiroaki Katsuragi from Nagoya University, devised a unique experiment to measure the relationship between the two phenomena.

“Everyone expected this result,” said Bandi, “but before us, no one was able to find a way to measure the relationship between disorder and metastability in granular materials.”

The relationship between metastability and disorder has previously been elusive since the measurements typically used in granular physics are not sensitive enough to detect metastability. Attempting to think out of the box, Bandi and colleagues borrowed a parameter, ‘S,’ from a different branch of physics called liquid crystal physics. They showed that S can be used to detect the orientation of the forces inside a granular material. These forces, called force chains, are the networks of forces that arise from the contact among grains.

Force chains give granular materials their physical proprieties. To study force chains, the scientists used special disks called photo-elastic disks which change their appearance when compressed.

Top: a photo-elastic disk at rest (left) and squeezed (right). Bottom: photo-elastic disks during an experiment. Granular force chains appear as multicolor bands on the surface of the disks. These photos were taken using a polarized lens. Credit: OIST
Top: a photo-elastic disk at rest (left) and squeezed (right). Bottom: photo-elastic disks during an experiment. Granular force chains appear as multicolor bands on the surface of the disks. These photos were taken using a polarized lens. Credit: OIST

After checking that in an ordered configuration there was no change in S, the scientists set up a random configuration of photo-elastic disks. They took disks of two different diameters and inserted them in a vertical structure. Inside such a configuration, the force chains point along any direction.

“Then, we tapped the structure, hitting it along the vertical direction,” Bandi said.

The configuration of the disks stayed the same, but the force chains slowly oriented themselves from the direction of gravity towards all random directions. The researchers then captured high-resolution images of this process, which showed the changing force chains.

The researchers translated the visual information into numbers, with resulting values of S confirming the change in the force chains. Thus, they proved the link between disorder, induced by tapping, and the metastable energy state of the granular material, which changed after each tapping like a bucket changing a pool in a strange well.

While this result was expected, the introduction of S, the parameter from liquid crystals, represents an important change in the study of granular materials. S switches the focus from the structure of granular materials, to the orientation of the force chains inside granular materials.

Such a switch of focus provides the researchers with a new way to conceptualize disorder and metastability in granular materials, hopefully leading to new advances in the understanding of these materials.


The article can be found at: Iikawa et al. (2016) Sensitivity of Granular Force Chain Orientation to Disorder-Induced Metastable Relaxation.

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Source: Okinawa Institute of Science and Technology Graduate University.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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