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."
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