Why Are Granular Materials Solids Yet Moving Like Liquids?

Granular materials are widely found both in nature and in industrial applications and although we use them every day, theoretical physicists and manufacturers are still puzzled by their weird behavior. Although they are solid, they refuse to comply to the laws of physics that govern the movement of solids and they don't bother about those of liquid motion, either.

Sand, cereals, coal, ice cubes, sugar, marbles, pills, they are all part of the same family whose movements are extremely difficult to predict, thus being a headache wherever they are employed, be it a massive grain silo, a pharmaceutical manufacturing plant or a coal hopper.

A new Brandeis University study aims to provide a new theoretical model for statistically predicting the properties of static, mechanically stable grain packings that cause many problems by jamming into industrial machines.

Physicist Bulbul Chakraborty analyzed the way small grains move in a pile and what causes them to lose energy when shaken, stirred or otherwise perturbed, unlike solid atoms and molecules.

"We asked a simple question," said Brandeis physicist Bulbul Chakraborty. "Could we construct a statistical framework to predict the probability of a certain arrangement of grains that have settled after having been disturbed? What's the probability of finding grains arranged in a hexagonal packing, say, compared to a square packing? How do these probabilities change when the grains are shaken more vigorously?"

By studying granular packings that have settled after being disturbed - due to the fact that they are prone to jamming - he's come up with a new way of predicting the appearance of jams, which are both annoying and costly, which could find practical applications in many industrial areas where grains are used.

"Jamming is a phenomenon that occurs in countless engineering and industrial applications; this framework is the first step to understanding and preventing unwanted and costly jams," said Chakraborty.