Reshaping our understanding of granular systems

Rochester researchers are discovering the unexpected role of grain shape in the mixing of granular systems such as pharmaceuticals, cereals and landslides.

Your morning cereal, a jar of nuts, the sand of distant planets, and even the concrete of your city are all examples of the granular systems that surround us. And such systems hold secrets that could change the way we mix things.

In a new article published in the Proceedings of the National Academy of Sciences, University of Rochester scientists including Rachel Glade, assistant professor of earth and environmental sciences and mechanical engineering; Fernando David Cúñez, former postdoctoral research associate in Glade’s lab and now postdoctoral research associate at the Rochester Institute of Technology; and Div Patel, studied granular materials and discovered the unexpected role that grain shape plays in the behavior of granular systems.

“Granular materials have particular behaviors,” explains Cúñez, “but we don’t know much about their exact behavior, because their behavior depends on many different circumstances.”

The Brazil nut effect

Granular materials such as grain, pharmaceuticals, sand, and concrete typically organize themselves in such a way that the grains separate based on size rather than mixing uniformly. For example, in a jar of nuts, the largest nuts are usually at the top, a phenomenon known as the “Brazil nut effect.”

The Brazil nut effect can be a nuisance to many industries, including food and medicine, because it prevents uniform mixing. This also has influences in nature, where grain segregation can alter the dynamics of geohazards such as landslides, erosion and debris flows.

Although the phenomenon is well known, it is not yet fully understood. Researchers have also traditionally focused on grain size, with most previous studies assuming that grains are spherical, a uniformity that rarely reflects reality.

Shape change dynamics

Glade and his team used advanced computer simulations comparing mixtures of spheres with mixtures of spheres and cubes in a rotating drum and in a river-like configuration to study how grain shape affects segregation under dry conditions and wet, respectively. Their research revealed that even small differences in grain shape can significantly change the dynamics of grain segregation.

Specifically, researchers discovered the following patterns in dry system mixtures:

• In a mixture of differently sized spheres: Segregation increases, with more larger spheres rising upwards, when the ratio of large volume spheres to small volume spheres is greater.
• In a mixture of same-sized spheres and larger cubes: segregation tends to be the same as in the case of spheres only, with the larger spheres rising upwards.
• In a mixture of same-sized spheres plus smaller cubes: segregation tends to decrease, with most of the larger spheres moving upwards but to a lesser extent than in the mixture of spheres only.

Interestingly, in a fluid system the trend is reversed: in a mixture of same-sized spheres and smaller cubes, the smaller cubes move upward.

“One way to look at it is that grain shape changes segregation both quantitatively (in the case of a dry drum, cubes decrease the amount of segregation) and qualitatively (in the case of a wet river, cubes change segregation patterns,” says Glade.

Reshaping industry and nature

Future research will explore why these changes in segregation are occurring. The researchers speculate that this is likely due to several factors, including forces on different particles that cause them to stick together and resist movement in different ways.

Regardless, the study shows the importance of grain shape in various fields.

Highlighting the broader implications, Glade says: “Our work demonstrates the importance of interdisciplinary research, drawing from physics, engineering and earth sciences. This collaboration paves the way for future work aimed at better understanding and predicting geohazards and mitigating segregation problems in industrial flows. , and improve our understanding of granular materials on Earth and other planets.