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Researchers have shown for the first time how hidden motions could control how granular materials such as soil and snow slip and slide, confirming a long-suspected hypothesis. The knowledge could help in understanding how landslides and avalanches work and even help the construction industry in the future.

Scientists have found sneaky swirls and loops of movement in materials such as soil and snow could influence how materials move. The knowledge could be invaluable in understanding how avalanches and landslides on Earth and Mars speed up or slow down. Understanding this phenomenon could also benefit various industries, from construction to the operation of silos during grain filling and discharge.

The findings, in Nature Communications, are a milestone in the field of granular physics.

Just like atoms in a river, when particles move in snow or soil, they do not always follow the path of their neighbors. It has long been theorized among researchers that underneath the surface of such materials, there are hidden currents and eddies that could impact the destructive power of avalanches and landslides.

Called "secondary flow," the process has never been observed under the surface, as it was not possible to see through the materials as they flow.

An international team of scientists led by the University of Sydney has now successfully mapped and captured this phenomenon within the bulk of flowing grains for the first time. This was achieved using DynamiX, a unique X-ray radiation technology built by the researchers to uncover the existence of secondary flow.

The team used a simultaneous three-directional X-ray system to look inside flowing soil masses in real-time. Specially designed algorithms were developed to process data and map the movement.

"Granular materials are everywhere. It's important to understand the physics of how they flow and interact: from tiny grains of sand or snow, or even pieces of rocks in minerals processing, granular materials can either behave like solids and flow like fluids, such as during landslides or when we discharge silos," said senior researcher Professor Itai Einav, from the University's School of Civil Engineering at the Faculty of Engineering.

"The existence of secondary flow has been an enduring theory in granular physics, but it has never been confirmed in 3D and in real-time. Uncovering secondary flow and understanding how it influences the movement of granular media will open new possibilities for industry and research," said Professor Einav, who is also director of the Sydney Centre in Geomechanics and Mining Materials (SciGEM).

Home-grown tech solving a mystery nearly a decade in the making

Behind an immense lead-lined door in a quiet corner of the School of Civil Engineering is an instrument custom-built to analyze granular physics, which played a key role in confirming secondary granular flow.

"We were determined to understand the fast flow of granular media, but there wasn't any equipment available on the market, so we decided to build it ourselves," said Professor Einav.

DynamiX was built over five years, but the idea came to Professor Einav's team nearly a decade ago.

A set of three perpendicular X-ray tubes and detectors mounted on a modular frame allows positioning of the X-ray pairs to examine any vessel of grains that is transparent to X-ray radiography. With DynamiX, the team can study almost any kind of flowing mixed material, from glass beads, soil to foams, wet or dry.

For the experiment, the team used a conveyor belt to drive a pile of glass beads against a wall, seeing how surface bumps and dips were formed. The lead-lined door protects researchers from the radiation emitted by DynamiX's three powerful X-ray tube-detector pairs that pointed at the particle vessel, to reveal movements hidden inside the material.

Observing from a control room, researchers watched as the grains swirled and rolled in complex 3D patterns underneath the flow's surface.

Professor Einav believes DynamiX is the only instrument of its kind to study granular flows both in real time and in 3D.

"The next mystery to solve is the secondary flow's origin, and whether its strength is influenced by the properties of the flowing material. Our goal is to develop models that can explain these questions mathematically," said Professor Einav.

First author Dr. Andres-Felipe Escobar-Rincon said the team initially wanted to study how granular flows (like avalanches or landslides) behave when they hit an obstacle, such as a retaining wall.

"However, once we noticed variations on the surface and examined their internal velocities with X-rays, we realized we were looking at complex interactions that occur beyond avalanches and landslides," said Dr. Escobar-Rincon. He conducted the study as part of his Ph.D. at the University of Sydney, in collaboration with the Université Grenoble Alpes, where he is now based.

"Now we are curious about what drives them," he said.