Landslides, avalanches, and construction safety may all benefit from new granular physics research that confirms a long-standing hypothesis about how particles, such as soil and snow, slip and slide.
The findings, reported in a new paper in Nature Communications, were made possible through the use of novel X-ray technology, which allowed a University of Sydney-led team of international researchers to closely examine the motion of snow and soil. This enabled them to discover how intricate swirls and loops affect the movement of these materials.
The researchers envision numerous industrial applications for this new research, including construction and grain storage, as well as future Martian habitation, to gain a deeper understanding of landslides and soil movement.
Secondary Flow
“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 co-author Professor Itai Einav, of the University of Sydney.
While it has long been known that particles in snow or soil do not always travel the same paths as those around them, what does dictate their individual movements has remained mysterious. For some time, scientists have suggested that difficult-to-observe eddies and currents may be dictating that movement and driving the destructive force witnessed in avalanches and landslides. Such a concept is called a “secondary flow” and has never been directly observed beneath the surface of such materials because researchers cannot see through them.
“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,” Einav added.
Observing Hidden Particle Movement
To observe these opaque conditions, the researchers had to build their own dedicated X-ray system, which they named DynamiX. Operating in three directions simultaneously, DynamiX can make real-time observation of penetrating soil masses.
“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,” Einav commented.
From an idea first conceived a decade ago, the team spent five years constructing DynamiX. The device’s frame is modular, designed to enable repositioning of the three X-ray tubes and detectors mounted on it. In their initial experiment, the researchers used a pile of glass beads on a conveyor belt to identify how bumps and dips formed in the overall mass, but their device is capable of more. Beyond just glass beads, DynamiX can see through any flowing mixed wet or dry material, such as soil or even foam.
Tracking Flow From X-ray Data
To these observational capabilities, the team added unique algorithms to process the tracking data and chart movement. Using the system, the team was able to map the obscured particle movement for the first time, finally capturing the subsurface grain flow.
“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,” Einav said.
Intriguingly, the team’s initial focus was somewhat different; instead, they looked at how granular flows, such as avalanches or landslides, behave upon impacting an obstacle.
“However, once we noticed variations on the surface and examined their internal velocities with X-rays, we realised we were looking at complex interactions that occur beyond avalanches and landslides,” said lead author Dr Andres Escobar-Rincon.
“Now we are curious about what drives them,” he added.
The paper, “Experimental Confirmation of Secondary Flows within Granular Media,” appeared in Nature Communications on August 26, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.