At first glance, a landslide, an avalanche, or even a heap of sand pushed by a bulldozer looks like a simple stream of grains moving downhill. However, beneath the surface lies a hidden world of swirling currents, where grains sneak sideways or loop in circles instead of following the obvious flow.
These invisible motions, called secondary flows, have long been suspected in computer simulations, but never directly observed in real materials. Now, scientists have captured them for the first time, using a powerful new X-ray imaging method.
Their discovery could transform our understanding of avalanches, landslides, and even how we handle everyday powders like flour, wheat, or medicine.
The trick to viewing the hidden swirls
For decades, researchers believed that when grains slide down a slope, most follow the steepest path downhill, known as the primary flow. However, theories and computer models suggested that not all grains follow the herd.
Some take detours, swirling sideways beneath the surface, subtly shaping how far and fast a landslide travels. These hidden flows were thought to exist, but proving them experimentally was nearly impossible. This is because stopping the grains for X-ray scans froze the motion, destroying the natural flow. Adding liquids to make grains transparent altered their behavior.
Therefore, researchers were stuck with indirect evidence from ripples on the surface or from simulations, never able to see the actual three-dimensional movement inside a flowing pile.
To overcome this problem, the study authors designed a conveyor-belt bulldozing experiment in a 0.55 m long, 0.10 m wide flume filled with three-millimeter glass beads. The belt pushed glass beads against a wall, forming a pile.
To look inside without disturbing the grains, they used a technique they developed called X-ray rheography, part of a new setup named DynamiX. Rheography works by taking rapid X-ray images of a moving pile of grains. As the grains flow, they block X-rays in shifting patterns of light and dark.
By tracking how these patterns move from frame to frame, scientists can calculate the speed and direction of the grains inside the pile, without ever stopping or disturbing the flow.
The first images from this setup revealed faint ripples on the surface of the pile. Previous studies had hinted that hidden swirls inside the flow might cause such ripples. However, for the first time, the researchers could link these ripples to real sideways motions beneath the surface.
To further strengthen the case, they mapped the surface from the X-ray data and tracked how grains moved through the full depth of the pile, not just along the main flow. What they found was sideways and swirling motions, providing the first direct experimental evidence of secondary flows in granular materials.
“We present an experimental confirmation of secondary kinematics within granular media using dynamic X-ray radiography, without needing to stop motion for tomography,” the study authors note.
The significance of watching secondary motion
The study shows that secondary flows are a fundamental feature of granular materials, whether in snowdrifts, sandpiles, or wheat silos. This finding is of great value.
For instance, in the case of natural hazards, models of landslides or avalanches that ignore secondary flows might underestimate how far debris can travel. Adding this detail could help engineers design better predictions and safety measures.
The findings also have implications in various other industries, ranging from pharmaceuticals to agriculture. Countless processes rely on moving powders or grains, so knowing that hidden sideways currents exist could improve how materials are stored, mixed, or transported.
However, the study also had some limitations. The team studied glass beads in a controlled bulldozing setup, not real snow or rocky landslides. Plus, the full three-dimensional picture of the flows is not yet complete, as the DynamiX system currently measures the motion along only one direction.
To overcome these, the researchers aim to expand the imaging methods and test more realistic materials in the future.
The study is published in the journal Nature Communications.