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The glass beads used in the bulldozing experiments and mapped with X-rays. Credit: Andr\u00e9s Felipe Escobar Rinc\u00f3n<\/p>\n
What looks like ordinary sand, rocks or snow flowing in one direction can actually hide swirling currents that move in multiple directions beneath the surface.<\/p>\n
When grains move in a landslide, most follow the steepest downhill path. This is the “primary flow,” where particles largely follow the herd. But some grains move sideways or swirl in hidden patterns, forming “secondary flows” that subtly influence how far and fast the material travels.<\/p>\n
Understanding how grains move beneath the surface could help explain the physics of avalanches and landslides, and even improve how we handle everyday materials like wheat in silos or powders in pharmaceuticals.<\/p>\n
However, nobody has been able to observe these hidden swirls\u2014until now. In a study published in Nature Communications<\/a> today, we have captured secondary flows inside granular materials for the first time with our advanced X-ray imaging setup.<\/p>\n A bulldozing experiment<\/p>\n The idea these secondary flows exist isn’t new. Researchers have seen these hidden movements in powerful computer simulations of grains between rotating cylinders<\/a> or in landslides<\/a> that model each particle’s movement using basic laws of physics. But that’s a purely theoretical way to study them.<\/p>\n Observing the flows in real granular materials\u2014such as sand, snow or similar\u2014has been almost impossible. To look inside the flow, you’d either have to keep stopping the grains for standard X-ray scans or add a liquid that makes them see-through. Unfortunately, both approaches change how the grains naturally behave.<\/p>\n We set out to study the flow of glass beads (a perfect granular material for a lab setting) in a bulldozing experiment, where a conveyor belt pushes grains into a wall, causing them to pile up.<\/p>\n