Secret life of grains: Hidden swirling motion observed for first time

Scientists have, for the first time, directly observed hidden swirling motions—known as secondary flows—within flowing granular materials like sand and snow, using a novel X-ray imaging technique called X-ray rheography. These secondary flows involve grains moving sideways or in loops beneath the surface, rather than simply following the primary downhill path. Previously, such motions were only predicted by computer simulations or inferred indirectly from surface patterns, as traditional methods either disturbed the flow or altered grain behavior. The researchers overcame these challenges by designing a conveyor-belt experiment with glass beads and capturing rapid X-ray images that allowed them to track grain movements inside the pile in real time without interrupting the flow. This breakthrough provides the first direct experimental evidence of secondary flows in granular media, confirming that these swirling currents are a fundamental characteristic of materials like snowdrifts, sandpiles, and grain silos. The discovery has significant implications for understanding natural hazards such as landslides and avalanches, where ignoring secondary flows in models may lead to