3D imaging furthers understanding of avalanches
Advanced 3D imaging techniques developed by researchers at Duke University are furthering the understanding of early warning signs of earthquakes and avalanches.
The international team of researchers collaborating at Duke University has developed a new way to measure the forces inside materials such as sand, soil or snow under pressure.
Described in Nature Communications, the technique uses lasers coupled with force sensors, digital cameras and computer algorithms to measure the forces between neighboring particles in 3D.
The work aids understanding of what happens inside granular materials when they are put under pressure, such as the force of gravity on a snow-covered mountain slope. Sand, for instance, flows through the gap in an hourglass like a liquid, yet supports the weight of a human like a solid.
Understanding fully the forces involved when granular materials are put under pressure has been surprisingly difficult. Study co-author Nicolas Brodu, now at the French institute Inria, along with physicists Robert Behringer of Duke University and Joshua Dijksman of Wageningen University in the Netherlands, described in the paper how they used simple tools to measure the network of forces at it spreads from one particle to the next.
The researchers used a solution of hundreds of translucent hydrogel beads in a Plexiglass box to simulate materials like soil, sand or snow.
A piston repeatedly pushed down on the beads in the box while a sheet of laser light scans the box, and a camera takes a series of cross-sectional images of the illuminated sections.
Like MRI scans used in medicine, the technique works by converting these cross-sectional slices into a 3D image.
Custom-built imaging software stacks the hundreds of thousands of 2D images together to reconstruct the surface of each individual particle in three dimensions, over time. By measuring the tiny deformations in the particles as they are squeezed together, the researchers are able to calculate the forces between them.
The new approach will help researchers better understand a range of natural and manmade hazards, such as why farm workers stepping into grain bins sometimes experience a quicksand effect and are suddenly sucked under.
‘This gives us hope of understanding what happens in disasters like a landslide, when packed soil and rocks on a mountain become loose and slide down,’ Brodu said. ‘First it acts like a solid, and then for reasons physicists don't completely understand, all of a sudden it destabilises and starts to flow like a liquid. This transition from solid to liquid can only be understood if you know what's going on inside the soil.’
The team has already used results from their technique to create a new model for the way particulate matter behaves, which is concurrently published in the journal Physical Review E.