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The force unleashed: how forces are transmitted through grain piles


The behaviour of granular materials under pressure is important for a wide variety of subjects such as pharmaceutical pill manufacturing, food processing and even natural events such as earthquakes. Deciphering how grains, like sand or wheat, move when compressed can help predict and model these natural and industrial processes. Until today scientists didn´t have a clear idea of how external loads are transmitted through these materials because there was no way to measure the forces between particles in 3D, but now an international team has shed some new light onto this problem.

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Researchers from Lund University (Sweden), Lawrence Livermore National Laboratory, the California Institute of Technology (USA) and the ESRF combined 3D x-ray diffraction, x-ray tomography and mathematical calculations to quantify forces in an assembly of quartz grains while it was slowly compressed. The team obtained a map of the contact forces between the particles.

The team managed to identify force chains or the strings of connected particles that span the length or width of the pile and carry the bulk of the stress. Force chains appear when some of the particles lock together and hold up more of the force. The results showed that as the load increases, the distribution of the forces in the sand pile evened out. 

The research was possible thanks to a collaboration between the ID11 beamline and Lund University. As Steve Hall, leader of the team from Lund University, says: "This research was made possible by a strong collaboration between experimentalists at Lund University and the ESRF to develop a new approach to the experimental analysis of granular materials. The key was the possibility to do combined tomography and 3DXRD at the ID11 beamline during in-situ loading of the sample. Furthermore, an essential component was the 3DXRD data processing developed at ID11 and the strong user support that, over a number of beam times, enabled the realisation of the published experiment and has paved the way for further developments in this field, extending the approach to more representative numbers of grains and more challenging in-situ tests."



A map of the contact forces between grains in a sand pile provides a first glimpse of how force is distributed in a 3D granular system. Credit: Hurley R. C., et al, Phys. Rev. Lett. 117, 098005




Hurley R. C., et al, Phys. Rev. Lett. 117, 098005

Top image: Credits: Siim Sepp.