Recently the research group led by Prof. Yujie Wang, School of Physics and Astronomy, SJTU, significantly advanced our understanding about the relaxation dynamics of granular materials on the level of the particles. This work, obtained in collaboration with Prof. Jie Zhang from SJTU, Haipeng Dong from Ruijin Hospital, SJTU School of Medicine and Prof. Walter Kob from the University of Montpellier, France has just been published in Nature with the title ”Granular materials flow like complex fluids” [Nature 551, 360–363(2017)] on Nov 1,2017.

Granular materials (sand, powders, etc.,) behave quite differently from traditional continuous medium such as elastic solids and simple liquids. This is because granular materials are by nature out of equilibrium systems because of the inelastic collisions and nonlinear friction between the particles. Despite the great need to have an accurate theoretical description of granular materials for engineering and geotechnical applications, there exist so far only empirical macroscopic descriptions of these materials. Therefore, it is of fundamental importance to develop a theory for granular materials starting from its microscopic dynamics.  

Left panel: Snapshot of the 3D ellipsoid system. Right panel: Time dependence of the translational mean squared displacement for different strain amplitudes gamma.

To probe the microscopic dynamics of granular materials the researchers in Prof. Yujie Wang’s group used the X-ray computational tomography (CT) technique to probe the positions of all the particles in a cell that was sheared. These experiments demonstrate that the relaxation dynamics of such "frictional" systems is qualitatively different from that of "thermal" systems and that granular materials are "marginal solids", i.e. stable if unperturbed but fluid under the slightest external perturbation. The mechanism that leads to this behavior is very general which indicates that these frictional out-of-equilibrium systems obey a new class of microscopic dynamics. It will help the development of a new continuum theory for granular materials and have deep impact for many research fields such as geoscience (e.g. earthquakes, landslide, and debris flow).

Paper Link: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature24062.html