We studied experimentally and numerically the compaction and subsequent expansion dynamics of a granular bed composed of cylindrical repelling magnets contained in a two-dimensional cell. The particles are firstly compressed vertically with a piston at a given strain rate until a maximum force is reached. The piston is then removed at the same strain rate while the bed expands due to the magnetic repulsion of the particles. In the experiments, two different initial configurations were generated, a standard and a loose packing bed. The standard packing bed was simulated, and modelling the dry friction between the magnetic particles and the walls of the cell was crucial for the correct description of the compression and expansion dynamics. We found that the force acting on the piston increases continuously and exponentially with the piston stroke during compression, being very sensitive to the initial packing conditions of the bed. In contrast, a history-independent exponential decrease of this force was found during the expansion phase. The hysteresis in the system was quantified in terms of the average displacement of the particles. The continuous compression contrasts with the sudden force drops observed during the compaction of granular materials with direct particle-particle contacts, where stick-slip motion is induced by friction and force chain breakage. Moreover, we found that the short range of magnetic interaction induces density inversion and crystallization of the system. Our results can be useful to develop a new kind of magnetic granular dampers.Graphical abstract

We introduce a new Magnetic Resonance Imaging technique to study the geometry of shear zones of soft, low-frictional and hard, frictional granular materials and their mixtures. Hydrogel spheres serve as the soft, low-frictional material component, while mustard seeds represent rigid, frictional grains. Some of the hydrogel spheres are doped with hbox {CuSO}_4 salt to serve as tracers. A cylindrical split-bottom cell is sheared stepwise and the shear profiles are determined from the differences of tomograms after successive shear steps, using Particle Imaging Velocimetry and Particle Tracking Velocimetry. We find that the shear zone geometry differs considerably between soft grains submersed in water and the same material without the embedding fluid.Graphic abstract

We study the transmission of compressive and tensile stresses, and the development of stress - induced anisotropy in entangled granular structures composed of nonconvex S-shaped hooks and staples. Utilizing discrete element simulations, we find that these systems exhibit fundamentally different behavior compared to standard convex particle systems, including the ability to entangle which contributes to a lower jamming packing fraction and facilitates the transmission of tensile stresses. We present direct evidence of tensile stress chains, and show that these chains are generally sparser, shorter and shorter-lived than the compressive chains found in convex particle packings. We finally study the probability distribution, angular density and anisotropic spatial correlation of the minor (compressive) and major (tensile) particle stresses. The insight gained for these systems can help the design of reconfigurable and recyclable granular structures capable of bearing considerable loads, without any need for reinforcement.Graphical abstract