Magnetic Janssen Effect

A pile of grains, even when at rest in a silo, can display remarkable physical properties. A classic example is the Janssen effect, named after the pioneering engineer who explained the saturation of pressure at the bottom of a container filled with corn due to the redirection of vertical stresses toward the sidewalls. This effect originates from frictional interactions between the grains within a disordered network of contacts and the vessel’s lateral walls, which partially support the weight of the column, thereby decreasing its apparent mass.

Here, we demonstrate that this stress redirection can be actively controlled using ferromagnetic grains subjected to an external magnetic field. The anisotropic interactions between magnetized grains generate a tunable radial force on the confining walls, giving rise to a controllable magnetic Janssen effect. Remarkably, this leads to a striking “more-is-less” phenomenon: for sufficiently strong vertical magnetic fields, adding more grains can reduce the measured mass at the bottom. Above a critical added mass, the granular column becomes effectively invisible to the bottom sensor, with a vanishing apparent mass. By varying the fraction and spatial arrangement of ferromagnetic grains, we further show that the apparent mass of the granular column can be finely tuned. Experiments combined with discrete-element simulations reveal a transition associated with the percolation of magnetic clusters that form a system-spanning network capable of channeling forces laterally. Beyond these striking static effects, our results open new perspectives for the design of programmable granular meta-materials with externally tunable mechanical and dynamical properties, for instance enabling the control and stabilization of silo discharge rates.