Hybrid magnetic Janssen effect arising from percolating ferromagnetic grain networks.

Abstract

When confining granular materials, vertical stresses are redirected toward the sidewalls, causing the pressure at the base of the granular column to saturate - a phenomenon called the "Janssen effect". This behaviour originates from the heterogeneous force-chain network within the packing, constrained by geometry and friction. Using ferromagnetic grains under an external magnetic field, we recently demonstrated the remote control of this stress redirection, giving rise to a "magnetic Janssen effect". Here, we go further by showing that the apparent mass of granular columns can be precisely controlled by varying the fraction of ferromagnetic grains and their spatial arrangement within the packing. Combining experiments with discrete-element numerical simulations, we uncover a hybrid magnetic Janssen effect: the apparent mass at the base decreases once the fraction of magnetic grains exceeds a critical threshold. This transition corresponds to the percolation of magnetic clusters, which form a system-spanning network capable of channeling forces laterally. Our findings establish a straightforward pathway toward programmable granular materials, in which load distribution can be actively tuned using external fields.