Two time scales drive the formation of transient networks in a ferrogranular experiment

Abstract

We are reporting experiments on the aggregation dynamics in a horizontally confined granular mixture composed of glass and magnetized steel spheres, under vertical vibration. Upon a sudden decrease of the shaking amplitude, magnetized particles undergo a transition from a dispersed to an aggregated state. For deep quenches, a transient, percolating network of magnetized spheres rapidly emerges and gradually coarsens into compact clusters. In contrast, moderate amplitude reductions lead directly to dense cluster formation without intermediate networking. Using structural and network metrics such as coordination number and its mean value, we identify two characteristic timescales: a fast one governing the head-to-tail chaining typical of dipolar hard spheres (DHS), and a slower one corresponding to the restructuring into compact aggregates. This progression is driven by the intrinsic susceptibility of the beads but is challenged by inter-sphere-friction. Thus, the employed susceptible dipolar hard spheres (SDHS) are a minimal model for phase separation with two intrinsic time scales in only one constituent.