Reconfiguring ferromagnetic microrod chains by alternating two orthogonal magnetic fields.

It is well-known that ferromagnetic microrods form linear chains under an external uniform magnetic field B and the chain length is strongly dependent on the applied field, the applied time duration, and the microrod density. When the chains become long enough and the B-field switches to its orthogonal direction, an irreversible morphological transition, i.e. a parallel linear chain array to a 2D network, is observed. The formation of the network depends on the ratio of the average chain length L and separation D, L/D, as well as the magnitude of the changed B-field. When the chain pattern has an L/D larger than a critical value, the network structure will be formed. Such a critical L/D ratio is a monotonic function of B, which determines the bending length of each magnetic chain during the change of B-fields. This morphological change triggered by external magnetic field can be used as scaffolds or building blocks for biological applications or design smart materials.