Aligned colloidal clusters: effect of magnetic relaxation time on the assembly of superparamagnetic colloidal beads

The application of magnetic fields to magnetically responsive colloidal systems provides a versatile approach to tune interparticle interactions and thereby drive the formation of different colloidal structures. The parameters of the magnetic field can be combined with the magnetic properties of the particles to obtain new kinds of particle arrangements. In this work, we harness the magnetic relaxation time of superparamagnetic colloidal beads to drive the assembly of aligned colloidal clusters by applying an alternating rotating magnetic field. These aligned colloidal clusters exhibit global orientational order and local crystalline packing. The anisotropic features of these clusters can be altered via the parameters of the alternating field. By comparing experiments and simulations of dimers (two particles) of superparamagnetic beads, we demonstrate the role of magnetic relaxation in the emergence of anisotropy. We explain how the combination of the alternating field and the magnetic relaxation produces an anisotropic time-averaged dipolar potential that predicts the alignment of dimers and clusters. This work opens up new avenues for colloidal assembly by accounting for the modulation of the magnetic field through the magnetic relaxation of the superparamagnetic beads.