Magnetic and hydrodynamic torques: Dynamics of superparamagnetic bead doublets

Rotating chains of magnetic microparticles have many applications in lab-on-a-chip technologies. The simplest such chain is the fluid-borne doublet, where two beads are in close contact, but remain unattached, allowing each bead to freely rotate. These beads typically have two components contributing to their net magnetic moment: (i) a superparamagnetic moment and (ii) a field-independent permanent moment. In a rotating magnetic field, there are magnetic torques that separately rotate the doublet and its constituent beads as well as a hydrodynamic torque from the bead-doublet coupling. This study investigates, through experiments and simulations, the dynamics of field-driven doublets. New dynamics were found for the case where the dominant torque stems from the hydrodynamic coupling.