Separating two attractive elongated particles by hydrodynamic interactions under alternating external field

In applications where magnetic particles are used to detect and dose targeted molecules, it is important to prevent particle clustering and aggregation. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a finite range of low frequencies, depending on the field and magnetization intensities.