Figure 2.

Quantification of the applied magnetic torque from unbound particle actuation. (a) The rotation frequency of an unbound particle as a function of the rotation frequency of the applied magnetic field, for field strengths of 5 mT and 10 mT. The particle continuously follows the magnetic field up to the breakdown frequency. (b) Above the breakdown frequency, the particle rotation frequency decreases due to remagnetization of the particle magnetic moment (characterized by the wiggles). The orientation of the magnetic field is plotted (shaded lines) and phase-shifted by multiples of 180° to show that the particle follows the magnetic field (5 mT) with a phase-lag of 180°. (c) At the breakdown frequency, i.e., right before the particle remagnetizes, the magnetic torque can be determined from the hydrodynamic drag.