Stabilization of the out-of-plane precession of magnetic nanorods in Magnetic Rotational Spectroscopy experiments*

Magnetic Rotational Spectroscopy (MRS) is a nanorheological technique offering analysis of the rheological response of a complex fluid on weak shear loads caused by rotating nanorods. MRS relies on imaging complete revolution of nanorods and thus offers high sensitivity. The original technique involves analysis of planar rotation of nanorods and determining a critical frequency of rotating magnetic field at which a synchronous rotation turns into an asynchronous rotation. Magnetic nanorods are subject to the out-of-plane perturbations of the ambient magnetic field and it becomes a real difficulty for an experimentalist to cancel the bias field: Earth’s field appears to be a significant obstacle for the MRS experiments. We theoretically predicted and experimentally validated that a bias field surprisingly acts favorably for MRS in viscous fluids as it stabilizes synchronous precession of the nanorod. The phase portrait of the associated dynamic system predicts unexpectedly complex dynamics of 3D rotation of nanorods. We report on the impact of these dynamics on studying viscosity of nanoliter droplets.