A Theoretical Analysis of Magnetic Particle Alignment in External Magnetic Fields Affected by Viscosity and Brownian Motion

Featured Application:& nbsp;Iron oxide nanoparticles with highly nonlinear magnetic behavior are attractive for biomedical applications, including biosensing using the rotational freedom of particles for detection of biomarkers for cancer cells and for contrast enhancement in magnetic resonance imaging (MRI). Hyperthermia therapy has been used for cancer therapy, and magnetic particle imaging (MPI) is a promising new imaging modality that can spatially resolve the concentration of nanoparticles. For the success of the technology, understanding the nanoparticle rotation mechanism is necessary. The presented computational model can be used in the study of magnetic particle alignment phenomena as the non-Markovian process with memory in the external force field as part of generalized Langevin theory. It can elucidate the significance of each kind of torque in this phenomenon, or can serve as the estimator of the characteristic time of magnetic particle alignment in a wide range of magnitudes of the external magnetic flux density field. Our results, therefore, have far-reaching implications for understanding and advancement of these emerging biomedical technologies. The interaction of an external magnetic field with magnetic objects affects their response and is a fundamental property for many biomedical applications, including magnetic resonance and particle imaging, electromagnetic hyperthermia, and magnetic targeting and separation. Magnetic alignment and relaxation are widely studied in the context of these applications. In this study, we theoretically investigate the alignment dynamics of a rotational magnetic particle as an inverse process to Brownian relaxation. The selected external magnetic flux density ranges from 5 mu T to 5T. We found that the viscous torque for arbitrary rotating particles with a history term due to the inertia and friction of the surrounding ambient water has a significant effect in strong magnetic fields (range 1-5T). In this range, oscillatory behavior due to the inertial torque of the particle also occurs, and the stochastic Brownian torque diminishes. In contrast, for weak fields (range 5-50 mu T), the history term of the viscous torque and the inertial torque can be neglected, and the stochastic Brownian torque induced by random collisions of the surrounding fluid molecules becomes dominant. These results contribute to a better understanding of the molecular mechanisms of magnetic particle alignment in external magnetic fields and have important implications in a variety of biomedical applications.