Interplay between steric and hydrodynamic interactions for ellipsoidal magnetic nanoparticles in a polymer suspension.

Magnetic nanoparticles couple to polymeric environments by several mechanisms. These include van der Waals, steric, hydrodynamic and electrostatic forces. This leads to numerous interesting effects and potential applications. Still, the details of the coupling are often unknown. In a previous work, we showed that, for spherical particles, hydrodynamic coupling alone can explain experimentally observed trends in magnetic AC susceptibility spectra [P. Kreissl, C. Holm and R. Weeber, Soft Matter, 2021, 17, 174-183]. Non-spherical, elongated particles are of interest because an enhanced coupling to the surrounding polymers is expected. In this publication we study the interplay of steric and hydrodynamic interactions between those particles and a polymer suspension. To this end, we obtain rotational friction coefficients, relaxation times for the magnetic moment, and AC susceptibility spectra, and compare these for simulations with and without hydrodynamic interactions considered. We show that, even if the particle is ellipsoidal, its hydrodynamic interactions with the surrounding polymers are much stronger than the steric ones due to the shape-anisotropy of the particle.