Energy conversion in ferrofluids: Magnetic nanoparticles as motors or generators

Abstract

We submit a ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ ferrofluid in rigid rotation (and then in a Couette flow) to an alternating magnetic field. Rotational viscosity and transverse magnetization are measured simultaneously. Depending on the relative values of fluid vorticity and field frequency, the magnetic particles behave as nanomotors or nanogenerators. It demonstrates the energy conversion between the magnetic and kinetic degrees of freedom of the particles. The effect of an hydrodynamic shear on the spectrum of relaxation times evidences an intimate structure of the ferrofluid consisting of small chains of dipoles.