Directing the Diffusion of a Nonmagnetic Nanosized Active Particle with External Magnetic Fields.

With the help of molecular dynamics simulations we show that an arbitrary nonmagnetic active particle with a size below one micrometer, being immersed in a suspension of magnetic nanoparticles, can diffuse faster along the direction of an applied field than perpendicular to the latter. This effect is demonstrated in monodisperse and polydisperse systems of magnetic nanoparticles for magnetic fields of moderate strength. The ability to direct a nonmagnetic active particle along the magnetic field stems from the formation of chains of magnetic nanoparticles aligned with the field direction. Such chains form effective channels through which the active particle can diffuse. We combine the investigations of the diffusion and transport efficiency of the active particle parallel and perpendicular to the field with the structural analysis of the magnetic nanoparticle system and find that the ability to direct an active particle of a given size can be maximized by changing magnetic particle concentration. The optimal transport efficiency is achieved at a concentration of magnetic material that provides a mean width of the effective tunnels that matches the effective size of the active particle.