Active particles are bodies able to self-propel through a certain environment by converting energy from their surroundings into oriented and/or persistent motion. Microscopic active particles include self-propelled cells and microorganisms, as well as artificial swimming colloids and nanomotors. These systems have become a subject of great research interest in recent years due to their relevance in such important fields as biology and biomedicine, nanoscience and nanotechnology.
Regarding nanotechnology and biomedicine applications, one important aspect in systems of microscopic active particles is to reach an effective external control of their motion. Whereas artificial active particles can be designed to allow such a external control, this feature can be much more complicated to achieve for self-propelled cells and microorganisms.
Here we explore theoretically the possibility to control the main direction of motion in a dispersion of generic microscopic active particles by adding to the carrier fluid a viscoelastic bath of magnetic nanoparticles. Liquid dispersions of magnetic nanoparticles, also known as ferrofluids, are smart materials that change their rheological properties under the influence of external magnetic fields due to the field-induced magnetic assembly of the nanoparticles. By means of extensive computer simulations, we study the influence of the field-assembled structures of nanoparticles on the motion of the active particles, characterizing the conditions that provide a higher control of the system.