Autonomously Motile “Moonwalking” Microbeads with Internalized Hierarchical Magnetic Architectures

Self-propelling or “active” particles directed by external fields continuously consume and dissipate energy, thereby creating localized field and flow gradients, and enabling autonomous movement. The development of colloidal rollers responsive to magnetic fields offers the potential for external control of their macroscopic motion using magnets. In the current study, we report a new pattern of dynamic active motion with our micro-scale architectures in non-Newtonian fluids where colloidal micro-rollers on a solid substrate rotate and translate in directions some of which are intuitively not expected. These micro-scale rotators are synthesized by internally embedding iron oxide nanoparticles (MNPs) inside microdroplets of a polydimethylsiloxane (PDMS) precursor and assembling the MNPs into organized structures using a static magnetic field. Rotating magnetic fields, even with a low field strength and low rotation frequencies, can induce a strong torque on these microbeads due to the presence of the organized structures. The microbeads can therefore exhibit clockwise or counter-clockwise rolling with forward or backward displacement. Surprisingly, when we placed these rollers in shear-thinning fluids, we established a fascinating new backward movement pattern, which we refer to as “moonwalking.” In this new dynamic pattern, the translation is opposite to the one expected by the rotational direction of the particles. We hypothesize that this reversive motion results from differing shear forces acting on the top and bottom of the particle. The “moonwalking” motion was found to increase with increasing viscosities until a critical rotational velocity where the microbeads lose alignment with the magnetic field and oscillate in place. This propulsion behaviour can help us to guide the forward or backward propulsion of the rollers without changing the field type and just by tuning the properties of the medium or the rotational frequency. The novel phenomenon in the single particle translation dynamics observed introduces a new physical principle of active propulsion. This new phenomenon has the potential to deepen our understanding of fundamental principles of autonomous materials and may open the door to future applications of active rollers in advanced biomedical applications.