Langmuir 2020, 36, 11125–11137.
Abstract
The motion of peanut-shaped magnetic microrods (PSMRs) with different magnetic moment (Ms) orientations φM under a nonuniform AC magnetic field has been investigated systematically. When gradually changing φM from 90° (perpendicular to the long axis of the PSMR) to 0°, the motion of the PSMR evolves from rolling to precession, then to tumbling. Systematic investigations on the translational velocity vp versus the magnitude of the applied magnetic field B and the angular velocity ωB show that the overall motion of the PSMRs can be divided into four different zones: Brownian motion zone, synchronized zone, asynchronized zone, and oscillation zone. The vp–ωB relationship can be rescaled by a critical frequency ωc, which is determined by Ms, B, and a hydrodynamic term. An intrinsic quality factor qm for the translational motion of a magnetically driven micro-/nanomotor is defined and is found to range from 0.73 to 13.65 T–1 in the literature, while the Fe PSMRs in the current work give the highest qm (= 25.48 T–1). High speed movies reveal that both the tumbling and precession motions of the PSMRs have a discretized nature. At the instances when the magnetic field changes direction, the PSMR performs an instantaneous rotation and the strong hydrodynamic wall effect would impose a driving force to move the PSMR translationally, and about more than 60% of the time, the PSMR neither rotates nor moves translationally. Based on this discretized motion nature, an analytic expression for qm is found to be determined by the shape of the surface walker, the hydrodynamics near a wall, and the magnetic properties of the surface walker. This work can help us to better understand the motion of magnetic surface walkers and gain insight into designing better micro-/nanomotors.