la0c02132_si_001.pdf (1.3 MB)
Discretized Motion of Surface Walker under a Nonuniform AC Magnetic Field
journal contribution
posted on 2020-09-04, 17:11 authored by Yanjun Yang, Yiping ZhaoThe
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.