Abstract
Locomotion via cyclic moves presents a challenge to mesoscopic objects in overdamped environments, where time reversibility may prevent directed motion. Most reported cyclic movers exploit anisotropic drag to push themselves forward. Under an oscillating drive, however, anisotropic drag enables locomotion only if the objects can change their shape. Here, we present a strategy that unexpectedly enables structurally invariant objects to move under oscillating fields. The objects are self-assembled clusters of magnetic particles that exhibit an off-centered dipole moment. By theoretical modeling and in experiments with magnetic Janus particles, we demonstrate that the interaction between such anisotropic particles in the cluster breaks time reversibility. Experimentally, we show that the magnetic configuration of a cluster determines its motion path. We realize stirrers and steerable movers with helical or directed path using the same particle system. The presented strategy based on internal interactions establishes a counterpart to locomotion via anisotropic drag.
Highlights
Locomotion is a fundamental requirement for any form of life to explore their environment
Those particles exhibit a stray field with dipolar characteristic [34], and the dipole moment points along the Janus director
The key point is that the interaction between spheres with this magnetic asymmetry breaks time-reversal symmetry under oscillating fields
Summary
Locomotion is a fundamental requirement for any form of life to explore their environment. We present a different actuation concept where internal rotations in multicomponent objects can cause propulsion. The magnetic interactions between the particles we employ lead to a nonreciprocal rotation under field actuation. We employ colloidal particles with a radius of rp = 2.27 μm, which are equipped with a hemispherical ferromagnetic thin film of [Co(0.28 nm)/Pd(0.9 nm)]8 [33] to form so-called capped or Janus particles After magnetic saturation, those particles exhibit a stray field with dipolar characteristic [34], and the dipole moment points along the Janus director. The key point is that the interaction between spheres with this magnetic asymmetry breaks time-reversal symmetry under oscillating fields. The findings demonstrate that internal rotations in multicomponent objects built from anisotropic particles enable new actuation strategies for cyclic movers that complement established schemes based on hydrodynamic drag
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