Enhanced and Robust Directional Propulsion of Light-Activated Janus Micromotors by Magnetic Spinning and the Magnus Effect

Abstract

Advances in the versatile design and synthesis of nanomaterials have imparted diverse functionalities to Janus micromotors as autonomous vehicles. However, a significant challenge remains in maneuvering Janus micromotors by following desired trajectories for on-demand motility and intelligent control due to the inherent rotational Brownian motion. Here, we present the enhanced and robust directional propulsion of light-activated Fe3O4@TiO2/Pt Janus micromotors by magnetic spinning and the Magnus effect. Once exposed to a low-intensity rotating magnetic field, the micromotors become physically actuated, and their rotational Brownian diffusion is quenched by the magnetic rotation. Photocatalytic propulsion can be triggered by unidirectional irradiation based on a self-electrophoretic mechanism. Thus, a transverse Magnus force can be generated due to the rotational motion and ballistic motion (photocatalytic propulsion) of the micromotors. Both the self-electrophoretic propulsion and the Magnus force are periodically changed due to the magnetic rotation, which results in an overall directed motion moving toward a trajectory with a deflection angle from the direction of incident light with enhanced speed, maneuverability, and steering robustness. Our study illustrates the admirable directional motion capabilities of light-driven Janus micromotors based on magnetic spinning and the Magnus effect, which unfolds a new paradigm for addressing the limitations of directionality control in the current asymmetric micromotors.

Achieving Control in Micro‐/Nanomotor Mobility

“…The latter can be observed in the work of Li et al or Shao et al Li and coworkers employed magnetic fields to reduce the rotational diffusion of magnetic, photo-catalytically driven particles and increase the directionality of movement (Figure 8c). [74] In Shao et al, a photo-responsive gold coating was placed on the surface of catalase-powered stomatocytes to enable propulsion at low H 2 O 2 concentrations. Fuel decomposition drove stomatocytes chemotactically in one direction.…”

“…With lowfrequency fields, the Janus spheres spin and counteract Brownian rotational diffusion, synergistically enhancing the directionality of MNM rotation (Figure 8b). [74] Alternatives in rotational movement are possible by employing spiral surface conformations that direct flows helically, as observed in Marangoni-powered, photo-switchable, chiral liquid crystal droplets of Lancia et al (Figure 9b). [31]…”

“…Several non-chemical fuelrelated engine combinations use motor design to achieve the direction of forces, ranging from acoustophoretic or lightresponsive components, magnetic materials with photocatalytic propulsion, and ultrasound/magnetic hybrids (Figure 11c). [74,84,91] Chemical fuel-related cases employ the combination of a catalyst and an external stimulus-sensitive component that can form fuel precursors or enhance catalytic rates, such as temperature. The most optimal and quickly actuating strategies include light-sensitive features that increase local concentrations of fuel in the motor.…”

“…Adapted from Ref. [74]. (c) Trajectory and torque‐driven reorientation mechanism of flask‐shaped motors in a chemical gradient.…”

“…Several non‐chemical fuel‐related engine combinations use motor design to achieve the direction of forces, ranging from acoustophoretic or light‐responsive components, magnetic materials with photocatalytic propulsion, and ultrasound/magnetic hybrids (Figure 11c). [74, 84, 91] …”