Three-dimensional drifting control of magnetic colloidal collectives. (A) The schematic diagram shows the motion mechanism of natural plankton. (B) The schematic diagram shows the colloidal collective climbing across a high obstacle under bimodal actuation fields (magnetic and optical fields). First, driven by the tailored rotating magnetic field, the settled ferrofluidic colloids self-assemble into a dynamic stable colloidal collective. Second, the optical field stimulates the colloidal collective to generate convective flow through the photothermal effect, thus allowing the colloidal collective to use currents for 3D drifting motion like the plankton. The proposed colloidal collectives can propel themselves in 3D space, transit between air-water surfaces, and move on the water surface. Credit: Science Advances, doi: 10.1126/sciadv.adj4201

Generation of the upward and downward movements of the colloidal collective. (A) Dispersed colloids (<1 μm) dynamically assemble into a colloidal collective inside deionized water upon being energized by the rotating magnetic field (f: from 10 to 50 Hz, Bm: 9 mT, θ: 0°). Scale bar, 100 μm. (B) Simulation results of the temperature and convective flow velocity distribution around the colloidal collective. The temperature difference between the collective and surrounding fluids (water) is 20 K. The background colors indicate the temperatures and velocities of the surrounding fluid. The white arrows represent the velocity vectors of flow. (C) Process in that the colloidal collectives rise and sink. The "M" and "O" labels indicate magnetic (f: 50 Hz, Bm: 9 mT, θ: 0°) and optical fields (λ: 808 nm, P: 2 W). The red and black arrows indicate the moving directions of the colloidal collective. Scale bar, 1 mm. Credit: Science Advances, doi: 10.1126/sciadv.adj4201

Controllable transition of the colloidal collective through the air-water interface. (A) Transition of the colloidal collective from underwater to the water surface. The "M" and "O" labels indicate magnetic (f: 50 Hz, Bm: 9 mT, θ: 0°) and optical fields (λ: 808 nm, P: 5 W). (B) Colloidal collective sinks into water with an inclined posture (f: 50 Hz, Bm: 9 mT, θ: from 0° to 20°). [(A) and (B)] Scale bars, 3 mm. Credit: Science Advances, doi: 10.1126/sciadv.adj4201

Adaptive locomotion of the microrobot collective. (A) Illustration of the microrobot collective locomotion underwater and at the air-water interface among 3D obstacles. The microrobot collectives can move underwater, maneuver on the water surface, dive into water, and make transitions between the water surface and the underwater environment. (B) Microrobot collective moves on the water surface under the magnetic field (f: 50 Hz, Bm: 9 mT, θ: 10°). (C) Microrobot collective climbs up the water meniscus under the optical field. (D) A collective crosses an obstacle with a height of 10 mm. (E) Microrobot collective passes through a channel with a diameter of 2.5 mm (f: 50 Hz, Bm: 9 mT, θ: 10°). (F) Microrobot collective crosses a gap with a width of 10 mm and climbs the high obstacle along the water-air interface. [(A) to (F)] Scale bars, 3 mm. Credit: Science Advances, doi: 10.1126/sciadv.adj4201