Rotational Dynamics of Magneto-optically Driven Macroscopic Objects through Photothermally Induced Ferrohydrodynamics of Nanofluid Assembly

The remote manipulation of large macroscopic objects using micro- and nanoswimmers in fluidic environments offers significant potential in robotics. Conventional optical control of molecules or microsystems based on light-generated angular momentum or recoil force from light–matter interaction is limited by the force generated by an electromagnetic field. A novel optomechanical system utilizing a laser-induced thermomagnetic force gradient in ferrofluidic microdroplets is developed to generate a larger torque, enabling the controlled rotation of macroscopic objects along any desired axis. This unique mechanism leverages the torsional motion of magnetically formed ferrofluidic liquid microdroplet domains controlled by spatially localized laser-induced photothermal effects. The microdroplets within the ferrofluid volume are coupled to macroscopic objects with extensive surfaces, allowing for the optical rotation of heavier payloads on the water surface without the need for mechanical actuators. The rotational direction can be switched from clockwise to counterclockwise by adjusting the laser’s position. Angular velocity is adjustable through laser intensity and depends on the object’s mass. Experimental results show that a giant torque of 6 nN·μm can be achieved with a 300 mW laser power, and the maximum carry-to-self-weight ratio approaches 140 times the weight of the droplet being trapped. It opens new avenues for remote-controlled robotics and fluidic manipulation, allowing for more efficient and versatile control of macroscopic objects in various environments.