Light‐Driven Rotational Control of Ferrofluid Liquid Marbles in Magneto‐Optical Traps for Optical and Acoustic Modulation

Abstract Optically induced rotation relies on dynamic optical forces acting on microscopic particles via spatiotemporal light modulation, angular momentum transfer, or recoil forces. However, rotating macroscopic particles using light fields with intensity, polarization, or phase variations is challenging, especially underwater, due to the low torque of electromagnetic fields. This study demonstrates stable optical trapping and switchable light‐induced rotation of millimeter‐scale liquid marbles (LMs) in a uniform, time‐invariant DC magnetic field. These LMs, composed of self‐assembled ferrofluid droplets encapsulated by superhydrophobic particles with high photothermal conversion efficiency, float on a fluid surface and are stably trapped via controlled surface tension forces. Light‐induced photothermal interactions within the magneto‐optical field generate significant angular momentum, driving rotation through the Kelvin body force—resulting from a thermo‐magnetic gradient—and surface tension forces. This enables optically induced rotation of magnetically trapped coupled LMs, with angular velocity tunable via magnetic and optical field strength, fluid viscosity, and surface tension. Torques exceeding 170 nN·µm are achieved with a 200 mW CW laser, modulating underwater optical transmission at 1–4 Hz. This approach advances reversible optical torque generation and optical communication in fluidic systems. Additionally, ultrasonic waves modulate rotation by altering the nonequilibrium force distribution within ferrofluid LMs.