Reversal Tuning of Liquid Metal Motor under Rotating Magnetic Field

Abstract Liquid metal motors are unconventional transformable machines whose motion control is important for microfluidics, drug delivery, and small‐scale robots. Without an external field, aluminum‐fueled liquid metal motors can achieve autonomous propulsion, but the random direction limits their applications. Here, the study reports a fundamental discovery of directionally reversing such liquid metal motors through a rotating magnetic field tuning approach. Systematic experiments reveal that the motor overcomes irregular Brownian motion and the volume limitation of liquid metals in magnetic control. Depending on its volume, the motor exhibits three different responses: moving opposite to the rotating magnet for smaller volumes, spinning in place at medium sizes, and moving with the magnet for larger ones. This phenomenon is attributed to the interaction of electromagnetic remote inductions, electrochemical reactions, and fluid dynamics. Rapidly changing magnetic fields induce currents and Ampere's force in the motor, restricting the locations of chemical reactions. Surface tension gradient and bubble recoil dominate the motion of small motors opposite to the magnet. As practical illustrations, this magnetically controlled motor displays advantages in rapid fluid mixing. Overall, the present finding significantly improves the controllability of liquid metal motors and would generate profound impacts on developing future small soft robots.