Not limited to shape transformation, the liquid metal can also be reversibly split up into several smaller liquid metal droplets by magnetic field actuation. This is achieved by a setup similar to the one shown in Figure 2g and S3. However, the circular magnet is not rotated along the vertical axis of the diameter of the ring magnet, but along the horizontal axis. Upon such a rotation and control of the distance between the composite and the magnet, the droplet robot gradually deforms (elongates) and finally splits into composite droplets, as shown schematically in Figure 3a. The simulation result in Figure 3b gives the variation of the magnetic field distribution before and after rotation. Splitting of a bigger droplet into 2, 3, and 8 droplets is shown in Figure 3c, 3d, and 3e, respectively (Movie S2). During the splitting process, one can observe the morphology change of the droplet surface because of the formed particle chains inside the droplet. The particle chains are generated by the dipole attractive forces under the magnetic field (as shown in Figure S4). (1) When the diameter of the liquid metal droplet is substantially larger than the characteristic size of the particle chains, the morphology of the liquid metal droplets before and after splitting will be consistent without obvious change. (2) When the diameter of liquid metal droplet is comparable with or even smaller than the characteristic size of the particle chains, the morphology of the liquid metal droplets will be affected by the incorporated particle chains. The splitting of the liquid metal droplet is due to a combination of high magnetic field strength and high vertical magnetic field gradient, which cannot be achieved with uniform magnetic fields. To trigger splitting of the liquid metal droplet (gravitational force of the droplet here is negligible compared with the magnetic force), the droplet diameter (D ) should be larger than the critical wavelength of the Rosensweig pattern (λ_C , Figure S5).^[38,57]

where σ is the surface tension of the liquid,μ₀ is the permeability of vacuum, and M is the magnetization. In our experiment, larger liquid metal droplets are much more susceptible to splitting. The droplet size of the split droplets appears to be comparable. Furthermore, the split droplets can be split again to obtain more magnetically moveable composite droplets. Thus, on demand breakup and disassembly of a robot into many smaller robots is achieved, which renders this approach useful to perform highly complex and sophisticated tasks.