Figure 2. Controlled deformation of the liquid metal droplets.(a) Schematic illustration of the magnetic field-controlled elongation
of the liquid metal droplet robot in water on an acrylic substrate. (b)
Simulation result showing the magnetic field distribution of a bar
magnet with the size of 5×1×0.5 cm3 (Side view). (c)
Simulation result showing the magnetic field distribution of a bar
magnet (5×1×0.5 cm3) at the distance of 4 mm and the
distance of 2 mm (Top view). (d) Magnetic field-induced reversible
elongation and deformation of a liquid metal soft robot upon
manipulation of the magnetic field by the distance of a bar magnet. (e)
Dependence of the magnetic field strength on the distance between the
bar magnet and the sample. (f) Dependence of the length of a composite
droplet on the magnetic field strength. The initial length and height
were 6.8 mm and 1.6 mm, respectively. (g) Schematic illustration of the
generation of a circular conductor via converging of a circular and
permanent magnet to an LM composite on acrylic substrate, followed by
rotating the magnet. (h) Simulation result showing the magnetic field
distribution of a circular magnet (outer diameter is 3.5 cm, inner
diameter is 1.5 cm, height is 0.5 cm) at the distance of 4 mm and the
distance of 2 mm (Top view). (i) Simulation result showing the magnetic
field distribution of a circular magnet (side view). (j) Optical
photographs of the shape transformation of a composite puddle into a
semi-circle and a circle, as shown schematically in (g).
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 (λC , Figure
S5).[38,57]
\begin{equation}
D>\lambda_{C}\approx 2\pi\sqrt{\frac{\sigma}{\frac{d}{\text{dz}}(\mu_{0}HM)}}\nonumber \\
\end{equation}
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.