The liquid metal-based soft robotic platform consists of the
abovementioned liquid and magnetic composite and an external magnetic
field setup (a permanent magnet but an electromagnetic can in principle
also be used). Initially, a bar magnet was used to characterize shape
transformation of the composite and exerted field strength to the
composite. The bar magnet was positioned below an acrylic sheet. The
liquid metal composite was placed on the acrylic sheet surrounded by
water, in order to mitigate adhesion of the oxide layer of the liquid
metal. The resistance to adhesion can be ascribed to a slip layer
between acrylic sheet and the liquid metal-based robot (oxide skin).[56] A scheme of the experimental setup can be viewed in Figure 2a.
The simulation result in Figure 2b describes the magnetic field
distribution around the bar magnet. More specifically, the magnetic
field distribution of the planes at certain distances along the vertical
axis are shown in Figure 2c. The strong magnetic field strength along
the bar magnet is the driving force for the elongation of the liquid
metal droplet robots. For example, with a bar magnet, gradually
increasing the field strength H and the vertical gradient
dH /dz acting on the droplet (by decreasing the gap between
the magnet and the liquid metal droplet) leads to variation of the
length of the composite robot, as shown in Figure 2d. In contrast, by
using a circular permanent magnet, the composite forms a semi-circular
conductor. Importantly, the elongation of the composite can be modulated
by the distance between the LM composite and the bar magnet. At long
distance between the composite and the bar magnet, the liquid metal
exhibits a spherical shape (due to interfacial tension) and the marginal
pulling force of the magnetic field exerted on the magnetic particles in
the composite (first image in Figure 2d). Upon decreasing the distance,
the pulling force (field strength of the bar magnet, as shown in Figure
2e) gets greater and deformation of the droplet toward a liquid metal
line can be observed. This deformation gets more pronounced the higher
the field strength gets. Interestingly, the shape transformation can be
reversed by slowly removing the bar magnet (Figure 2d). The observed
stretching and curl up of the composite resemble the shape change of a
leech. In Figure 2f, the length of the composite is plotted versus the
magnetic field strength. Initially, the LM composite is shaped ovaloid
with a length of around 6.8 mm and a height of 1.6 mm. Upon application
of the magnetic field, the composite elongates to around 22-23 mm at 100
- 160 mT, which is ascribed to the pulling force of the bar magnet’s
magnetic field. Upon converging of the bar magnet and the composite, the
field strength rises to 200 mT and the length increases further to 25
mm. Thus, a 3.5-fold increase in length can be achieved by this means.
Importantly, this approach is not limited to shape transformation into
elongated structures, but can also be used to generate bend structures,
as shown in the scheme in Figure 2h. To generate a circular composite
conductor pattern on a surface, a circular magnet can be used. The
circular magnet generates a magnetic field with a strong field strength
above its circular shape and forms a weak magnetic field at the circular
center, resulting in bending of the liquid metal droplet robots (Figure
2h and i). Upon rotating the circular magnet along the vertical axis of
its diameter, a circular conductive trace is generated on the acrylic
substrate, as shown in Figure 2j. This shape morphing is interesting as
we show later that the shape transformation is reversible. Generally,
the extend of shape transformation achievable by this means is greater
for a bigger composite droplet, and droplets with a volume of 50 μL or
above work well.