The control of the droplet shape is not limited to elongation, bending,
and on-demand droplet splitting and merging. Patterns can even be
achieved on a surface without a necessitating a water layer by first
elongating a droplet with a centimeter-sized bar magnet on a
superhydrophobic and liquid metal phobic (lyophobic) surface (SEM images
of the surface morphology is given in Figure S6), followed by
magnetically steering of one end of the elongated liquid metal-based
robot with the tip of the bar magnet, as schematically shown in Figure
5a. In general, the oxide skin of gallium-based liquid metal is
considered to be sticky and actuation on dry medium is therefore
difficult. Adhesion of liquid metal in such an instance can be avoided
by decorating the liquid metal with nano-/microparticles. Yet, this
would not be conducive in this experiment, as the deformation would
generate new surfaces, which would be exposed and could adhere to the
surface. The role of the liquid metal phobic surface is to reduce
friction and adhesion of the surface towards the soft robots by reduced
contact area (viz., by roughness) in combination with the solid-like
behavior of the oxide skin.[58,59] Due to the oxide layer on the
robot surface, which exhibits a yield stress, the droplet maintains the
shape after the bar magnet is removed. Figure 5b signifies that the
liquid metal-based robots can be steered. For example, it is feasible to
draw Arabic numbers 1 to 10, and thus, this method offers a general way
to deform the liquid metal droplet into complex shapes in a
reconfigurable and reversible fashion, which can be leveraged for
dynamically reconfigurable and recyclable switches in complex circuits
and electronics. Moreover, the phase transition temperature of the
liquid metal can be exploited to fix a desired shape after the
programmable shape encoding.
Finally, the phase transition temperature of the liquid metal (Gallium)
can be exploited to fix a desired shape or interconnection. For example,
a connection can be established at room temperature, followed by
freezing the liquid metal in place (which results in a highly increase
modulus (solid state)). Once reconfiguration gets necessary, the robot
can be melted, yielding the soft and reconfigurable state again (Figure
5d, Movie S4). To change the melting and solidification temperature, Ga
can be alloyed with In and Sn to reduce the melting temperature or Cu to
increase the melting temperature.[60]