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]