Importantly, the on-demand splitting of the LM composite robots is
reversible. To trigger the
coalescence process of liquid metal droplets, the system should overcome
the energy barrier between two liquid metal droplets that are generated
by the surface oxide layer. The oxide layer is in our case removed by
the alkaline medium. Afterwards, the coalescence process is a
spontaneous process because the liquid metal droplets tend to shrink
into spherical shape so as to minimize the surface energy. Upon moving
two or more droplets together they merge (coalesce), which is fueled by
the high interfacial tension of the liquid metal (and liquid metal
oxide), as shown schematically in Figure 4a. Here, the droplets are
steered and actuated by a bar magnet located below the acrylic sheet.
Upon converging of the two droplets in Figure 4a, the LM-based robots
quickly coalesce and the coalescence process is completed in less than 1
second. Moreover, the coalescence of several droplets (see Figure 4b,
Movie S3), an array of droplets (see Figure 4c), and a pattern (see
Figure 4d) can be done swiftly due to the good precision of the magnetic
actuation and the quick coalescence kinetics. This process can be
leveraged to remove and recycle the liquid metal-based robots to a high
degree.