In recent years, room-temperature liquid metal has attracted much attention because of its versatile uses in energy management, chip cooling, printed electronics, 3D printing and biomedical applications. Self-fueled biomimetic liquid metal mollusk (Advanced Materials, 27: 2648, 2015, selected as inside Front Cover) has been proposed by the group led by Liu Jing at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS) and Tsinghua University. It can spontaneously convert chemical energy into mechanical activity to induce autonomous locomotion, and is thus an excellent candidate for making self-powered machines, detectors/sensors, and new robots.

In a recent Liu’s publication, “Transient state machine enabled from the colliding and coalescence of a swarm of autonomously running liquid metal motors” (DOI: 10.1002/smll.201501364), a new conceptual liquid metal machine style was defined and demonstrated. The proposed machine can work as either a large robot, or partial running elements, or just be divided spontaneously into running swarm of tiny motors. According to need, the discrete droplet machines generated through injecting the stream of a large liquid metal machine can combine back again to the original one. Throughout this process, each tiny machine keeps its running, colliding, bouncing, or adhesion states until finally assembling into a single machine. Unlike rigid machines, such a transient state system can be reversible in working shapes. Depending on their surface tension, the autonomously traveling droplet motors can bounce and collide before undergoing total coalescence, arrested coalescence, or total bounce. This finding would help mold unconventional robot in the sense of transient state machine that could automatically transform among different geometries such as a single or swarm, small or large size, assembling and interaction, etc. It refreshes people's basic understandings on machines, liquid metal materials, fluid mechanics, and micro-motors.

In addition, Liu’s group has made progress regarding the fundamental phenomena of liquid metal. They reported that the magnetic field can make up a boundary or trap to restrict the motion of aluminum-powered liquid metal motors, which suggests an important way to control their behavior (Applied Physics Letters, 107: 071904, 2015). The magnetic field combined with electricity was also found to be able to regulate and control rotation of a liquid metal sphere or pool within a solution (Proceedings of the Royal Society A, 471: 0150177, 2015, Cover paper). Further, the group disclosed the interiorly driven macroscopic Brownian motion behavior of self-powered liquid metal motors, the autonomous convergence and divergence phenomena of the self-powered soft liquid metal vehicles, and the oscillating coalescence and ejection of contacting liquid metal droplets. They went on to describe the surface tension mechanisms involved (Science Bulletin, 60: 1203, 2015; 60: 943, 2015, Cover paper; 60: 648, 2015). The work was partially supported by the Research Fund of the Chinese Academy of Science.