Soft robotics potential: New liquid metals can move and stretch both horizontally and vertically

In the blockbuster Terminator movie franchise, an evil robot morphs into different human forms and objects and oozes through narrow openings, thanks to its “liquid-metal” composition. Although current robots don’t have these capabilities, the technology is getting closer with the development of new liquid metals that can be manipulated in 3D space with magnets.

Reported in ACS Applied Materials & Interfaces, the materials could someday find applications in soft robotics.

Metals that are liquids at room temperature, such as gallium and certain alloys, have unique properties including high conductivity, low melting point and high deformability. These properties make them attractive for use in soft robots and flexible electronics. By adding magnetic particles, such as nickel or iron, researchers can produce liquid metals that they can manipulate with magnets. However, because of their high surface tension, most magnetic liquid metals can only move horizontally, and they must be immersed completely in liquid to avoid forming a paste. Liang Hu, Jing Liu and colleagues wanted to make a magnetic liquid metal that they could move and stretch both horizontally and vertically, without needing to put the material completely in a liquid.

To do so, the researchers first worked with the material submersed in liquid. They added iron particles to a droplet of a gallium, indium and tin alloy immersed in hydrochloric acid. A gallium oxide layer formed on the droplet surface, which lowered the surface tension of the liquid metal. When the team applied two magnets in opposite directions, they could stretch the droplet to almost four times its resting length. They could also manipulate the liquid metal to connect two immersed, horizontal electrodes and, by virtue of its conductive properties, light up an LED bulb. The liquid metal could even stretch vertically and then move horizontally to connect two electrodes –– the upper one exposed to air, and the lower one in the hydrochloric acid. This demonstrated that the material didn’t have to be fully immersed in liquid. In this way, the magnetic liquid metal was reminiscent of an upright walking amphibian, the researchers say.

In the present study, a magnetic liquid metal droplet (MLMD), which can be stretched in large scales both horizontally and vertically in the free space, is introduced. This MLMD is fabricated based on a multimaterial system including liquid metals, iron particles, and electrolytes. Such remarkable stretching capacity is reversible, long-lasting, and can be repeated for multiple times. The seemingly contrary properties, the good stretchability and the mechanic strength for three-dimensional (3D) stretch, should owe to the surface oxide over the MLMD. On the basis of the 3D stretching ability of the MLMD, an intelligent scalable conductor was achieved, which can make electrical connections at various directions in the 3D free space. Moreover, the vertically stretched MLMD can move horizontally with its half body in the solution and the other half in the air, which resembles the nature of an upright walking amphibian.

All the behaviors can be precisely, conveniently, and contactlessly controlled by the magnetic field provided by permanent magnets. With all the appealing properties, this MLMD presents a fundamental and promising platform for the liquid metals to further develop the multi-freedom actuation in free space and eventually lead to the dynamically reconfigurable intelligent and biomimetic soft robots in the future.

Learn more: ‘Terminator’-like liquid metal moves and stretches in 3D space