The shape-shifting robots from Terminator 2 may be in for a reboot on the high seas. A liquid metal alloy less dense than water has been made by injecting the material with glass beads and it could be used to make lightweight exoskeletons or transformable robots. Like mercury, which has the lowest melting point of pure metals at -38.8°C, liquid metal alloys dont solidify at room temperature. They are also eutectic, meaning that they melt at a lower temperature than the individual melting points of the metals they are made from. Jing Liu at Tsinghua University in China and his colleagues have created such a material by mixing pure gallium and indium to create a liquid metal alloy with a melting point of 15.7°C. To decrease its density, they stirred tiny glass bubbles filled with air into the liquid. The loose beads, which were 75 micrometres in diameter or smaller, clustered together in the mixture. Oxygen mixes in with the liquid metal, which helps the glass beads stay suspended, says Liu. Those beads decreased the density of the gallium-indium alloy by up to 97 per cent without altering its other properties it has high electrical conductivity and it can be shaped and deformed without breaking. These features make it ideal for making exoskeletons that arent too heavy for practical use, or flexible robots that could transform depending on temperature, says Liu. Depending on the size of glass beads used, the liquids density varied between 0.45 grams per cubic centimetre less than half the density of water and 2.01 grams per cubic centimetre. He says this could come in handy for controlling the buoyancy of underwater devices. The team plans to test the effect of using different variations of beads, such as glass beads with a vacuum inside instead of air, or plastic beads.
newscientist.com, 5 March 2020