Coloration of Liquid-Metal Soft Robots: From Silver-White to Iridescent.

Gallium-based room-temperature liquid metals are becoming increasingly attractive and outstanding candidates for designing soft robots because of their remarkable electrical conductivity, superior flexibility, excellent stability, and low toxicity. However, the color of liquid metals is limited to shiny silver-white with high reflectivity, which is not helpful for camouflage, like that found in natural soft animals such as cephalopods. Herein, a biomimetic chromatic liquid-metal soft robot with tunable structural colors is reported. Colors ranging from white to gold and black appear on the surface of liquid metal when placed on a graphite substrate and mixed with Al foil in an electrolyte solution. A stable liquid-metal functional material with a rainbow-like appearance is realized under the regulation of an electric field. Further composition and structure characterization reveals that it is a nanoscale GaO film that displays the multicolor characteristic. The nanostructural film indicates that light scattering of GaO occurs when the liquid metal is on the graphite surface, and thin-film interference triggers iridescence when the liquid metals are subjected to electrolysis, respectively. These results provide a route to create kaleidoscopic and colorful liquid metals, which are expected to have diverse applications, especially in reinforcing soft robot design with intelligent camouflage function.