Liquid metals are appealing materials that combine the fluidity of a liquid with the electrical conductivity and optical reflectivity of a metal. This hybrid behavior is useful for developing flexible, self-healing, or reconfigurable optoelectronic devices. Among such materials, the eutectic gallium-indium alloy (EGaIn) is of particular interest because, in addition to the properties mentioned above, its surface tension can be widely tuned by oxidizing or reducing its surface. Such tuning is achieved by immersing the EGaIn sample in an electrolyte and applying a suitable voltage. This has been applied to tailor the morphology of EGaIn droplets, or to guide EGaIn wires in space, for example through a maze. Here, Keisuke Nakakubo and coworkers further explore the fascinating applications of EGaIn. They harness its redox properties to design reversibly reconfigurable optical surfaces. Upon changing the voltage sign, the EGaIn surface switches between oxidized and reduced. This affects markedly its morphology that switches between rough and smooth. This switches its optical response between that of a scattering medium and that of a mirror. Such changes occur in less than one second, upon application of a ~1 V voltage. This is appealing for the design of fast and energy efficient reversibly reconfigurable devices.  

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