Researchers at North Carolina State University have developed a method to control the surface tension of liquid metals by applying extremely low voltages, opening the door to a new generation of reconfigurable electronic circuits, antennas and other technologies. This method relies on the fact that the oxide “skin” of the metal, which can be deposited or removed, acts as a surfactant, reducing the surface tension between the metal and the surrounding liquid. googletag.cmd.push(function() { googletag.display(‘div-gpt-ad-1449240174198-2′); });
        The researchers used a liquid metal alloy of gallium and indium. In the substrate, the bare alloy has an extremely high surface tension, about 500 millinewtons (mN)/meter, which causes the metal to form spherical patches.
        “But we found that the application of a small positive charge – less than 1 volt – caused an electrochemical reaction that formed an oxide layer on the surface of the metal, which significantly reduced the surface tension from 500 mN/m to about 2 mN/m.” said Michael Dickey, Ph.D., associate professor of chemical and biomolecular engineering at North Carolina State and senior author of the paper describing the work. “This change causes the liquid metal to expand like a pancake under the force of gravity.”
        The researchers also showed that the change in surface tension is reversible. If the researchers change the polarity of the charge from positive to negative, the oxide is removed and the high surface tension returns. The surface tension can be tuned between these two extremes by changing the stress in small increments. You can watch the video of the technique below.
        “The resulting change in surface tension is one of the largest ever recorded, which is remarkable given that it can be controlled at less than a volt,” Dickey said. “We can use this technique to control the movement of liquid metals, which allows us to change the shape of antennas and make or break circuits. It can also be used in microfluidic channels, MEMS, or photonic and optical devices. Many materials form surface oxides, so this work can be extended beyond the liquid metals studied here.”
        Dickey’s lab has previously demonstrated a liquid metal “3D printing” method that uses an oxide layer that forms in air to help the liquid metal retain its shape – similar to what an oxide layer does with an alloy in an alkaline solution. .
       “We think oxides behave differently in basic environments than in ambient air,” Dickey said.
       Additional information: The article “Giant and switchable surface activity of liquid metal through surface oxidation” will be published on the Internet on September 15 in the Proceedings of the National Academy of Sciences:
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