Composed of liquid metal, the robot assassin in Terminator 2 could change its shape at will. In boring real life, surface tension makes forming non-spherical liquid shapes impractical—at least until now. New research has yielded a technique that makes it possible to manipulate liquid metal into multiple configurations.
In a study just published in the Proceedings of the National Academy of Sciences, researchers from North Carolina State University describe how they've succeeded in controlling the surface tension of liquid metals by applying very low voltages—opening the door to a new generation of reconfigurable electronic circuits (see video below).
The scientists used a liquid metal alloy of gallium and indium—which has a remarkably high surface tension, causing it to bead up into a spherical blob. But, by applying less than one volt to the metal when it was immersed in water, they created an oxide "skin" that lowered the surface tension between the alloy and the surrounding fluid. This change allowed the liquid metal to spread out like a pancake, due to gravity.
The researchers also found that the change in surface tension is reversible. If they flip the polarity of the charge from positive to negative, the oxide is eliminated and high surface tension is restored. The surface tension can be tuned between these two extremes by varying the voltage in small steps.
"The resulting changes in surface tension are among the largest ever reported, which is remarkable considering it can be manipulated by less than one volt," says Michael Dickey, a professor of chemical and biomolecular engineering "We can use this technique to control the movement of liquid metals, allowing us to change the shape of antennas and complete or break circuits." Many materials form surface oxides, he adds, so the work could extend beyond the liquid metals presented in their study.