Researchers control surface tension to manipulate liquid metals

The researchers used a liquid metal alloy of gallium and indium. In base, the bare alloy has a remarkably high surface tension of about 500 millinewtons (mN)/meter, which causes the metal to bead up into a spherical blob.

"But we discovered that applying a small, positive charge -- less than 1 volt -- causes an electrochemical reaction that creates an oxide layer on the surface of the metal, dramatically lowering the surface tension from 500 mN/meter to around 2 mN/meter," says Dr. Michael Dickey, an associate professor of chemical and biomolecular engineering at NC State and senior author of a paper describing the work. "This change allows the liquid metal to spread out like a pancake, due to gravity."

The researchers also showed that the change in surface tension is reversible. If researchers 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. Video of the technique in action is available online (http://www.youtube.com/watch?v=mB2ZqO5E1Zo).

"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," Dickey says. "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. It could also be used in microfluidic channels, MEMS, or photonic and optical devices. Many materials form surface oxides, so the work could extend beyond the liquid metals studied here."

Dickey's lab had previously demonstrated a technique for "3-D printing" liquid metals, which used the oxide layer formed in air to help the liquid metal retain its shape -- the exact opposite of what the oxide layer does to the alloy in a basic solution.

"We think the oxide's mechanical properties are different in a basic environment than they are in ambient air," Dickey says.

The paper, "Giant and Switchable Surface Activity of Liquid Metal via Surface Oxidation," will be published online in the Proceedings of the National Academy of Sciences during the week of September 15. Lead authors of the paper are Mohammad Rashed Khan and Collin Eaker, Ph.D. students at NC State. The paper was co-authored by Dr. Edmond Bowden, a professor of chemistry at NC State.

The research was supported by National Science Foundation (NSF) CAREER grant number CMMI-0954321 and the Research Triangle NSF Materials Research Science and Engineering Center on Programmable Soft Matter grant number DMR-1121107.