It was a finding that surprised researchers at North Carolina State University: By applying low voltage, a liquid metal — gallium indium (EGaIn) — can be induced to spread and form fractal patterns.

The work, which appears in Physical Review Letters, has implications for shape control of liquid metals — and the development of reconfigurable electronic, electromagnetic and optical devices that take advantage of their metallic properties.

What makes the finding particularly remarkable is that gallium indium at room temperature has the highest surface tension of any known liquid. Surface tension is the force exerted by the liquid's surface, which causes it to "bead up" or form droplets. Because liquid metals generally have very high surface tensions, they almost always manifest in spherical shapes.

Applying voltage, as chemical and biomolecular engineering professor Michael Dickey explains, forms a thin layer of oxide on the surface of the metal, effectively lowering the surface tension. It also appears to create compressive stresses, which are the opposite of tension.

“Normally, the tension of liquids can be lowered by adding surfactants — like putting soap or detergent in water — to the liquid," Dickey points out. "It’s easy to put soap into water, but hard to get the soap out. In contrast, the use of voltage to control the tension is interesting because it is reversible, and incredibly effective.”

“We also found that if you apply higher amounts of voltage to the metal, it stops spreading and beads up again,” adds physics professor Karen Daniels. “That’s due to the amount of oxide produced: A small amount lowers the surface tension, but too much forms a crust over the metal and stops it spreading. So controlling the voltage is a nice way to control the spreading of the metal.”

The fractals formed by EGaIn appear to be unique: They do not match any currently described fractals. In order for them to form, says Daniels, the surface tension of the liquid metal must be close to zero.

“We now have a tool to apply compressive forces directly to the surface of a liquid. These properties give us greater control over the metal’s behavior," says Dickey.