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Materials

Liquid metals went to work

Unusual properties of gallium alloys opened a door to stretchable electronics and soldering without heat

by Mitch Jacoby
December 13, 2016 | A version of this story appeared in Volume 94, Issue 49

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Credit: Michael Dickey/NCSU
Dickey and coworkers create random patterns by dispensing a gallium-based liquid metal from a nozzle. The metal shapes are stable and free-standing thanks to a thin oxide shell that forms spontaneously in air.
A photo shows a free-standing pattern of liquid metal balls.
Credit: Michael Dickey/NCSU
Dickey and coworkers create random patterns by dispensing a gallium-based liquid metal from a nozzle. The metal shapes are stable and free-standing thanks to a thin oxide shell that forms spontaneously in air.

Liquid metals, which have long been treated as scientific curiosities, drew renewed attention this year with novel applications, for example in flexible, stretchable electronics for wearable and implantable devices.

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Credit: Sci. Rep.
Thuo’s group scrapes away part of a thin shell of a Bi-In-Sn alloy (orange, false color) with an ion beam to allow encapsulated liquid metal droplets (left) to flow and immediately solidify (right), like solder.
These micrographs shows liquid droplets in their encapsulated state and after flowing and coalescing.
Credit: Sci. Rep.
Thuo’s group scrapes away part of a thin shell of a Bi-In-Sn alloy (orange, false color) with an ion beam to allow encapsulated liquid metal droplets (left) to flow and immediately solidify (right), like solder.
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Credit: Adv. Mater.
Gallium-based liquid-metal circuitry created by Lacour and coworkers, ink-jet printed on a glove, tracks subtle motions of the fingers by measuring stretch-induced strain.
A thin stretchy glove onto which liquid metal circuitry has been printed.
Credit: Adv. Mater.
Gallium-based liquid-metal circuitry created by Lacour and coworkers, ink-jet printed on a glove, tracks subtle motions of the fingers by measuring stretch-induced strain.

This small group of materials mainly includes gallium and a few of its alloys. On exposure to air, the liquid spontaneously forms a thin oxide skin that mechanically stabilizes droplets and arbitrary patterns that researchers create. If the material is jostled, the skin breaks and the metal flows momentarily until the skin re-forms around the liquid.

A team led by Michael D. Dickey of North Carolina State University took advantage of this unusual property to make 10-μm-wide polymer-encased wires of eGaIn, a eutectic mixture of gallium and indium that’s a liquid at room temperature. Unlike ordinary wires, the ones made with eGaIn can easily be stretched, bent, and shaped while maintaining electrical conductivity (Extreme Mech. Lett. 2016, DOI: 10.1016/­j.eml.2016.03.010).

In another example from this year, Stéphanie P. Lacour and coworkers at the Swiss Federal Institute of Technology, Lausanne (EPFL), devised a method for making a two-phase material consisting of solid AuGa2 clusters interspersed with microscopic liquid gallium droplets. They used the material to fabricate stretchable devices containing stacked layers of LEDs and sensors embedded in gloves that can track the subtle motions of fingers (Adv. Mater. 2016, DOI: 10.1002/adma.201506234).

Martin Thuo’s group at Iowa State University exploited the spontaneously forming oxide skin of bismuth-indium-tin and related alloys to keep microscopic liquid metal droplets from solidifying, even at temperatures below their melting points. The researchers showed that applying a gentle force to the droplets breaks the shells, causing the metal to briefly flow before the skin re-forms. They used that property to bond metal parts together at room temperature, in effect soldering without electricity or heat (Sci. Rep. 2016, DOI: 10.1038/srep21864).


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