A pioneering work by an international team of scientists from Australia and Switzerland is setting the foundation for moving beyond solid state electronics towards flexible and reconfigurable soft circuit systems.

Modern electronic technologies like smart phones and computers are mainly based on circuits that use solid state components, with fixed metallic tracks and semiconducting devices.

But engineers dream of being able to create truly elastic electronic components – soft circuit systems that can act more like live cells, moving around autonomously and communicating with each other to form new circuits.

Liquid metals, in particular non-toxic alloys of gallium, have so far offered the most promising path for realizing that dream.

Prof. Kourosh Kalantar-zadeh from the School of Engineering at RMIT University and his colleagues from the Queensland University of Technology, RMIT University and ETH Zürich used Galinstan, a eutectic alloy of gallium, as the model liquid metal.

“Gallium has low toxicity, negligible vapor pressure and is relatively safe for practical application,” they explained.

“Gallium itself melts above room temperature at 29.8 degrees Celsius; however, when combined with other metals its melting point can be significantly lowered to below zero degrees Celsius such as is the case with Galinstan, which is a eutectic alloy of 68.5% gallium, 21.5% indium and 10% tin.”

As well as being incredibly malleable, any droplet of liquid metal contains a highly-conductive metallic core and an atomically thin semiconducting oxide skin – all the essentials needed for making electronic circuits.

To work out how to enable liquid metal to move autonomously, the scientists first immersed droplets of Galinstan in water.

“Putting droplets in another liquid with an ionic content can be used for breaking symmetry across them and allow them to move about freely in three dimensions, but so far we have not understood the fundamentals of how liquid metal interacts with surrounding fluid,” Prof. Kalantar-zadeh explained.

The team then adjusted the concentrations of acid, base and salt components in the water and investigated the effect.

“Simply tweaking the water’s chemistry made the liquid metal droplets move and change shape, without any need for external mechanical, electronic or optical stimulants,” Prof. Kalantar-zadeh said.

“Using this discovery, we were able to create moving objects, switches and pumps that could operate autonomously – self-propelling liquid metals driven by the composition of the surrounding fluid.”

This work lays the foundation for being able to use ‘electronic’ liquid metals to make 3D electronic displays and components on demand, and create makeshift and floating electronics.

“Eventually, using the fundamentals of this discovery, it may be possible to build a 3D liquid metal humanoid on demand – like the T-1000 Terminator but with better programming,” Prof. Kalantar-zadeh said.

The team’s results appear this week in the journal Nature Communications.

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Ali Zavabeti et al. 2016. Ionic imbalance induced self-propulsion of liquid metals. Nature Communications 7, article number: 12402; doi: 10.1038/ncomms12402