'Chameleon metals' change surfaces in response to heat

Just like a chameleon changes its skin colour in response to its environment, liquid metal — and potentially solid metal — can change its surface structure in response to heat.

That’s according to engineers from Iowa State University, who have reported in the journal Angewandte Chemie that treating particles of liquid metal alloys with heat causes them to roughen their surfaces with tiny spheres or nanowires. Control the heat and you can control the surface patterns.

The research team started with a liquid metal alloy of gallium, indium and tin synthesised into particles covered with a smooth oxide shell that has been chemically stabilised. As the particles are heated, the surface thickens and stiffens and begins to behave more like a solid.

Eventually the surface breaks, allowing the liquid metal inside to come to the surface. The most reactive, gallium, breaks through first. More heat brings indium to the surface. The highest heat — about 870°C — brings out florets of tin. This movement from the under-layer to the surface allows a liquid metal particle to “continuously invert its composition under thermal stimuli”, the researchers wrote.

“The particles are responding to a certain level of heat and releasing a specific element based on temperature, just as a chameleon responds to the colour of its environment,” said Assistant Professor Martin Thuo, a co-founder of start-up SAFI-Tech and lead author of the paper. “That’s why we say they’re chameleon metals — but responding to heat, not to colour as the reptile does.”

Doctoral student Winnie Kiarie, a co-author on the paper, said the metal particles are responding to a very controlled environment — time, temperature and oxygen levels are carefully controlled by the researchers. That allows the researchers to predict and program the exact surface texture of the particles.

Asst Prof Thuo said the technology could be used to fine-tune a metal’s performance as a catalyst or its ability to absorb compounds. The researchers also claim the technology will work with other metal alloys, writing that it could “inspire design of ‘smart’ alloy systems that evolve the surface patterns and their composition with temperature (or analogous stimuli) for applications ranging from sensing to catalysis”.

“This is not unique to these materials,” Asst Prof Thuo said. “This is a behaviour of metals in general. Other metals subject to the same treatment should do this. This is a universal property of metals.”

That could make chameleon metals a very interesting and useful technology. As noted by Asst Prof Thuo, “When you talk about smart materials, polymers come to mind. But metals can do this, too. But it’s a big beast — you just need to know how to tame it.”

Illustration shows how a particle of ‘chameleon metal’ reacts to higher and higher temperatures by sequentially changing its surface structure. Image by Andrew Martin.

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