Scientists have developed a thin, lightweight and flexible film that can outfox infrared cameras, allowing hot bodies to appear cool and cold items to appear warm. The invention can also help camouflage an object by making it appear the same temperature as its background.
The design was inspired by the colour-shifting capabilities of cuttlefish, says Coskun Kocabas, a co-author of the research from the University of Manchester.
The approach involves using electricity to alter the properties of the film, so that it changes from acting more like a “black body” – which absorbs and emits electromagnetic radiation but does not reflect it – to becoming more like a metal, which reflects radiation but is not good at absorbing or emitting it.
Kocabas said the film could have a number of uses. “One obvious application is of course camouflage, but the novelty in this is it is adaptive camouflage,” he said, adding it could also be useful for covering radiators on satellites, allowing them to be tweaked to reflect heat when facing the sun and emit excess heat when facing deep space.
Writing in the journal Nano Letters, Kocabas and colleagues in the US and Turkey reveal how they created the material using a stack made of nylon, gold, polyethylene soaked in a liquid composed of charged molecules, and multiple layers of graphene.
Graphene, a honeycomb sheet of carbon one atom thick that has been dubbed a “wonder material”, is very good at emitting heat in the form of radiation. As a result, when the device is rested on a person’s hand, the heat it gains from the hand is emitted as infrared light, meaning that when a thermal camera is pointed at the device, an image of the person’s hand can be seen.
But there’s a twist. The team found that when a voltage was applied across the device, the charged molecules in the polyethylene layer became incorporated within the graphene layers. That, they found, reduced the level of infrared light the graphene could emit.
“They go into the graphene and they change [the] conductivity of graphene and they change [the] optics of graphene,” said Kocabas. “We are basically changing graphene into metal.”
As a result, when the team applied three volts to the device, the image of the person’s hand could no longer be seen when the device was viewed through the infrared camera.
The team then set up a sensor to measure the changing temperature of a surface, and used that information, via an algorithm, to adjust the voltage applied to the device – which itself was kept at a constant temperature.
As a result, they were able to change the apparent warmth of the device as seen through an infrared camera, allowing it thermally blend into the surface in less than five seconds, even though its actual temperature did not change. The team say the system can currently work between temperatures of 25C and 38C.
Further work revealed that incorporation of the charged molecules into the graphene doesn’t just affect the device’s ability to emit infrared light – it also changes how reflective the device is.
The team found when the device itself is kept at a temperature hotter than its surroundings, increasing the voltage decreases the apparent temperature of the device. That is because the device is now less able to emit infrared light and, although it is now more reflective, there is little infrared light to reflect.
However, Kocabas said, when the device is kept cooler than the surroundings and a voltage is applied, the device appears warmer than it really is because there is now more infrared light to reflect from its surroundings.