Colour vision

Galinski cites a further example. In a joint project with Empa, ETH researchers applied the same sensor concept to textiles. “We gave the textile fibres a thermochromic coating that changes colour when the material is damaged by heat,” he explains. This kind of visual warning could mean the difference between life and death for people who are regularly exposed to hazardous situations, such as firefighters. When materials such as ropes or garments overheat due to fire or friction, this impairs their function. The change in colour warns users that the item is damaged and should no longer be used.

These examples show how colour can be harnessed as a sensor system to monitor functionality. “After all, we humans constantly use colour to assess our surroundings. When we see a red traffic light, for example, we know it means ‘stop’,” says Galinski. The researchers are building on the same principle, as Spolenak explains: “In reality, the 
damage to the material is microscopically small, but our colour-coating system amplifies the effects, making them visible to the naked eye.” The system used on the textile fibres consists of multiple coatings. However, only the uppermost layer – which is just 20 nanometres thick – reacts to temperature by crystallising and changing colour.

Less material, more light

“We’re also interested in how light interacts with larger surfaces, specifically in relation to thin coatings,” says 
Spolenak. Developments in this area could be a game-changer: if scientists could find a way to capture lots of light in a small amount of material until the light is completely absorbed, this could have huge benefits for solar cells and other materials used in the energy industry. “If we can take the same method of concentrating light that we use to create colour and make it work in a small volume of material, then the efficiency will be very high,” says Galinski.

Recently, Spolenak’s group developed a principle for using nanoscale networks to capture light efficiently. These networks, which are made of a special alloy, enable the absorption of up to 
99 percent of the light – at practically any angle of incidence.

A few years ago, the group collaborated with an international team of researchers. Together, they successfully developed a principle for producing metal coatings in different colours. The coating consists of a special microstructure made up of two different layers. The bottom layer comprises a network of metals permeated by tiny pores, while the upper part of the coating is a thin oxide layer. The colour is produced primarily through the interaction of light with the disordered interface between the two materials. The thickness of this interface region determines the colour; for example, 12 nanometres gives the material a green tone, 24 nanometres makes it yellow, and 48 nanometres makes it blue.

Claudiadele Polinari from Rämibühl secondary school also produced structural colours as part of her baccalaureate paper. Rather than creating just a few individual colours, she set out to obtain the broadest possible palette. The green tones pushed the young researcher’s dual-layer principle to its limits. Nonetheless – or perhaps as a consequence – she learned a great deal during her short research visit at ETH. Some of her many successful colour specimens were framed and now hang on the wall above the meeting table – a striking 
example of how research results can provide a real visual treat!