“Asking why chocolate is brown is like asking why the sky is blue,” says Ralph Spolenak, Professor of Nanometallurgy in the Department of Materials at ETH Zurich. In both cases, particles scatter the light in such a way that only a certain part of it reaches our eyes: particles of cocoa powder make chocolate appear brown, while air molecules make the sky look blue. Particles are also responsible for a colour’s intensity: the higher the cocoa content, the more the cocoa particles will influence how the light scatters, and the darker the chocolate will appear.
Henning Galinski, a physicist in Spolenak’s research group, has been delving into the science of chocolate. “We were focusing on the optical properties of chocolate and wanted to find out if we could change the colour of chocolate without adding any extra ingredients,” he explains. To answer this question, Galinski worked closely with ETH research groups in Complex Materials and Food Process Engineering and with the University of Applied Sciences and Arts Northwestern Switzerland.
The team started by studying the effects of both scattered and reflected light. Unlike scattering, reflection occurs when a ray of light is reflected at a fixed angle after striking a surface such as a piece of metal or a mirror. “We imprinted a specific pattern on the surface of the chocolate to create a diffraction grating, which bends the reflected light,” says Galinski. The nanostructured pattern splits the incident light into its spectral components, causing the chocolate to shimmer in beautiful rainbow hues without any coatings or chemical modifications.
Visible to the naked eye
“Beauty is in the eye of the beholder, but I think the iridescent chocolate looks great!” says Spolenak. Yet colours are more than just easy on the eye. They can also be used to make objective observations. “We wondered whether we could use colours to gauge a material’s properties,” Galinski explains. For example, could they serve to signal how the hardness of an alloy changes when it gets hot?
Galinski offers the example of a wind turbine. If the turbine overheats, this could damage the material and make it unstable. But using helicopters to monitor turbines in offshore wind farms is a costly and time consuming business. “We developed a system to continuously monitor changes in a material’s properties using a simple optical measurement,” says Galinski. “We were able to use changes in colour to directly indicate changes in a material’s hardness or its electrical resistance.”
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!