Non-iridescent structural colour ink

Researchers at Kobe University in Japan developed non-fading structural colours that do not depend on the viewing angle and can be printed.

An object has colour when light of a specific wavelength is reflected. With traditional pigments, this happens when molecules absorb other colours from white light. However, over time this interaction makes the molecules degrade and the colour fade.

Structural colours, on the other hand, arise when light is reflected from parallel nanostructures set apart at just the right distance so that only light of certain wavelengths will reflect while others are cancelled out. These colours can be found in nature, for instance in the wings of a butterfly. A major advantage is that these colours do not fade. However, these structures are difficult to replicate on an industrial scale, which makes them hard to paint or print.

The team from Kobe developed a new way of creating structural colours. Instead of parallel nanostructures, they use tiny crystalline silicon spheres. Through a phenomenon that is called Mie resonance, these nano-scale spheres reflect certain wavelengths much more strongly than others. These wavelengths vary with the size of the particles, which means that changing the size of the particles means changing the colour of the material. The colour is not iridescent because the spheres reflect the light in all directions.

The researchers proved that ink with the nano spheres can be printed. Interestingly, the reflectance is highest when the individual particles are separated instead of when tightly packed.

The coating could be applied to for example airplanes. Normally, airplane coatings weigh several hundreds of kilograms. With the new coating, this could be less that 10 per cent of that.

Photo: Kobe University