06/07/2018
New material that changes colour under a flow of human breath
Our clothes, food and everyday devices get their colour from dye or pigments that absorb light. These colours fade over time, as laundry detergent ads never fail to remind us. But a different, more lasting type of colour exists as well.
Structural colours
Peacock tail feathers, for instance, are pigmented brown, yet they also reflect blue and green light due to their microscopic structure. These blues and greens are so-called structural colours. The wings of butterflies have structural colour as well, as do buttercup petals and some textiles. Structural colours don't fade, but they may temporarily change under the influence of external stimuli such as humidity, temperature and light.
Creating optical effects with crystals
Structural colour is found in nature but can also be artificially created with so-called colloidal photonic crystals (CPCs). The tiny particles that make up these crystals have the same size as the wavelength of light. This creates beautiful optical effects similar to the ones found in opal gemstones or peacock tail feathers. These effects, however, depend on the difference between the optical properties of the particle and the air that surrounds it. So when the space between these particles is filled with water, for instance, the optical effects disappear.
New material
This is not the case for a new CPC-based material produced by Dr Kuo Zhong and Professor Koen Clays from the KU Leuven Department of Chemistry, together with Professor Kai Song from the Chinese Academy of Sciences. The researchers used crystals made of hollow particles. This gives the crystals a unique optical property: when the space between the hollow particles fills with water, the optical effects don't disappear, they increase. This is because the hollow part remains hollow, while the difference in optical properties increases.
Human breath
For their experiments, the team assembled a layer of hollow CPCs and treated it with a water-repellent agent. They then etched a non-water-repellent pattern into this layer. Under ambient conditions, this pattern has the same blue colour as the water-repellent layer, making it invisible to the naked eye. But when someone blows onto the sample, the structural colour of the pattern changes into green, making it visible. This is because, as the scientists were able to show, the amount of water in our breath and our speed of breathing makes the human breath the ideal external stimulus to change the structural colour of the hollow CPCs.
Reversible process
The colour change of the hidden pattern is only temporary: the colour simply changes back when the water evaporates, so that the pattern disappears again. This is still the case after 200 cycles. The technique holds potential for anti-counterfeiting, identification and encryption devices. After all, it is quick, simple and reversible. It is similar to the fluorescent pattern on banknotes and passports, which needs UV light to become visible. For the demo in the below mentioned video, the researchers etched a hidden pattern into a sample of their new material. When someone blows onto it, the pattern turns green and reveals the letters "KUL" - a tongue-in-cheek reference to the former name of KU Leuven.
Source: KU Leuven