This is the problem that Droguet’s team set out to solve using cellulose derived from commercially available wood pulp. First, they must figure out how to set up the crystal in the correct way.They automatically form a structure, but which one The structure depends on the ionic composition of the water in which they are located. To change this composition, “actually, you only need to add salt,” Vignolini said. Salt changes the way molecules attract each other and determines the shape they form and the color of the flashes they produce subsequently. Just adding 5 milligrams will change the color of a full kilogram of cellulose, causing the crystal to refract shorter wavelengths, such as green and blue. With less salt, they will refract longer wavelengths, such as red.
The team also figured out how to carefully control the production process so that they can now use a roll-to-roll machine (a common industrial device) to make a one-meter-long glitter. The machine rolls the strands of the polymer matrix or “net” while the dispenser sprays a uniform amount of nanocrystal solution. The mixture must be dilute enough to be easily deposited on the roll, but viscous enough to leave a deep and uniform color.
At this point, the mixture is transparent, so the team cannot be sure whether they have successfully produced a good batch of products until they run the web through a hot air dryer. After the water evaporates, only a layer of nanocrystals remains. The color suddenly appeared and deepened. “At the last minute, it was really fast,” Droguet said, and he made green, blue, red, and gold sparkle. The film can then be peeled off the web and ground into a craft flash or mixed into the paint. Compared with the manufacture of plastic glitter, the process requires less energy. Even if the final product is mixed with soapy water, ethanol and oil, it can maintain its luster, which means it can be used in cosmetics and even food. “I think we have now shown that these principles can work on a large scale,” Droguet said.
But they have not yet tried industrial production. Using Cambridge equipment, it currently takes about two months for Droguet to produce one kilogram of flash. In order to increase production, he needs capital and access to commercial premises with larger roll-to-roll machines. So far, getting companies involved has been a challenge. Vignolini said that manufacturers are excited but hesitant because this material is very different from the materials they currently use. “This is brand new,” she said, and the company wants to make sure it works.
Vignolini and Droguet also want to test to understand how this material decomposes during its life cycle and how this decomposition affects the environment. They collaborated with Dannielle Green, an ecologist at Ruskin University in Anglia, UK, who studied other cellulose-based glitters to understand how they affect the growth of algae.