By Max Planck Institute of Colloids and Interfaces
Cellulose can be extracted from natural sources in the form of thin, rigid rods known as cellulose nanocrystals (CNCs). CNCs are less than a micron long—too small to be seen by the human eye, even under an optical microscope. Still, upon drying, they can form films with dazzling colours. This phenomenon arises from the spontaneous self-assembly of the particles into periodic structures called liquid crystal phases. More precisely, the CNCs form a stair-like (helical) structure made from rotating layers of locally aligned CNCs. Light interference within this periodic structure leads to the selective reflection of certain wavelengths and not others. The reflected wavelength is proportional to the spacing between the CNC layers: a larger spacing reflects longer wavelengths, so the colour is red-shifted, while a smaller spacing reflects shorter wavelengths, causing a blue shift. In recent years, researchers have developed techniques to guide the self-assembly of CNCs, enabling precise control over film colouration. Today, CNCs are commercially available and can be used to create colourful films, droplets and particles derived from natural feedstocks. In our lab, we have made significant advances in scalable pigment production through the use of an industrial roll-to-roll coating system. This technology allows the fabrication of cellulose-based photonic films from nano to meter-scale, which can be ground into vibrant pigments and glitter. These innovations offer a sustainable alternative to conventional pigments, delivering intense colour and shimmer while being biodegradable and biocompatible. They can replace plastic-based glitter in textiles and household products, and can compete with environmentally problematic titania and mica-based effect pigments in cosmetics, food, paints, and packaging. While significant progress has been made, further research is essential to fully realize the potential of these innovative materials. Ongoing work focuses on improving the sustainability of CNC production and enhancing the homogeneity of their resulting structural colouration.
