Anomalous‐Diffusion‐Assisted Brightness in White Cellulose Nanofibril Membranes

The understanding of the interaction between light and complex, random structures is the key for designing and tailoring the optical appearance and performance of many materials that surround us, ranging from everyday consumer products, such as those for personal care, paints, and paper, to light diffusers used in the LED‐lamps and solar cells. Here, it is demonstrated that the light transport in membranes of pure cellulose nanofibrils (CNFs) can be controlled to achieve bright whiteness in structures only a few micrometers thick. This is in contrast to other materials, such as paper, which require hundreds of micrometers to achieve a comparable appearance. The diffusion of light in the CNF membranes is shown to become anomalous by tuning the porosity and morphological features. Considering also their strong mechanical properties and biocompatibility, such white coatings are proposed as a new application for cellulose nanofibrils. Cellulose nanofibrils (CNFs) allow sustainable, biodegradable, biocompatible, and renewable materials for efficient light‐scattering media. By tuning the membrane porosity and morphological properties of the nanofibrils, the light transport can be controlled from standard to anomalous diffusion.