Enhanced homogeneity and flexibility in a humidity sensor using cellulose nanocrystal-based composite film with circular shear flow

Cellulose nanocrystal (CNC) film is known to be one kind of dynamic color-sensing material, capable of reversible color changes in response to varying humidity levels. However, the brittleness, low hygroscopicity and poor homogeneity of these films have hindered their development. To address this limitation, we present a novel approach where we combine natural deep eutectic solvents (NADES) with sorbitol under the influence of circular shear flow to craft a CNC humidity-sensitive film with enhanced flexibility, hygroscopicity and homogeneity. The inclusion of sorbitol and NADES enhances hygroscopicity and improves the flexibility. Surprisingly, the introduction of circular shear flow was found not only to improve homogeneity, macroscopically and microscopically, but also to further enhance flexibility, toughness, and water absorption capability. The resulting composite films demonstrated highly reversible color changes across the whole visible spectrum depending on the relative humidity, showing their capability to be reliable humidity-sensing materials. Thanks to the improved homogeneity and flexibility, the obtained humidity-sensing composite film can be employed in its entirety without the need for cutting, making it a promising candidate for various applications.