Nano-captured water affects the wettability of cellulose nanofiber films

In this study, we report that the wettability of cellulose nanofiber films varies with their thickness, and this variation is attributed to the amount of water captured within the hydrogel-like film. To investigate this, we conducted water contact angle measurements on cellulose nanofiber films with different thicknesses. The wettability of the films was found to change from hydrophilic (approximately 20°) to that of the underlying silicon substrate (approximately 80°) by decreasing the mean film thickness within a range of 150 nm or less. Transmission electron microscope observations revealed that even under high-vacuum conditions (< 10−5 Pa), water was captured in regions where the film was thick enough to form a dense, three-dimensional network. However, in regions where the film was thin, the network structure became sparse and two-dimensional, and no water was observed. We propose that the wettability of cellulose nanofiber films changes based on the area ratio of water captured in thick 3D network regions, the underlying substrate, and the cellulose nanofibers. Furthermore, we quantitatively evaluated the relationship between the area ratio and the wetting state of the composite surface based on the Cassie-Baxter equation, then determined the minimum thickness at which water is retained within the cellulose fiber network.