Figure 6|. Structural colour of anisotropic nanofibrils.

a | Artificial chiral nematic cellulose nanocrystal suspensions and films feature structural colour when p/2 corresponds to a wavelength of visible light (400–700 nm), in which the pitch, p, is the distance between two planes with the same fibril orientation. The photograph shows an aqueous dispersion of cellulose nanocrystals (0.50 (w/v)) with birefringent domains caused by cross-polarized light. The optical microscopy and scanning electron microscopy (SEM) images illustrate the fractured surface across the film. The high-resolution SEM image of the fractured surface is shown in the inset. b | The photographs show organosilica and mesoporous resin films of chiral nematic cellulose nanocrystal templates. Strips (centimetre scale) of mesoporous resin films show the different colours achieved by treatment with binary water/ethanol mixtures. The left and right ends of the strips were treated with HCl and CH₂O, respectively, and the middle region remained untreated. c | Polarizing optical microscopy (POM) images of a chiral nematic cellulose nanocrystal droplet with a diameter of 154 μm and a height of 118 μm show the multilayer structure and the typical pattern of concentric rings and crosses. d | Droplet-based microfluidic systems can be applied to generate droplets in the chiral nematic cellulose nanocrystal phase, as illustrated in the optical microscopy image. The POM image shows the microfluidic generation of water-in-oil droplets using a 14.5% (w/v) suspension of cellulose nanocrystals. e | Representative POM images of chiral nematic cellulose nanocrystals show the structural evolution of nanocrystal droplets or tactoid microgels. Left panel: cellulose nanocrystal droplets with sizes ranging from radius R = 130 μm (first image), 40≤R≤ 115 μm (second image), 10 ≤R≤ 40 μm (third image) to R ≤10 μm (fourth image). Middle panel: microgels of cellulose nanocrystal tactoids with diameters of 69 μm, 136 μm, 142 μm and 141 μm. Right panel: shrinkage evolution of confined chiral nematic cellulose nanocrystal droplets. Scale bars = 50 μm. Part a is reproduced from REFS 104,113, Macmillan Publishers Limited. Part b is reproduced with permission from REF. 126, American Chemical Society; and reproduced with permission from REFS 119,122, John Wiley and Sons. Part c is reproduced from REF. 133, CC-BY-4.0; and reproduced with permission from REF. 135, John Wiley and Sons. Part d is reproduced from REFS 132,133, CC-BY-4.0. Part e is reproduced from REFS 132,133, CC-BY-4.0; and reproduced with permission from REF. 135, John Wiley and Sons.