The transfer of chirality across length-scales is an intriguing and universal natural phenomenon. However, connecting the properties of individual building blocks to the emergent features of their resulting large-scale structure remains a challenge. In this work, we investigate the origins of mesophase chirality in cellulose nanocrystal suspensions, whose self-assembly into chiral photonic films has attracted significant interest. By correlating the ensemble behaviour in suspensions and films with a quantitative morphological analysis of the individual nanoparticles, we reveal an inverse relationship between the cholesteric pitch and the abundance of laterally-bound composite particles. These ‘bundles’ thus act as colloidal chiral dopants, analogous to those used in molecular liquid crystals, providing the missing link in the hierarchical transfer of chirality from the molecular to the colloidal scale.

Daytime radiative cooling (DRC) materials offer a sustainable approach to thermal management by exploiting net positive heat transfer to deep space. While such materials typically have a white or mirror-like appearance to maximize solar reflection, extending the palette of available colors is required to promote their real-world utilization. However, the incorporation of conventional absorption-based colorants inevitably leads to solar heating, which counteracts any radiative cooling effect. In this work, efficient sub-ambient DRC (Day: −4 °C, Night: −11 °C) from a vibrant, structurally colored film prepared from naturally derived cellulose nanocrystals (CNCs), is instead demonstrated. Arising from the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade-resistant coloration, while maintaining a low solar absorption (≈3%). Additionally, a high emission within the mid-infrared atmospheric window (>90%) allows for significant radiative heat loss. By coating such CNC films onto a highly scattering, porous ethylcellulose (EC) base layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color simultaneously. Finally, scalable manufacturing using a commercially relevant roll-to-roll process validates the potential to produce such colored radiative cooling materials at a large scale from a low-cost and sustainable feedstock.

The structural coloration of arthropods often arises from helicoidal structures made primarily of chitin. Although it is possible to achieve analogous helicoidal architectures by exploiting the self‐assembly of chitin nanocrystals (ChNCs), to date no evidence of structural coloration has been reported from such structures. Previous studies are identified to have been constrained by both the experimental inability to access sub‐micrometer helicoidal pitches and the intrinsically low birefringence of crystalline chitin. To expand the range of accessible pitches, here, ChNCs are isolated from two phylogenetically distinct sources of α‐chitin, namely fungi and shrimp, while to increase the birefringence, an in situ alkaline treatment is performed, increasing the intensity of the reflected color by nearly two orders of magnitude. By combining this treatment with precise control over ChNC suspension formulation, structurally colored chitin‐based films are demonstrated with reflection tunable from blue to near infrared.

Cellulose nanocrystals (CNCs) can spontaneously assemble into chiral nematic films capable of reflecting circularly polarized light in the visible range. As many other photonic materials obtained by bottom-up approaches, CNC films often display defects that greatly impact their visual appearance. Here, we study the optical response of defects in photonic CNC films, coupling optical microscopy with hyperspectral imaging, and we compare it to optical simulations of discontinuous cholesteric structures of increasing complexity. Cross-sectional SEM observations of the film structure guided the choice of simulation parameters and showed excellent agreement with experimental optical patterns. More importantly, it strongly suggests that the last fraction of CNCs to self-assemble, upon solvent evaporation, does not undergo the typical nucleation and growth pathway, but a spinodal decomposition, an alternative self-assembly pathway so far overlooked in cast films and that can have far-reaching consequences on choices of CNC sources and assembly conditions.

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