Orientation Relaxation Dynamics in Cellulose Nanocrystal Dispersions in the Chiral Liquid Crystalline Phase

Hyperspectral Imaging of Photonic Cellulose Nanocrystal Films: Structure of Local Defects and Implications for Self-Assembly Pathways

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.

Printing of Colorful Cellulose Nanocrystalline Patterns Visible in Linearly Polarized Light

This study addresses the inkjet printing approach for fabrication of cellulose nanocrystalline (CNC) patterns with tunable optical properties varied by the thickness of deposited layers. In particular, forming functional patterns visible only in linearly polarized light is of the primary interest. The possibility of controlling the bright iridescent color response associated with the birefringence in the chiral anisotropic structure of inkjet-printed layers of CNC with sulfo-groups (s-CNC) has been thoroughly investigated. In this connection, we have elaborated an appropriate synthesis sequence for deriving printable inks in the form of sedimentation-stable s-CNC colloids with various concentrations of solid phase and experimentally determined the optimal regimes of their inkjet printing. For this purpose, the rheological parameters and s-CNC particle concentration have also been optimized. The study is accomplished with a comprehensive optical characterization of the deposited s-CNC layers with variable thickness, drying conditions, and the polarization state. The experimental results demonstrate the feasibility of inkjet printing technology to perform the precise fabrication of optically active s-CNC patterns with variable optical properties. These results are particularly relevant for applications requiring special conditions of color demonstration in security printing for such as anticounterfeiting applications, polygraphy decoration printing, and color photo filters.

Relaxation dynamics in bio-colloidal cholesteric liquid crystals confined to cylindrical geometry

Para-nematic phases, induced by unwinding chiral helices, spontaneously relax to a chiral ground state through phase ordering dynamics that are of great interest and crucial for applications such as stimuli-responsive and biomimetic engineering. In this work, we characterize the cholesteric phase relaxation behaviors of β-lactoglobulin amyloid fibrils and cellulose nanocrystals confined into cylindrical capillaries, uncovering two different equilibration pathways. The integration of experimental measurements and theoretical predictions reveals the starkly distinct underlying mechanism behind the relaxation dynamics of β-lactoglobulin amyloid fibrils, characterized by slow equilibration achieved through consecutive sigmoidal-like steps, and of cellulose nanocrystals, characterized by fast equilibration obtained through smooth relaxation dynamics. Particularly, the specific relaxation behaviors are shown to emerge from the order parameter of the unwound cholesteric medium, which depends on chirality and elasticity. The experimental findings are supported by direct numerical simulations, allowing to establish hard-to-measure viscoelastic properties without applying magnetic or electric fields.

Chiral Structure Formation during Casting of Cellulose Nanocrystalline Films

A Landau-de Gennes formulation coupled with a mass-transfer equation was used to track the evaporation front and the development of chiral microstructures during the casting of sulfuric acid-hydrolyzed cellulose nanocrystal (CNC) films. These simulations are compared to thin-film casting experiments that used analogous processing parameters and environments. The results show that the initial concentration, chiral strength, surface anchoring, speed of drying, and the influence of initial shear alignment all affect the uniformity of the microstructure and the orientation of the chiral director. In this report, we aim to show that under optimal casting conditions, the lateral size of planar microstructural domains that exhibit uniform selective reflection can be achieved on the order of millimeters.

The angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

Cellulose nanocrystals (CNCs) are bio-sourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.