Kinetic Access to Focal Conic Domains in Photonic Cellulose Nanocrystal Films

Cellulose nanocrystals (CNCs) self-assemble into cholesteric structures that enable sustainable photonic films, but controlling their pitch, domain morphology, and angular color response remains challenging. Here, we investigate how salt concentration and sonication dose jointly regulate the evaporation-induced self-assembly of CNC suspensions. Across 24 dish-cast films, polarized optical microscopy, time-lapse imaging, rheology, electron microscopy, and angle-resolved spectroscopy reveal that these processing parameters affect not only the cholesteric pitch but also tactoid nucleation, transport, coalescence, and the onset of kinetic arrest. Increasing NaCl concentration generally blue-shifts the photonic response, whereas sonication red-shifts it by fragmenting CNC bundles, with the magnitude of this effect decreasing as ionic content increases. At intermediate ionic content, low sonication doses produce extended cholesteric domains with comparatively narrow spectral responses, while the abundance of focal conic domains (FCDs) increases with sonication dose, culminating in FCD-rich films at moderate sonication. Refined parameter scans show that FCDs form within a finite processing window. Time-lapse imaging shows that FCDs appear abruptly after tactoid growth and coalescence, as macroscopic motion becomes strongly restricted during late-stage evaporation. Concentration- and residence-time-dependent rheology further demonstrates that the material response evolves throughout the assembly pathway. The FCD-rich film retains a narrow local reflection band while exhibiting the smallest angular displacement of the reflection maximum among the films examined. These findings show that salt and sonication regulate the kinetic accessibility of elastically driven focal conic structures, providing a route to tune domain morphology, spectral bandwidth, and iridescence in CNC photonic films.