Spray-Assisted Fabrication of Cellulose Photonic Pigments on Superhydrophobic Surfaces.

Photonic pigments, especially those based on naturally-derived building blocks like cellulose nanocrystals (CNCs), are emerging as a promising sustainable alternative to absorption-based colorants. However, the proposed manufacturing methods for CNC pigments, via either grinding films or emulsion-based production, usually require several processing steps. This limits their commercialization by increasing the costs, timescales, and environmental impacts of production. Toward addressing these challenges, it is reported that photonic pigments can be produced in a single process by drying microdroplets of aqueous CNC suspension on a superhydrophobic surface. Such liquid-repellent substrate ensures the microdroplets maintain a near-spherical shape, enabling the radial self-organization of the cholesteric phase. Upon drying under ambient conditions, the CNC mesophase becomes kinetically arrested, after which the strong capillary forces induced by water evaporation result in extensive buckling of the microparticle. This buckling, coupled with prior tuning of the CNC formulation, enables photonic pigments with adjustable color across the visible spectrum. Importantly, the elimination of an emulsifying oil phase to create microdroplets enables a much faster drying time (≈40 min) and improved color stability (e.g., polar solvents, elevated temperatures), while the reduction in reagents (e.g., oils, surfactants) and post-processing steps (e.g., solvent, heat) improves the sustainability of the fabrication process.