Hierarchical Organization of Structurally Colored Cholesteric Phases of Cellulose via 3D Printing

Structural color—a widespread phenomenon observed throughout nature, from the iridescent blue wings of the Morpho butterfly, to the circularly polarized shells of the green Chrysina gloriosa beetle and deep metallic blue skin of Pollia condensata berries —is caused by light interference from ordered phases of matter. While state-of-the-art nanofabrication techniques can produce structural organization in small areas, we still lack a cost-effective, scalable, and simple technique that can generate tunable color at sub-micron length scales. In this work, we produced wrinkled structurally colored hydroxypropyl cellulose (HPC) filaments with a suppressed angular color response by 3-d printing. Our systematic study of the morphology of the filaments reveals the key stages of the cross-linking mechanism of the HPC and demonstrates the induction of a two-degree hierarchical order through 3-d printing. The first degree of order originates from the changing of the cholesteric pitch at a few hundred nm scale via chemical modification and tuning of the solid content of the lyotropic phase. Upon 3-d printing, the secondary hierarchical order of periodic wrinkling was introduced through the Helfrich-Hurault deformation of the shear-aligned cholesteric phases. Our work reveals the mechanism of the wrinkling behavior evidenced by detailed morphological characterization using SEM. The filaments have four morphological zones and the ordered wrinkles in the core section were found to have 5 μm to 20 μm wavelengths with 2 μm to 10 μm amplitude. While it is possible to modify the wrinkling behavior by changing the printing conditions, the optical microscopy and angular imaging also revealed that the color response was homogenous and non-directional within angles of 0 to 60 degrees.