Hierarchical organization of structurally colored cholesteric phases of cellulose via 3-d printing

Structural color—a widespread phenomenon observed throughout nature 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, and scalable techniques to generate tunable color at sub-micron length scales. In this work, we produced structurally colored hydroxypropyl cellulose filaments with a suppressed angular color response by 3-d printing. Our systematic study of the morphology of the filaments reveals the key stages in 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. In single layered filaments, we identified four morphological zones with varying order of wrinkles. Through this work, we demonstrate the possibility of modifying the wrinkling behavior and thus the angle dependence of the color response by changing the printing conditions.