Cholesteric Liquid Crystal Elastomer Tubes with Highly Strain-Sensitive Structural Colors and Adjustable Spatial Configurations

In nature, living organisms such as chameleons and brown algae alter the structural colors (SC) on their skin by changing the spacing of periodic nanostructures for signaling or adapting to their environment. Researchers have emulated these dynamic color changes in stretchable SC materials by applying strain or stress, which provides the fastest response compared to other stimuli. Most studies on strain-responsive SC materials have focused on films, which can quickly change color through uniaxial or biaxial stretching, bending, or compression. However, the flat geometry of films limits their spatial configuration and confines color changes to 2D. Enhancing the configurational flexibility of strain-sensitive platforms has been pursued by fabricating SC materials into fibers. Yet, color change in fibers occurs only via uniaxial stretching, restricting the color shift to a single dimension. Furthermore, large uniaxial strains are necessary to shift the color from red to blue, which can degrade the color quality due to the unwinding of the supramolecular helix, or even cause material failure. Thus, a platform that combines the spatial flexibility of SC materials with multidimensional color-changing capability is needed to achieve SC shifts in both 2D and 3D configurations.
In this work, we introduce the first SC tube made from cholesteric liquid crystal elastomers (CLCEs), formed by leveraging interfacial flow to create an air-oligomer interface inside an elastomer channel. CLCEs, consisting of consecutive rotating layers of aligned liquid crystal (LC) molecules crosslinked into a soft elastomer, are ideal candidates for a highly sensitive color response to mechanical strain due to their inherent anisotropy and flexibility. We also capitalize on the ultrahigh viscosity of the CLCE oligomer to delay Rayleigh-Plateau instability, while addressing a modified Bretherton’s problem. The formation of an air channel surrounded by the oligomer allows for rapid and uniform coloration, which is then photocured before the instability affects the axial uniformity of the wall thickness. Notably, the SC tubes display advanced optical properties in terms of reflectance and color-change sensitivity when both inflated and stretched, which we verify through experiments and theoretical calculations. Our findings reveal that CLCE tubes exhibit a greater blue shift during stretching at the same axial strain as CLCE fibers, and tube inflation results in even greater color sensitivity. These effects are quantified by examining the geometrically confined Poisson’s ratios of CLCE fibers and tubes when stretched or inflated.
Moreover, the tubular geometry offers arbitrary spatial configurations and is reconfigurable while maintaining its color-changing properties. The sensitivity of the color change is heightened when the tubes are initially bent. Inflation of a bent tube induces a larger color shift compared to a straight one, as it combines elongation in multiple directions and compression. By harnessing the highly sensitive color change and reconfigurable properties of SC tubes, we design an innovative reflective alphabet display for communication activated by both inflation and elongation of the tubes. Additionally, the tubes can wrap around arbitrary 3D objects and function as curvature sensors or strain-sensitive 3D surfaces, maintaining uniform colors based on tube positioning. This novel SC geometry paves the way for reconfigurable, multidimensional SC platforms, including reflective displays and soft robotics applications.