Surface Relief Changes in Cholesteric Cyclosiloxane Oligomer Films at Different Temperatures

A transparent cyclo-linear polyphenylsiloxane elastomer integrating high refractive index, thermal stability and flexibility

A cyclo-linear structured transparent polyphenylsiloxane elastomer combining high refractive index, high thermal stability and superior flexibility was prepared by a one-pot hydrosilylation reaction.

Biomimetic design of iridescent insect cuticles with tailored, self-organized cholesteric patterns

Replicating biological patterns is promising for designing materials with multifaceted properties. Twisted cholesteric liquid crystal patterns are found in the iridescent tessellated cuticles of many insects and a few fruits. Their accurate replication is extremely difficult since discontinuous patterns and colors must coexist in a single layer without discontinuity of the structures. Here, a solution is demonstrated by addressing striped insect cuticles with a complex twisted organization. Geometric constraints are met by controlling the thermal diffusion in a cholesteric oligomer bilayer subjected to local changes in the molecular anchoring conditions. A multicriterion comparison reveals a very high level of biomimicry. Proof-of-concept prototypes of anti-counterfeiting tags are presented. The present design involves an economy of resources and a high versatility of chiral patterns unreached by the current manufacturing techniques such as metallic layer vacuum deposition, template embossing and various forms of lithography which are limited and often prohibitively expensive.

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures

Ongoing research on chiral liquid crystals takes advantage of the peculiar behavior of twisted structures subject to curvature. We demonstrate the fine tunability of the characteristics of the bandgap of a cholesteric structure in which the orientation of the helix axis spatially changes. To date, the spectral resolution of the order of 6 nm, herein reached by hyperspectral imaging, has not been solved in tilted helices. A correlation between spectral shifts and spatial twists is thus made possible.

Manipulation of Structural Colors in Liquid-Crystal Helical Structures Deformed by Surface Controls

Structural colors from cholesteric liquid crystals (CLCs) are manipulated by changing the only surface anchoring energy of an alignment layer. This behavior comes from the fact that weak surface energy of the perfluoropolymer induces the tilting of the cholesteric helix. Such deformed CLC structures with durability are successfully demonstrated without any external field applications and additional solidification processes. In addition, electrical tunings of structural colors from the deformed CLCs occur at very low operating voltages, compared to those of conventional CLC structures. On the basis of easy and simple fabrication, high durability, electrical tunability at low operating voltages, and the unique optical characteristics, the new deformed CLC structure could lead to extension in applications of CLCs, including multifunctional sensors, displays, and lasers.

Multiwavelength micromirrors in the cuticle of scarab beetle Chrysina gloriosa

Beetles from the genus Chrysina show vivid reflections from bright green to metallic silver-gold as a consequence of the cholesteric liquid crystal organization of chitin molecules. Particularly, the cuticle of Chrysina gloriosa exhibits green and silver stripes. By combining confocal microscopy and spectrophotometry, scanning electron microscopy and numerical simulations, the relationship between the reflectance and the structural parameters for both stripes at the micro- and nanoscales are established. Over the visible and near IR spectra, polygonal cells in tessellated green stripes behave as multiwavelength selective micro-mirrors and the silver stripes as specular broadband mirrors. Thermoregulation, conspecifics or intra-species communication, or camouflage against predators are discussed as possible functions. As a prerequisite to bio-inspired artificial replicas, the physical characteristics of the polygonal texture in Chrysina gloriosa cuticle are compared to their equivalents in synthetic cholesteric oligomers and their fundamental differences are ascertained. It is shown that the cuticle has concave cells whereas the artificial films have convex cells, contrary to expectation and assumption in the literature. The present results may provide inspiration for fabricating multiwavelength selective micromirrors or spatial wavelength-specific light modulators.

STATEMENT OF SIGNIFICANCE

Many insects own a tessellated carapace with bumps, pits or indentations. Little is known on the physical properties of these geometric variations and biological functions are unknown or still debated. We show that the polygonal cells in scarab beetle Chrysina gloriosa behave as multiwavelength selective micromirrors over the visible and infrared spectra, with a variety of spatial patterns. In the context of biomimetic materials, we demonstrate that the carapace has concave cells whereas the artificial films have convex cells, contrary to expectation in the literature. Thermoregulation, communication or camouflage are discussed as advanced functions. Results may provide inspiration for fabricating spatial wavelength-specific light modulators and optical packet switching in routing technologies.