The helical structure gives rise to Bragg diffraction that is selective not only in wavelength but also in polarization: only the circularly polarized component of light with the same handedness as the cholesteric helix is Bragg-diffracted. This is achieved particularly easily using cholesteric liquid crystals (CLCs), since these long-range ordered and birefringent liquids can be processed into spherical droplets just like with any other liquid, for instance using microfluidic or other emulsification techniques, and since they spontaneously organize with a helical organization of the optic axis that modulates the refractive indices with a period p that is easily adjusted from ≈100 nm to several tens of microns by varying the CLC mixture composition. The first problem is much reduced if the structurally colored material is produced not in the form of flat films-as is usually the case-but rather in the shape of spheres, with radial orientation of the symmetry axis. Second, structural color is purely spectral, hence it does not cover the entire perceivable color space which includes mixed colors, most notably white. ![]() First, the wavelengths of constructive and destructive interference depend on the incidence angle with respect to the symmetry axis of the periodic structure, meaning that a flat structurally colored surface will change in color depending on how the sample is illuminated and observed (iridescence). Structural color has two important limitations, however. Even in artificial materials, it can be produced fully sustainably from renewable biosources like cellulose or chitin. It is frequently used in nature -typically in combination with absorbing pigments-and in some artificial products such as glittery foils. Structural color, arising from wavelength-selective interference phenomena (Bragg diffraction) in periodic structures with characteristic lengths on the order of visible light wavelengths, is an alternative method to create color, often resulting in vibrant and strongly saturated spectral colors. Moreover, many inorganic pigments rely on rare earth elements or toxic chemical compositions. Concerns have also been raised about the health impact of the TiO 2 particles frequently used to scatter light for strong whites. While the biological world manages this very well, in a fully sustainable manner, the industrial use of human-made dyes and pigments has severe negative environmental impact. Most often, color is generated by a combination of absorption across parts of the visible spectrum (except for black, where all visible light should be absorbed) and strong scattering of all colors, to produce white or to enhance colors defined by absorption. The approach can be used to create arbitrary colors, including nonspectral ones, without any absorption or nonselective scattering, opening doors to decorating surfaces as desired while minimizing light loss.īecause our world is so colorful, the ability to control the color by which an object appears is of great importance, the impact ranging from the survival and proliferation of animals and plants to proper function and market success of human-designed industrial products. By embedding the CSRs in an index matching transparent medium, nonselective specular reflections and scattering are avoided. ![]() ![]() Mixing them in equal proportions gives a white surface. A method of creating densely packed monolayers of CSRs with red (R), green (G), and blue (B) retroreflection is developed. This allows them to be used as pixels for generating nonspectral colors, following the principle of digital displays. Exhibiting omnidirectional selective retroreflectivity of well-defined color, CSRs are discrete “packages” of structural color. Herein, it is demonstrated that these challenges can be overcome by using cholesteric spherical reflectors (CSRs), spheres of polymerized cholesteric liquid crystal with radial alignment of the self-assembled helical structure. It also normally suffers from a strong viewing angle dependence, making color definition difficult. While structural color is a powerful means of obtaining saturated and durable pigments that minimize absorption, scattering, and negative environmental impact, appearing naturally in animals and plants as well as in carefully designed artificial composites, it is fundamentally limited to spectral colors, leaving white and other mixed colors elusive.
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