Iridescent colors

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

Liquid crystals are mainly used for decorative purposes in cosmetics. Cholesteric liquid crystals are particularly suitable because of their iridescent color effects, and find applications in nail varnish, eye shadow, and lipsticks. The structure of these thermotropic liquid crystals changes as a result of body temperature, resulting in the desired color effect. In recent times, such thermotropic cholesteric liquid crystals have been included in body care cosmetics, where they are dispersed in a hydrogel. Depending whether this dispersion requires stirring or a special spraying process, the iridescent liquid crystalline particles are distributed statistically in the gel (Estee Lau-... 

Owing to the simphcity and versatility of surface-initiated ATRP, the above-mentioned AuNP work may be extended to other particles for their two- or three-dimensionally ordered assemblies with a wide controllabiUty of lattice parameters. In fact, a dispersion of monodisperse SiPs coated with high-density PMMA brushes showed an iridescent color, in organic solvents (e.g., toluene), suggesting the formation of a colloidal crystal [108]. To clarify this phenomenon, the direct observation of the concentrated dispersion of a rhodamine-labeled SiP coated with a high-density polymer brush was carried out by confocal laser scanning microscopy. As shown in Fig. 23, the experiment revealed that the hybrid particles formed a wide range of three-dimensional array with a periodic structure. This will open up a new route to the fabrication of colloidal crystals. 

The organized structures give to the aqueous phases new macroscopic properties like iridescent colors, viscoelasticity, gel character, a yield stress, and, between crossed polarizers, beautifully colored patterns that make the order in the samples visible. The self-organization of the surfactant molecules is simply a result of the hydrophobic and electrostatic interaction between the individual molecules and the micellar structures. The size of the micellar structures, as in the case of small imUamellar vesicles, can be extremely monodisperse, even though one vesicle consists of hrmdreds of surfactant molecules. 

Thunig C, Hoffmann H, Platz G (1989) Iridescent colors in surfactant solutions. Progr Colloid Polym Sci 79 297-307... 

Synthetic or natural pigments used to achieve lustrous, brilliant, or iridescent color effects by interference on thin optical films are called nacreous or pearlescent pigments [5.190]—[5.194]. They were originally used to simulate the appearance of... 

Nacreous and interference pigments are used as colorants or part of color formulations for all applications where traditional pigments are used, but where additional color depth, brilliance, iridescence, color shift (flop), and other spectacular effects are desired [5.253]. Mica-based pigments dominate their combination of pearl and interference effects, brilliance, stability, and weather resistance is unsurpassed. Furthermore, they are non-toxic [5.221],... 

Lahradoriie is the characteristic feldspar of the more basic rock types like diorite. gahbro. andesite or basalt and it is usually associated with some one of the pyroxenes or amphiholes. Lahradoriie frequently shows a heauliful play of iridescent colors due lo minute inclusions of another mineral. However, the labradorescent phenomenon has not been fully determined. The classic location for this mineral is. of course. Labrador, whence its name. Il is a constituent there of the rock anorthosite and is found in the anorthosites of the Provinces of Quebec and Ontario, and in the Adirondack region in New York Stale. 

The iridescent colors on this piece of metal are produced by anodizing different parts of the surface at different voltages. 

The degraded film specimens often impart an iridescent color. This effect may be attributed to the interference of visible light due to diffused reflection between the protuberances formed on the surface of the films, which is interpretable in terms of a thin-layered gradation of the refractive index near the film surface. 

Thunig, C., Hoffmann, H. and Platz, G. (1998) Iridescent colors in surfactant solutions. Prog. Colloid Polym. Scl, 79, 297-307. 

Special effect pigments, which can be natural or synthetic, show outstanding qualities of luster, brilliance and iridescent color effects based upon optically thin layers [5.122-5.125]. This visual impression develops by reflection and scattering of light on thin multiple layers. In nature this is not limited to pearls and mussel shells alone there are a multitude of birds, fish, precious stones and minerals, even insects, that... 

This difference in refractive indices, arising equally on the interface between an air/oil film or oil film/water, is a prerequisite for the well-known iridescent color images in these media. Small highly refractive platelets of pearl luster pigments align themselves parallel in optically thin systems such as paints, printing inks, or plastics. Interference effects develop when the distances of the various layers or the thicknesses of the platelets have the right values. 

Cholesteric oily streak textures could again be clearly observed for these LC polymers containing chiral spacers. In general, the pitch of the helical packing increased in a regular manner not only with temperature, as judged by an iridescent color, but also with the amount of achiral component in the copolymers. 

Fig.48 The Grandjean plane texture of the chiral nematic phase of supermolecule 43. There is a blue iridescent color which is due to the selective reflection of light from the helical macrostructure (xlOO)...

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