Appearance and Light Scattering of Liquid Crystals

As we know, the scattering of matter is caused by the fluctuation of their refractive index (or optical dielectric constant). The local compression or dilution of mass density in bulk isotropic liquids modifies in turn the local refractive index. The change can be expressed in the form of the dielectric constant as 

Liquid crystals are optically anisotropic media with the exception of the cubic phases, such as the D-phase and blue phase. The refractive index or dielectric constant of liquid crystals varies according to the orientation of the molecular axes (or the optical axes). For example, the nematic liquid crystals are optically uniaxial and exhibit remarkable birefringence, that 

The ratio reaches up to one million times. This explains why the liquid crystal is in fact very turbid while ordinary liquid is transparent. The light scattering varies for different phases of the liquid crystals. For example, owing to the suppression of molecules into layers the light scattering of the smectic A phase is less than the nematic liquid crystal. For the smectic C phase, the fluctuation of the projection of tilted molecules onto smectic layers (the c-vector) causes stronger scattering than that in the smectic A phase. 

The above analysis is one example of the anisotropic properties of liquid crystals. It is these anisotropic properties that make liquid crystals and provide for its wide applications. 

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