Liquid crystal

Schlieren texture of Liquid Crystal. This photo was taken with a polarizing filter and a microscope

Liquid crystals are special substances that are usually liquid, but that show certain properties of a solid crystal. A liquid crystal (LC) may flow like a liquid, but its molecules may be rotated in a crystal-like way. There are many different types of liquid crystal phases. Different phases are different because their optical properties are different. Using a microscope and a polarized light source, different liquid crystal phases will have different textures. This is also shown in the image.

The contrasting areas in the texture each correspond to a domain where the LC molecules are rotated in a different direction. Within a domain, however, the molecules are well ordered. LC materials may not always be in an LC phase (just as water is not always in the liquid phase: it may also be found in the solid and gaseous phase).

Liquid crystal displays use the optical properties of liquid crystals to control light.

Liquid Crystal Media

• 

  Chemical structure of cholesteryl benzoate molecule

• 

  Der deutsche Physiker Otto Lehmann in seinem Labor an der TH Karlsruhe.

• 

  Chemical structure of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) molecule

• 

  Alignment in a nematic phase

• 

  Phase transition between a nematic (left) and smectic A (right) phases observed between crossed polarizers. The black color corresponds to isotropic medium.

• 

  Schematic of alignment in the smectic phases. The smectic A phase (left) has molecules organized into layers. In the smectic C phase (right), the molecules are tilted inside the layers.

• 

  Schematic of ordering in chiral liquid crystal phases. The chiral nematic phase (left), also called the cholesteric phase, and the smectic C* phase (right).

• 

  Chiral nematic phase. The numerator p refers to the chiral pitch (see text).

• 

  A planar cell, filled with achiral LC host doped with an optically active Tröger base analog, placed between a pair of parallel (A) and crossed (B) linear polarizers. This doped mesogenic phase forms self-organized helical superstructures, that allow specific wavelengths of light to pass through the crossed polarizers, and selectively reflects a particular wavelength of light.