0- Mesogen (or Liquid Crystal) is a compound that displays liquid crystal properties and "mesogenic" is the term given to a substance that induces the liquid crystalline state (also known as the liquid crystal phase).
- A material is called lyotropic if it forms liquid crystal phases upon the addition of a solvent. Historically, the term was used to describe materials composed of amphiphilic molecules.
- A twisted nematic (TN) display is a common type of liquid-crystal display (LCD) that consists of a substance called a nematic liquid crystal that is confined between two plates of polarized glass.
- Disk-shaped LC molecules can orient themselves in a layer-like fashion known as the discotic nematic phase. If the disks pack into stacks, the phase is called discotic columnar. The columns themselves may be organized into rectangular or hexagonal arrays. Chiral discotic phases, similar to the chiral nematic phase, are also known.
- Liquid crystals find wide use in liquid crystal displays, which rely on the properties of certain liquid crystalline substances in the presence or absence of an E.
- In a typical device, a liquid crystal layer (typically 4 μm thick) sits between two Polarizers that are crossed (oriented at 90° to one another).
- The liquid crystal alignment is chosen so that its relaxed phase is a twisted one. This twisted phase reorients light that has passed through the first polarizer, allowing its transmission through the second polarizer (and reflected back to the observer if a reflector is provided).
- The device thus appears transparent. When an electric field is applied to the LC layer, the long molecular axes tend to align parallel to the electric field thus gradually untwisting in the center of the liquid crystal layer. In this state, the LC molecules do not reorient light, so the light polarized at the first polarizer is absorbed in the second polarizer, and the device loses transparency with increasing voltage.
- In this way, the electric field can be used to make a pixel switch between transparent or opaque on command. Color LCD systems use the same technique, with color filters used to generate red, green, and blue pixels.
- Chiral smectic liquid crystals are used in ferroelectric LCDs which are fast-switching binary light modulators. Similar principles can be used to make other liquid crystal based optical devices.
