• The liquid crystal state is a distinct phase of matter observed between the crystalline (solid) and isotropic (liquid) states. There are many types of liquid crystal states, depending upon the amount of order in the material.

Nematic Phases

• One of the most common LC phases is the nematic.
• The word nematic comes from the Greek word ‘nema’, which means "thread". This term originates from the thread-like topological defects observed in nematics, which are formally called 'disclinations'.
• Nematics also exhibit so-called hedgehog topological defects.
• In a nematic phase, the calamitic or rod-shaped organic molecules have no positional order, but they self-align to have long-range directional order with their long axes roughly parallel.
• Thus, the molecules are free to flow and their centre of mass positions are randomly distributed as in a liquid, but still maintain their long-range directional order.
• Most nematics are uniaxial: they have one axis that is longer and preferred, with the other two being equivalent (can be approximated as cylinders or rods). However, some liquid crystals are biaxial nematics, meaning that in addition to orienting their long axis, they also orient along a secondary axis.
• Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field.
• Aligned nematics have the optical properties of uniaxial crystals and this makes them extremely useful in liquid crystal displays (LCD).

Smectic Phases

• The word "smectic" is derived from the Greek word for soap.
• This seemingly ambiguous origin is explained by the fact that the thick, slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal.
• The smectic state is another distinct mesophase of liquid crystal substances.
• Molecules in this phase show a degree of translational order not present in the nematic.
• In the smectic state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes.
• Motion is restricted to within these planes, and separate planes are observed to flow past each other. The increased order means that the smectic state is more "solid-like" than the nematic.

Cholesteric Phases

• The cholesteric (or chiral nematic) liquid crystal phase is typically composed of nematic mesogenic molecules containing a chiral center which produces intermolecular forces that favour alignment between molecules at a slight angle to one another.
• This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those above and below. In this structure, the directors actually form in a continuous helical pattern about the layer normal

Columnar Phases

• Columnar liquid crystals are different from the previous types because they are shaped like disks instead of long rods.
• This mesophase is characterized by stacked columns of molecules.
• The columns are packed together to form a two-dimensional crystalline array. The arrangement of the molecules within the columns and the arrangement of the columns themselves lead to new mesophases.