What are Liquid Crystals?

They are substances, organic compounds, that have physical properties between those associated with crystalline solids and those of liquids. 

These substances are usually classified under two schemes: 

•   Shape of the molecules.
•   How the molecules interact to form spatial arrangements. 

Many liquid crystalline substances are organic molecules with a long non-polar chain (calamatic) with a polar group on one of its ends and commonly possess aromatic rings. The molecules can also be disc-like (discotic) in shape. 

Calamatic:

Bridging Group   X

- N = N -

- N = NO -

- CH = CH -

- C  C -

- CH = N(O) -

- CH = N -

- O - CO -

-

End Groups   R, R'

- CnH2n+1    n = 1 to 9

- OCnH2n+1  n = 1 to 9

- C  N

- OOC - OCnH2n+1  n = 1 to 9

Compounds with the above structure can self-assemble to form nematic and smectic phases. 

Discotic:

Molecular models of a few good liquid crystals are shown below. The different kinds of atoms in these molecules are colored as described below. 
 

Spatial Structures formed

They are liquids but their rod-like molecules are arranged into an orderly array like a crystal. They are ordered along specific directions but the molecular units are not spaced in a periodic manner. This orientational ordering is in one and two dimensions. 

Scanning Tunneling Microscopy image of octadecanol molecules adsorbed on a graphite surface, forming domains. The molecules are chain-like and polar, thus align with each other. 
 
 

Scanning Tunneling Microscopy (9 nm x 9nm scan) image of 8-CB liquid crystal molecules assembling on graphite.  Molecular details such as the linked benzene rings and a portion of its tail are shown in this image.