What is Liquid Crystal?

What are Liquid Crystal Phases?

          It is easy to guess from their names that liquid crystal phases are states of matter intermediate between those of liquid and crystal.  In crystal phase, molecules are organized in exact positions and orientations.  In other words, they have both positional and orientational order.  In liquid phase, however, molecules can flow around randomly, losing both positional and orientational order.  In liquid crystal phases, molecules can flow around like liquid but maintain long-range orientational order. 

              Liquid crystal molecules are usually rod-like in shape.  We describe the molecular field of liquid crystal by a vector n (call a director), which is a unit vector representing a local averaged orientation of the long-axis of the molecules.  Light polarized along the director n experiences different index of refraction than light polarized perpendicular to n.  This optical birefringent property as well as an ability to response of electric and magnetic field make liquid crystals perfect candidates in display industry.

Thermotropic liquid crystals change their phases with temperature.  The ordering of the phases increases as the temperature decreases.  Examples of some thermotropic liquid crystal phases are shown in the Figure.  An isotropic phase or liquid phase is the state when liquid crystals lose both positional and orientational order.  In a nematic phase, the molecules are roughly oriented along their long axis n.  In smectic phases, the molecules form layers.  They are not allowed to flow between the layers, but they can flow freely in 2D within the layers.  In smectic A phase, the molecules orient along the layer normal, while in smectic C phase the molecules are tilted with respected to the layer normal.