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.


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