What are liquid crystals    

The Discovery
The well-known three states of matter are solid, liquid and gas.  When cooled, gas condenses to form a liquid as you see in a warm room in winter where water vapor forms dew on glass windows cooled by the cold air outside.  In the gas state, molecules are free to move around pretty much independent from each other except for occasional collisions.    Molecules in the liquid state are less mobile and closer to each other.   Frequent collisions between molecules make the liquid more viscous, yet it can still flow like "liquid."  As the liquid is further cooled, say at the freezing point of water 0℃ (32℉), it is transformed to a solid, which is rigid; water freezes to become ice at
0℃ (32℉).  Until two scientists in Europe, Friedrich Reinitzer and Otto Lehmann, discovered liquid crystals in the late 19th century, these three are the only states of matter that humans have ever known.

Liquid crystal is the fourth state of matter that occurs between solid and liquid.   While studying the function of cholesterol in plants, Friedrich Reinitzer, an Austrian botanist, found an unusual melting that was always accompanied by the presence of cloudy liquid state before the clear liquid appears.  This cloudy liquid is what is now known as "liquid crystal."  Intrigued by this usual observation, Reinitzer sent the sample1 to a renowned German crystallographer, Otto Lehmann.   Through his careful observations of the melting of the substance using his state-of-the-art microscope with a gas heating stage, Lehmann was convinced that the cloudy state is truly a new state of matter that differs from solid, liquid and gas.   The year 1888 Reinitzer found this double melting phenomenon is officially recognized as the year of discovery of liquid crystals. 

Friedrich Reinitzer
Austrian botanist, born February 25, 1857 in Prague, and died February 16, 1927 in Graz.

Otto Lehmann
German physicist, born January 13, 1855, in Konstanz and died June 17, 1922, in Karlsruhe.

What you see in this small glass vial is a liquid crystal.  It is opaque and flows like ordinary oil.   To demonstrate the phase change from the liquid crystal state to the ordinary liquid, the glass vial is inserted in hot water.   As it is heated, the cloudy look disappers, rapidly taken over by a clear liquid.   Finally, it becomes completely transparent.  It is now taken out of the hot bath and is kept in air.   As it is cooled down, the initial opacity reappears.     Get Adobe Flash player

Further Readings:
  • Hans Kelker "History of Liquid Crystals"  Mol. Cyrst. Liq. Cryst. 21,1-48(1973).
  • Hans Kelker "Survey of the Early History of Liquid Crystals" Mol. Cyrst. Liq. Cryst. 165,1-43(1988).
  • Peter M. Knoll, Hans Kelker, "Otto Lehmann" BoD – Books on Demand, 2010.
Notes and References:
  1. The samples Reinitzer sent to Lehmann were cholesterol-acetate (1)  and cholesterol-benzoate (2), which have a cholesteric (or twisted nematic) phase above 100

    (1) (2)