Lessons from the discovery of liquid crystals

I recently learned a little about the history of the discovery of liquid crystals, stimulated by Soft Matter: A Very Short Introduction by Tom McLeish. Besides being a fascinating story there are lessons about the importance of curiosity-driven research, interdisciplinarity, serendipity, and the long road to technology.

Friedrich Reinitzer (1857 - 1927) was a botanist and chemist who worked at the Institute of Plant Physiology in Prague. He was studying cholesterol with the aim of determining its molecular weight. He produced crystals of cholesteryl benzoate and measured their heat capacity as a function of temperature. Aside: For chemists today this measurement is known as differential scanning calorimetry (a constant source of heat is added and the temperature measured as a function of time). 

In 1888, Reinitzer observed that the crystal melted at 145.5 degrees Celsius (signified by absorption of heat), forming a milky liquid. However, at 178.5 degrees Celsius, there was a second absorption of heat, and the liquid became transparent. This suggested that there were two melting transitions. Puzzled by this Reinitzer consulted the physicist and crystallographer, Otto Lehmann, who promoted the idea that this was a new state of matter, which he dubbed a "liquid crystal" (or flowing crystal).

Today, cholesteryl benzoate is classified as a chiral nematic liquid crystal, which is also sometimes known as a cholesteric liquid crystal, in honour of the first one. A schematic of the ordering is shown below.

The milkiness was not explained until the 1960s by Pierre-Gilles de Gennes, who exploited an analogue with a superconductor in a magnetic field.

More detail is in the paper

Liquid‐Crystal Science from 1888 to 1922: Building a Revolution 

Michel Mitov 

This discovery of liquid crystals was the first of many cases where a new state of matter was discovered by a thermodynamic measurement. Others include superfluid 4He (the lambda transition) and superfluid 3He, as I have recently highlighted.