Liquid Crystals

- Sep 26, 2018-

Liquid crystals were discovered by accident in 1888 by Austrian botanist Friedrich Reinitzer. He showed that a plant derivative, cholesteryl benzoate, had two melting points, becoming a cloudy liquid at 145 °C and turning clear at 179 °C. To seek an explanation, he passed his samples to physicist Otto Lehmann. Using a microscope fitted with a heating stage, Lehmann showed that the in-between cloudy state had optical properties typical of some crystals, yet was a liquid – and so the term “liquid crystal” was born.


It is understood that most liquid crystals, like cholesteryl benzoate, consist of molecules with long, rod-like structures. It is the combination of the attractive forces that exist between all molecules coupled with the rod-like structure that causes the liquid-crystal phase to form. However, the interaction is not quite strong enough to hold the molecules firmly in place. Many different kinds of liquid-crystal structures have since been discovered. Some organise further into layers, while others are even disc-shaped and form columns.

 

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Throughout the 1920s and 1930s, researchers studied the effects of electric and magnetic fields on liquid crystals. In 1929, Russian physicist Vsevolod Freedericksz showed that liquid-crystal molecules, in a thin film sandwiched between two plates, changed their alignment when a magnetic field was applied. This was the forerunner of the modern voltage-operated LCD. The first patent for a liquid-crystal device was taken out by the UK Marconi Wireless Telegraph company in 1936. However, it was not until after the Second World War that LCDs generated serious interest. As physicists started to develop ever-smaller electronic devices and integrated circuits for everyday appliances, it became clear there was a need for a compatible display technology. LCDs became a candidate.

 

The first devices, which were developed in the late 1960s, consisted of a thin film of liquid crystal sandwiched between glass slides coated with transparent electrodes. An applied electric field disrupted the liquid-crystal alignment, transforming its appearance from transparent to opaque. These and subsequent devices were rather sensitive, for example, to temperature and did not last long. However, the breakthrough came in the UK when physicist Peter Raynes at the Royal Signals and Radar Establishment (RSRE) collaborated with chemists George Gray and Ken Harrison of the University of Hull in developing novel LCD materials that worked, were stable at room temperature and were suitable for mass-production. This interdisciplinary collaboration was crucial in advancing LCD technology. The RSRE research programme led by a physicist, Cyril Hilsum, resulted in a number of key device inventions, including the supertwisted nematic LCD, thin-film transistors (TFTs) for driving LCDs, the defect-free twisted nematic device and the zenithal bistable display. TFT LCDs, which incorporate a thin-film silicon transistor, are now the main technology used in TVs and computer monitors.