St Cross College, Oxford

College Seminar given on 30 November 2000

Most of us feel comfortable with the concepts of liquids and crystals. We know that wine is a liquid and we can pour it from the bottle into a glass, and correctly think of a liquid as containing a totally random arrangement of mobile atoms or molecules. On the other hand we know that in a solid there is a fixed array of atoms or molecules and that in a crystal there is a particular pattern to this array which results in a distinctive and attractive geometric shape. When we think of a crystal we think of a snowflake, or perhaps of the copper sulphate crystal we grew as a child. The regular order within a crystal also imparts a number of distinctive properties. X-rays are diffracted in a manner which allows the crystal structure to be determined and light, particularly polarised light, can be influenced by the structure of the crystal as it passed through it. We also know that it is easy to melt a snowflake into liquid water, making it loose forever its distinctive crystalline features. So when we hear of liquid crystals, how can we reconcile the concept of a liquid comprised of mobile random molecules with a crystal containing a fixed regular array of molecules? We feel it must be a contradiction in terms.

The liquid crystal phase was first observed in 1888 by an Austrian botanist, Reinitzer. As he heated a simple derivative of cholesterol, he noticed that effects on polarised light typical of a solid crystal persisted for a small range of temperature above the melting point.

Over the next twenty to thirty years many other materials were found to exhibit the same phenomenon, and they acquired the descriptive but contradictory name of 'liquid crystals'. The ordering present became understood as a combination of random liquid-like behaviour of the centres of gravity of the molecules with an ordering of their long axes. A similar combination can be seen if a thin layer of rice grains is gently shaken in the bottom of a bowl, or in logs floating on a river. Over the middle part of the 20th century many new materials, physical properties and structural theories were developed. It also became understood how widespread liquid crystals were. A liquid crystal phase can be found in 1% of all organic materials, and in areas as diverse as soap solutions, cake mixtures and even in the human body. However, the feeling developed that everything interesting about liquid crystals was known, and most groups stopped their liquid crystal activities and turned to other areas.

Throughout this period, occasional observations had been made of strange electro-optic effects in liquid crystals. It was quite easy to persuade liquid crystals to alter their influence on polarised light by the application of quite a small voltage. Indeed, the first ever patent for one of these liquid crystal switching effects was filed in the UK in 1934 by the Marconi Company. In the mid 1960s, a small number of industrial groups around the world started to think that it was time that display devices moved out of the dark ages and caught up with the tremendous advances being made in silicon integrated circuit technology. Already miniature integrated circuits consuming virtually no power could perform the electronic operations needed by a number of every day items such as digital watches or calculators. However, how was the information to be displayed? The only really viable display devices were based on large evacuated glass bottles known as cathode ray tubes, hardly what one wanted for portable low power equipment! Some of these industrial groups decided to re-investigate the electro-optic effects in liquid crystals. None of these effects were particularly useful for use in display devices, but new effects were quickly invented, new liquid crystal materials synthesised and a range of practical problems solved. Progress was rapid, undoubtedly benefiting from the firm scientific foundations laid earlier in the century. Liquid crystal displays soon became widespread in watches, calculators and a wide range of office and domestic equipment. The contradictory name 'liquid crystals' and the common acronym used for the display devices, LCDs, have both slipped into everyday language. Totally new products such as laptop computers and mobile phones have grown out of the availability of LCDs. It is estimated that in the year 2000, over 1.5 billion LCDs were fabricated, not quite one for each member of the human rase, but getting close.

Is the explosive growth in LCD technology now complete? I think not. The range of options available is so diverse that I believe that we are at the most only half way through the development of technology using these strange materials with the contradictory name.

Peter Raynes