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The Nobel Prize in Physics 1991
Pierre-Gilles de Gennes

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Award Ceremony Speech

Presentation Speech by Professor Ingvar Lindgren of the Royal Swedish Academy of Sciences, December 10, 1991

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen.

This year's Nobel Prize in Physics has been awarded to Pierre-Gilles de Gennes, College de France, Paris, for his investigations of liquid crystals and polymers. De Gennes has shown that mathematical models, developed for studying simpler systems, are applicable also to such complicated systems. De Gennes has discovered relations between different, seemingly quite unrelated, fields of physics - connections which nobody has seen before.

Liquid crystals and polymers can be regarded as intermediate states between order and disorder. A simple crystal, such as ordinary salt, is an example of almost perfect order - its atoms or ions are located in exact positions relative to each other. An ordinary liquid is an example of the opposite, complete disorder, its atoms or ions seem to move in completely irregular fashion. These examples represent two extremes of the concept order-disorder. In nature, there are more subtle forms of order and a liquid crystal is an example of that. It can be well ordered in one dimension but completely disordered in another. De Gennes has generalized the description of order for media of this type and been able to see analogies with, e.g. magnetic and superconducting materials.

The discovery of the remarkable substances we now call liquid crystals was made by the Austrian botanist Friedrich Reinitzer slightly more than a hundred years ago. In studying plants, he found that a substance related to the cholesterol had two distinct melting points. At the lower temperature, the substance became liquid but opaque and at the higher temperature completely transparent. Earlier, similar properties had been found in stearin. The German physicist Otto Lehmann found that the material was completely uniform between these temperatures with properties characteristic of a liquid as well as a crystal. Therefore, he named it "liquid crystal".

All of us have seen liquid crystals in the display of digital watches and pocket calculators. Most likely, we shall shortly see them also on the screen of our TV sets. Applications of this kind depend upon the unique optical properties of the liquid crystals and the fact that these can easily be changed, e.g. by an electric field.

It has been known for a long time that liquid crystals scatter light in an exceptional way, but all early explanations of this phenomenon failed. De Gennes found the explanation in the special way the molecules of a liquid crystal are ordered. One of the phases of a liquid crystal, called nematic, can be compared with a ferromagnet, where the atoms, which are themselves tiny magnets, are ordered so that they point in essentially the same direction - with slight variations. These variations follow a strict mathematical rule, which near the so-called critical temperature, where the magnet ceases to be magnetic, attains a very special form. In the liquid crystal the molecules are ordered in a similar way at every temperature, which explains its remarkable optical properties.

Another large field, where de Gennes has been very active, is that of polymer physics. A polymer consists of a large number of molecular fragments, monomers, which are linked together to form long chains or other structures. These molecules can be formed in a countless number of ways, giving the polymer materials a great variety of chemical and physical properties. We are quite familiar with some of the applications, which range from plastic bags to parts of automobiles and aircraft.

Also in these materials, de Gennes has found analogies with critical phenomena appearing in magnetic and superconducting materials. For instance, the size of the polymer in a solution increases by a certain power of the number of monomers, which is mathematically analogous to the behavior near a critical temperature of a magnet. This had led to the formulation of scaling laws, from which simple relations between different properties of polymers can be deduced. In this way, predictions can be made about unknown properties - predictions which later in many cases have been confirmed by experiments.

Major progress in science is often made by transfering knowledge from one discipline to another. Only few people have sufficiently deep insight and sufficient overview to carry out this process. De Gennes is definitely one of them.

 

Professor de Gennes,

You have been awarded the 1991 Nobel Prize in Physics for your outstanding contributions to the understanding of liquid crystals and polymers. It is my privilege to convey to you the heartiest congratulations of the Royal Swedish Academy of Sciences, and I now ask you to receive the Prize from the hands of His Majesty the King.

From Nobel Lectures, Physics 1991-1995, Editor Gösta Ekspong, World Scientific Publishing Co., Singapore, 1997

 

Copyright © The Nobel Foundation 1991
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