Superconductivity, superfluidity, liquid crystals, hydrodynamic instabilities and turbulence, porous media, granular systems and others are the themes on which the French physicist Etienne Guyon, who died on July 13 at the age of 88, made major contributions, widely recognized by the international scientific community.

But to mention only this very important aspect of his scientific life would be to neglect his personality, which is rare in this community. Etienne Guyon has distinguished himself as a teacher, writer, scientific animator, popularizer and institutional actor. Professor at the University of Paris-Orsay and at the School of Industrial Physics and Chemistry of Paris (ESPCI), he was also Director of the Palais de la Découverte from 1988, a position he left in 1990 to take over the management of the Ecole Normale Supérieure (ENS) until 2000, before returning as Emeritus Researcher at the Laboratory of Physics and Mechanics of Heterogeneous Media (PMMH) of ESPCI, where he continued to work until his death.

In 1955, he passed the competition for the Ecole polytechnique and the ENS and chose the latter. In these times of war in Algeria, he was part of a group of Catholic students (the “talas”), committed pacifists. In 1961, he began an experimental thesis, which he defended in 1965, on superconductivity (electrical conduction without resistance at very low temperature) with a young physics professor from the University of Orsay named Pierre-Gilles de Gennes, future Nobel Prize winner, of whom he was to be the first doctoral student and faithful friend.

After his thesis, Etienne Guyon went to the University of California at Los Angeles, to work on superfluid helium (flowing without viscosity at very low temperature). Part of his experiments inspired the theory of Kosterlitz and Thouless, awarded in 2016 by the Nobel Prize in Physics.

Back in Orsay, he joined a new activity launched by Pierre-Gilles de Gennes on liquid crystals. His work on the movement of liquid crystals subjected to a temperature difference makes him an international reference in this field. Etienne Guyon then became one of the leading figures of the French research community on instabilities, chaos and turbulence, which would have a strong international impact between the 1980s and 1990s.

In 1978, he joined the ESPCI, to teach the course in physical hydrodynamics. Soon he opened a new laboratory, which would later become the PMMH. This Grande Ecole, attached to the City of Paris, trains engineers by favoring experimentation and research. It has a rich past in the history of physics: Pierre and Marie Curie discovered radium there in 1898. The establishment was managed by Paul Langevin, among others, and when Etienne Guyon joined him, Pierre-Gilles de Gennes assumed the management.

The courses given at ESPCI by Pierre Bergé and Etienne Guyon have radically changed the teaching of hydrodynamics. They provide them based on recent research results and make them the basis of an educational text of which Etienne Guyon was the co-author, under the title of Physical Hydrodynamics (1991). This book will accompany generations of students around the world.

A dazzling dynamism

At ESPCI, he developed research on hydrodynamic instabilities and porous media, and opened a research activity on soft matter. Endowed with a remarkable capacity for work, coupled with a dazzling dynamism, Etienne Guyon played a very important role in the genesis of a pioneering style of research, combining physics and mechanics, and inspired by statistical physics.

He was interested in disordered matter and was the founder of the research group “Macroscopic random environments” which structured, and still structures today, the French community of studies on soft matter.

In 1975, on the occasion of the National Congress of the French Physical Society, in Dijon, he was asked to install an open-air physics animation entitled “Physics in the Street”, an installation for which he confided that he was inspired by May 68 and which was a great success. He will rely on this experience when he chairs the orientation committee of the Cité des sciences de la Villette, when it opens, and then at the Palais de la Découverte.

The model of the Palais de la Découverte (exhibitions, activities, conferences), with continuous relations with experiments and the world of research, is very suitable for this experimental physicist. During his short mandate, he revitalized the scientific clubs of young people (“Les petits débrouillards”), to which he will always remain very attached.

He completed his institutional experience from 1990, crossing the 200 meters separating the ESPCI from the rue d’Ulm (still in the 5th arrondissement of Paris) to direct the ENS for ten years. His broad culture and his sensitivity for the human sciences made him particularly attentive to literary students, and he intervened in particular to digitize the library and develop IT in the literary departments.

One of its most innovative initiatives was the creation of the “European” entrance examination, allowing direct access to the normalien student category after an examination in one of the five authorized languages ​​(English, German, French, Spanish and Italian). Currently, unfortunately, this selection only gives access to scholarships.

The pen as a weapon

Etienne Guyon energetically defended the precise use of the language in the world of science, insisting even in the recurring debates in the laboratory on the use of French. This is how he was appointed expert to the General Commission of Terminology and Neology. By cultivating bilingualism, he promoted linguistic diversity.

A man of combat, he wielded the pen like a weapon and, speaking with ease to everyone, his voice carried far, literally and figuratively. Etienne Guyon was, in his last years, a prolific author, writing a dozen books in which he exposed, in simple and accessible language, the latest results of the physics of disorder. Many of its co-authors were young scientists marked by this exercise of working together.

His family home in Limours (Essonne) was open to anyone who needed a roof. He, who had suffered the pain of losing two of his four children, beamed with pleasure, years later, announcing urbi et orbi the birth of a ninth great-grandchild.