Obituary: Jan Beenakker, 1926-1998

Jan
Beenakker, painted by Carla Roodenberg (1990)

Jan Beenakker passed away on July 23, 1998. A brief illness brought an unexpected end to fifty years of dedicated service to physics in Leiden and Dutch science in general.

Jan Beenakker was born in Koog aan de Zaan (North-Holland) on February 1, 1926, as the first child of a railway employee from Leiden. As his family moved from one train station to another, he grew up first in Zeeland and then in Rotterdam. There he concluded his high school education at the Saint Francis Gymnasium (1943), but could not continue with university because of the war. After the liberation in 1945 he started the study of physics in Leiden. He would commute from Rotterdam and in that city was an active youth member of the labor union. After a break for military service (as a meteorologist), his masters degree in physics followed in 1951 and his Ph.D. in 1954. His thesis "The influence of the Helium isotope with mass 3 on the properties of liquid Helium II" was done under the direction of Cor Gorter, the head of the Kamerlingh Onnes Laboratory in Leiden. (Altough his actual mentor was Krijn Taconis.) Beenakker stayed at Leiden University until his retirement in 1991, interrupted only for a sabbatical leave at MIT in 1969/1970. He was promoted to Reader in 1959 and to full Professor in 1963, and also held a position as visiting professor at the Catholic University of Leuven (1961-1992).

While the topic of the thesis still followed the Leiden tradition in low temperature physics, it already contained the seed of what would become Beenakker's broad interest in thermodynamic and transport properties of molecular liquids and gases. In the early sixties Jan Beenakker, motivated by Gorter, entered the field in which he and his group would play a pioneering role: The transport properties of non-spherical molecules. Using highly sensitive techniques, measurements were performed of the heat conductivity and viscosity of molecular gases in a transverse magnetic or electric field. This led to the discovery of new phenomena that have a certain similarity to the Hall effect and stimulated extensive theoretical research. It was believed from early work of Senftleben that an external magnetic field would only influence paramagnetic molecules such as NO and O2, but Beenakker (working with his younger colleague Hein Knaap) showed experimentally that it is a much more general property of non-spherical molecules. Also diamagnetic gases such as N2 and CH4 are influenced by an external field as a result of precession between collisions. Such field effects are now known as Senftleben-Beenakker effects.
Here you can read how Jan Beenakker recalled the excitement of his discovery twenty-five years later.

Later Beenakker extended his research to transport properties in the rarefied gas regime, where boundary layer effects are essential. New phenomena such as a viscomagnetic heat current and thermomagnetic pressure difference were observed. For the first time it became possible to measure directly the nonequilibrium molecular velocity distribution in a heat conducting gas. This experimental research program was carried out in close collaboration with a variety of theorists, who would be regular and long-term visitors to Leiden. The international scientific collaboration culminated in the monumental monograph "Nonequilibrium Phenomena in Polyatomic Gases" (Clarendon, Oxford, 1991), which Beenakker co-authored with Fred McCourt (Waterloo), Walter Köhler (Erlangen), and Ivan Kuscer (Ljubljana).

In parallel to his scientific research, Beenakker contributed to Dutch science policy at the request of the minister Pais of Science and Education. In Leiden he served as Dean of the Faculty of Sciences and Rector Magnificus of the University (1985-1991). More than many of his colleagues, he had a good eye for the political reality that constrains the desires of the scientific community. After his retirement he chaired the Dutch Science Foundation FOM, without losing his interest and involvement in research: He finished the manuscript of his last paper (with Sergei Krylov) just a few weeks before his death.

His long-time collaborator and friend Jo Hermans gave the following characterization of Jan Beenakker as a person: "Jan was modest and a bit shy and those who did not know him well could find it difficult to make contact. But he formed unusally close bonds with the members of his group. Not only was he an inspiring thesis advisor, quick and creative in the scientific debate; He was also a wise counsel and a father figure in the classic sense of the word: Generous with praise and honest with criticism. He posessed a highly developed sense of social responsibility, and would be especially attentive to the needs of the non-scientific staff. The result of all this was that the members of the Beenakker group kept strong ties to their teacher long after they had left Leiden."

Beenakker was a member of the Royal Netherlands Academy of Arts and Sciences and a Doctor Honoris Causa from the University of Waterloo (Canada). He was knighted both in the Dutch and Belgian royal orders. Dutch physics in general and Leiden physics in particular has much for which to thank Jan Beenakker. We will miss his wisdom.

Johan van der Waals

published (in Dutch) in "Levensberichten en Herdenkingen" (KNAW, Amsterdam, 2000).

...hier is een In Memoriam in het Nederlands...