In Memoriam professor Rudolf de Bruyn Ouboter
PHYSICS
With great sadness, we have learned of the passing of professor Rudolf de Bruyn Ouboter. As Professor of Experimental Physics, he made fundamental contributions to our field over the course of several decades.
His entire academic career was spent in Leiden, beginning in 1951 with his study of Physics and culminating in the award of his doctorate in 1961 for his thesis ‘Thermodynamic Properties of Liquid ³He–⁴He Mixtures’.
In 1961, in his doctoral thesis, De Bruyn Ouboter proposed that dilution refrigerators could operate more effectively using a mixture of ³He and ⁴He. At the time, scientists were not yet aware that the phase separation process involved (which extracts heat) remains effective down to temperatures close to absolute zero. De Bruyn Ouboter’s prediction proved correct, and the ³He–⁴He mixture continues to form the basis of modern dilution refrigerators to this day.
In 1974, De Bruyn Ouboter was appointed Professor of Experimental Physics. His inaugural lecture, entitled *Flow Without Friction*, reflected the central theme of his research.
In the 1980s, the De Bruyn Ouboter group conducted pioneering experiments in the field of macroscopic quantum tunnelling — research of relevance to the 2025 Nobel Prize in Physics.
We are grateful for De Bruyn Ouboter’s contributions to physics in Leiden. We wish his loved ones strength and comfort in coping with this loss.
On behalf of the university community,
Sense Jan van der Molen, Scientific Director, Leiden Institute of Physics
Jasper Knoester, Dean, Faculty of Science
Sara de Rijcke, Rector Magnificus Universiteit Leiden