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Lorentz Professor Tom Lubensky: pioneer in soft matter

Professor Tom Lubensky from the University of Pennsylvania is visiting Leiden University as the 64th Lorentz Professor at the department of Theoretical Physics. He is a pioneer in the field of theoretical soft matter physics and winner of the prestigious Buckley Condensed Matter Prize. We spoke with him about his time in Leiden and how he became part of the invention of soft matter physics.

Your field is theoretical soft matter physics. How do you explain this to people outside of science?

‘Soft matter encompasses a broad range of materials, such as colloids, emulsions, soaps or regular fluids. I work on the theoretical part, which means that I try to explain for example how soft materials change phases or how they flow. Liquid crystals form a big part of the playground in my field.

How did you get interested in this field?

‘When I started off as an undergraduate at Caltech, soft matter physics didn’t even exist. I wanted to study astronomy. But then I saw what astronomers actually did: they were calculating the effect of Jupiter on the orbit of Mercury up to five decimal points. I realized that this wasn’t for me. Then I decided to do particle physics, but I didn’t like that either. I went to Paris on a fellowship to work on magnetism. However the professor was caught up in the aftermath of the 1968 student revolt and his group was not functioning well. Instead I met Pierre-Gilles de Gennes and joined his liquid crystal group. In the early 1980s we became part of the invention of soft matter physics.

Since the 1980s the field has been developing. What is currently the hottest topic?

‘What is really big right now, is what they call active matter. This is matter made up of components with a source of energy that allows them to move on their own, like flocks of birds, robot swarms or traffic flows. This creates many interesting opportunities for theory. There is a whole string of questions: how does active matter differ from matter in equilibrium? Are there completely new concepts? Well, the answer is yes. For example a set of spheres, with an asymmetry that defines a particular direction of motion, will form dynamic clusters, stabilized by the inward motion of the spheres at the periphery.

What do you expect from your Lorentz professorship?

‘It is an opportunity to visit a new place and strike up new relations and build possible collaborations. Leiden has a good soft matter group. We might collaborate for example on studying an unusual new state of matter within our field, called a ferronematic phase. When you add nanomagnets to certain liquid crystals, like the ones in your tv screen, the magnetic moment of the nanomagnets align to form a fluid ferromagnet. So you have a magnet in the daily sense of the word, the ones that kids play with, but in liquid form.

You are part of a long list of Lorentz Professors since 1955. Whose work do you admire most?

‘Of course De Gennes. But also David Nelson with whom I’ve been in friendly competition with for years. And other people, like Bert Halperin and David Mermin. You certainly have the cream of the crop visiting your physics institute.

You have experienced Leiden for over a week now. How do you like the city?

‘Leiden is a delightful town. It feels alive, it is not overrun by traffic, and there seems to be plenty to do. My wife and I already visited the St. Pieterskerk and the botanical gardens. Plus we went out of town to see the hyacinths in the fields. I will be here until June, so I have plenty of time to see more. The Rijksmuseum in Amsterdam is certainly on my list.’

Since 1955, the department of theoretical physics has invited a distinguished scientist to be a guest professor in Leiden over the summer; the Lorentz chair. The list of Lorentz professors includes 15 Nobel laureates. Among them is Kip Thorne, who won the 2017 Nobel Prize for the detection of gravitational waves. Each year, the Lorentz Professor signs the historic signature wall in the Oort building.

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