- Beata Turoňová1,2,*,
- Mateusz Sikora3,*,
- Christoph Schürmann4,*,
- Wim J. H. Hagen1,
- Sonja Welsch5,
- Florian E. C. Blanc3,
- Sören von Bülow3,
- Michael Gecht3,
- Katrin Bagola6,
- Cindy Hörner4,7,
- Ger van Zandbergen6,8,9,
- Jonathan Landry10,
- Nayara Trevisan Doimo de Azevedo10,
- Shyamal Mosalaganti1,2,
- Andre Schwarz1,
- Roberto Covino3,11,
- Michael D. Mühlebach4,7,
- Gerhard Hummer3,12,†,
- Jacomine Krijnse Locker13,†,
- Martin Beck1,2,†
Wednesday, June 14, 2023
Demonstrating polymer entanglement
Thursday, July 8, 2021
Is condensed matter physics too abstract?
Condensed matter physics is about the properties of real materials. Real stuff that you can see and touch and that you can use to make very practical things like TV screens and mobile phones. Yet, I find it fascinating and somewhat ironic that in condensed matter theory very abstract ideas and mathematical techniques keep cropping up (and being extremely useful): variable spatial dimensions, imaginary frequencies, topology, Chern numbers, conformal invariance, ...
Yet, there is a danger with abstraction. Theoretical condensed matter is not pure mathematics. Perhaps, too often fancy and beautiful mathematics is prized over physical intuition and insight. Theory may take precedence over experiment. How does one find the appropriate balance?
This is part of broader issues about the role of abstraction and formality in education.
Pierre de Gennes (1932-2007) was arguably the founder of soft matter as a research field, as recognized by the Nobel Prize in Physics in 1991. He began his career working on superconductivity and went on to develop a unified framework to understand soft matter (liquid crystals, polymers, foams, colloids...), introducing ideas such as order parameters, scaling, renormalisation, and universality.
After his Nobel, de Gennes gave many lectures in French high schools, which were then published as a book, Fragile Objects: Soft Matter, Hard Science, and the Thrill of Discovery. I highly recommend it, both as a popular introduction to soft matter, but also to hear the perspective of a great scientist on education and research.
de Gennes spent almost his whole life living and working in France. In the book he rants about the French system, particularly its obsession with entrance exams, mathematics, formality, and the abstract.
“Manual skills, visual acumen, the sense of observation, an interest for the physical world which surrounds us, are all qualities that are neglected or downgraded.”
“To work in a garage seems to me the best initiation to a professional life.”
“Ignorance of the real world causes grave distortions.”
“the positivist prejudice”
I found this fascinating because one thing de Gennes is famous for is showing how some properties of a polymer can be understood by considering a theory involving a vector of dimension n, where n was a continuous variable, in the limit where n approaches zero! That is pretty abstract! But, I guess the point is that he is not against abstraction, exams, and mathematics, per se. Rather, he is against them taking on a life of their own.
de Gennes concerns are also shared by Henri Alloul (well known for beautiful NMR experiments on strongly correlated electron materials) author of Introduction to the Physics of Electrons in Solids. In the Preface, he writes,
In many countries, teaching traditions have always given pride of place to a formal, and essentially deductive, presentation of the physics, i.e., starting from formal hypotheses and leading up to observable consequences. This deductive approach leaves a purely a posteriori verificational role to observation, and hides the thinking that has gone into building up the models in the first place. Here we shall adopt the opposite approach, which begins with the fact that in science in general, and in solid state physics in particular, the qualitative understanding of a phenomenon is an important step which precedes the formulation of any theoretical development. We thus urge the reader to carry out a careful examination of the deeper significance of experimental observations, in order to understand the need for specific models and carry out realistic approximations.
The debate about abstract mathematics is also central to contrasting views about the Institute for Advanced Study at Princeton.
de Gennes's views would have also resonated with Harry Kroto who shared The Nobel Prize in Chemistry for the discovery of buckyballs. He credited playing with Meccano as a child as very important in his scientific development.
Friday, December 18, 2020
Lessons from the discovery of liquid crystals
I recently learned a little about the history of the discovery of liquid crystals, stimulated by Soft Matter: A Very Short Introduction by Tom McLeish. Besides being a fascinating story there are lessons about the importance of curiosity-driven research, interdisciplinarity, serendipity, and the long road to technology.
Friedrich Reinitzer (1857 - 1927) was a botanist and chemist who worked at the Institute of Plant Physiology in Prague. He was studying cholesterol with the aim of determining its molecular weight. He produced crystals of cholesteryl benzoate and measured their heat capacity as a function of temperature. Aside: For chemists today this measurement is known as differential scanning calorimetry (a constant source of heat is added and the temperature measured as a function of time).
Thursday, November 26, 2020
Signatures of soft matter
What is soft matter?
Soft Matter: A Very Short Introduction by Tom McLeish has just been published.
McLeish identifies six characteristics of soft matter.
1. Thermal motion
They exhibit large local spatial rearrangements of their microscopic constituents under thermal agitation. In contrast, "hard" materials experience only small distortions due to thermal motion.
2. Structure on intermediate length scales
There are basic units ("fundamental" structures), typically involving a very large number (hundreds to thousands) of atoms, that are key to understand soft matter behaviour. These basic units are neither macroscopic nor microscopic (in the atomic sense), but rather mesoscopic (meso from the Greek word for middle). The relevant scales range from several nanometres up to a micrometer. An example of these length scales is those associated with (topological) defects in liquid crystals, such as those shown below.
Image is from here.
3. Slow dynamics
The mesoscopic length scales and complex structures lead to phenomena occurring on time scales of the order of seconds or minutes.
4. Universality
The same physical properties can arise from materials with quite different underlying chemistries.
This characteristic is of significant practical relevance. Solving a problem for one specific material can also solve it for whole families of materials. This universality is also of deep conceptual significance as understanding a general phenomenon is usually more powerful than just a specific example.
5. Common experimental techniques
The dominant tools are microscopy, scattering (light, x-rays, neutrons), and rheometers which measure mechanical properties such as viscosity (rheology).
6. Multi-disciplinarity
Soft matter is studied by physicists, chemists, engineers, and biologists.
The chapter titles in the book are
Milkiness, muddiness, and inkiness [Colloids]
Sliminess and stickiness [Polymers]
Gelification and soapiness [Foams and Self-assembly]
Pearliness [Liquid crystals]
Liveliness [Active matter]
I highly recommend the book. Hopefully, later I will write a review.
Tuesday, October 20, 2020
The physics of the SARS-CoV-2 virion
Some progress is being made in understanding the structure and dynamics of the SARS-CoV-2 virions (virus particles) that are responsible for the pandemic. A nice starting point for the non-expert is a recent article in The New York Times.
A fundamental question is what is the structure and symmetry of the virion? In particular, does it have the icosahedral symmetry possessed by many virions, as discussed in a talk I gave earlier this year and in a recent review (with lots of nice pictures). As far as I am aware, there are still no definitive results on the overall structure and symmetry.
This preprint has some really nice images and videos such as the video below.
SARS-CoV-2 structure and replication characterized by in situ cryo-electron tomography
The paper below shows that the nucleocapsid protein (N) is similar to that for SARS-CoV and MERS. The protein can form dimers and tetramers, steps in the self-assembly of the whole virion.
Specific viral RNA drives the SARS CoV-2 nucleocapsid to phase separate
Some nice soft matter physics is in the preprint below. It argues that the N protein can undergo liquid-liquid phase separation with the viral genome. Aside: even before covid, liquid-liquid phase separation was quite a hot topic in cell biology, as recently discussed by Tom McLeish.
Architecture and self‐assembly of the SARS‐CoV‐2 nucleocapsid protein
Qiaozhen Ye, Alan M. V. West, Steve Silletti, Kevin D. Corbett
Finally, the paper below combines molecular dynamics simulations with experiments to argue that the stalk of the spike protein has three hinges giving the head of the spike unexpected orientational freedom so it can scan the host cell surface.
The two-state model for spin crossover in organometallics
Previously, I discussed how spin-crossover is a misnomer for organometallic compounds and proposed that an effective Hamiltonian to describe...

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