Reflections on graduate education in soft matter

Overview of soft matter

The term “soft matter” (or matière molle), though popularized in the 1991 Nobel lecture of Pierre Gilles de Gennes, was first introduced in the 1970s by Madeleine Veyssié,¹ a close collaborator of de Gennes, to describe materials that exhibit large deformations in response to modest forces. An equivalent definition is that, in contrast to our usual vision of solids, soft matter is “soft” rather than “hard” to the touch, i.e. it gives when pressed. Another definition of soft matter that is sometimes used, particularly when there is a desire to distinguish it from “hard” matter like metals and insulators, which are the traditional subject of solid state physics, is that since its characteristic energies per particle are of the order of room-temperature thermal energy, its properties generally do not require quantum mechanics (i.e. ħ) to describe them. This definition, which in a sense extends soft matter to include all of classical physics, is perhaps too broad, but it does correctly convey the reality that soft matter is not just about materials and their properties; it is also about the (often nonequilibrium) phenomena associated with them, such as the wetting of a surface by a fluid, the deformation of a water droplet as it emerges from a faucet, the aging of a glass, or the swimming of a bacterium.

Under any definition, the range of materials that fall under the heading of soft matter is almost limitless. The list begins with polymers, liquid crystals, colloids, emulsions, foams, pastes, and complex fluids formed by mixtures of water with surfactants or amphiphiles; but it also includes ferrofluids, granular materials, the vast array of biological or biologically inspired materials, the very vibrant new area of active fluids, and much more. Understanding the phenomena associated with soft matter requires a passing familiarity with classical elasticity and fluid mechanics and their generalizations to systems with broken symmetries, thermodynamics and statistical mechanics, non-equilibrium processes, physical chemistry, how complex molecules like block co-polymers and dendrimers are synthesized, biomolecules from DNA to motor proteins, and the list goes on.

The popularity of soft matter is growing. Institutions and individual scientists around the world have formed centers of varying types devoted to soft matter. Over 90 groups around the world have self-identified as soft matter in the informal database on the softmatter.org website run by Linda Hirst of UC Merced, and the Wikipedia entry on soft matter lists over 30 multi-investigator groups and 20 single-investigator groups in soft matter. These figures clearly underestimate the total.

Soft matter and controlling the phenomena associated with it are critical to industry. Clearly, understanding polymers is important to the chemical and pharmaceutical industries. Liquid crystals in essence define the modern display industry, although newer technologies like that of the colloid-based E-ink technology in the Amazon Kindle are providing some competition. Colloids and emulsions are essential to the paint, pharmaceutical, and metallurgical industries and assist in processes like water purification and sewage disposal. A large fraction of the raw materials used in the chemical industry are in granular form, as are food products like wheat and corn. The cosmetics industry, which seeks to deliver products in a controlled form over extended periods, requires exquisite control of vesicular and related encapsulating media. New uses of soft materials, possibly in conjunction with new technologies like microfluidics, are invented every day and serve as the basis of start-up companies around the world. And new needs for soft materials crop up as well: the latest in the news is the use of the thickening agent guar in fracking fluids.

Current educational landscape

The educational landscape for graduate study in soft matter is almost as varied as the field itself, ranging from traditional coursework to inter-laboratory collaborations and international conferences. To my knowledge, there are no university departments of soft matter. Nevertheless, soft matter or soft matter related courses are taught in many institutions. Departments like polymer science, materials science, and chemical engineering offer soft matter courses on polymers, colloids, rheology, and complex fluids as part of their core curricula. Other departments, such as physics and mechanical engineering offer soft matter courses, either as a part of a specialized program or as advanced electives. A quick Google search reveals that universities around the world, including Ben-Gurion and Weizmann in Israel, Chalmers in Sweden, CUHK in Hong Kong, ENS in France, the University of Manchester in the UK, and Colorado and Harvard in the US offer courses with soft matter in their titles. Usually these courses exist not because there is a university or department policy to do so, but because faculty members who do research in soft matter have created them. Soft matter topics are often covered in courses not specifically designated as soft matter. For example Brownian motion and Langevin theory or Poisson–Boltzmann theory are often covered in standard statistical mechanics classes.

Summer schools provide a vital source of pedagogical courses in soft matter. Since its inception in 2000, the Boulder Summer School for Condensed Matter and Materials Physics has run seven programs related to soft matter topics, including complex fluids, non-equilibrium statistical mechanics, hydrodynamics, and biophysics. The University of Massachusetts Summer School on Soft Solids and Complex Fluids has offered week-long programs on topics ranging from topological defects to polymer dynamics since 2008. The Les Houches Physics School has run at least five programs on soft matter since 2009. The Enrico Fermi International School of Physics, the Cargèse Summer Schools, and the ICTP in Trieste have all held soft matter courses. These courses are generally open on a competitive basis to advanced students from around the world, although financial support to attend them is not always available.

Courses are important, but the bulk of graduate education occurs after Ph.D. candidates begin their research. Typically, students, particularly experimental ones, become part of a research group that holds weekly meetings in which ongoing research is discussed. Students and postdocs review their latest progress to the rest of the group, which provides feedback and suggestions for new directions. Students practise talks to be presented at professional meetings in front of their peers, who provide critical advice about presentation.

Many institutions have created research centers with a focus on soft matter, often in conjunction with large grants like the US NSF Materials Research and Engineering Centers (MRSECS). These include the NYU center for soft matter, the Brandeis Complex Fluids Groups, the Institute for Condensed Matter and Complex Systems at the University of Edinburgh, and many more. At their best, these centers bring together researchers in soft matter from various departments within a university. Regular meetings foster communication, sharing of ideas, lab equipment, and research projects. Participation in these cross-disciplinary encounters allows graduate students to learn about and to appreciate different approaches to soft matter science and the different priorities of participating departments. It is in these encounters that the true interdisciplinary nature of soft matter becomes apparent.

There are an increasing number of regional organizations, including the New England Complex Fluids Workshop, the Middle Atlantic Soft Matter Workshop (MASMX), the International Research Training Group (IRTG) “Soft Matter Science: Concepts for the Design of Functional Materials” of the Universities of Freiburg and Strasbourg, that hold regular meetings in which faculty, postdocs, and graduate students present and share their work and that run other programs like summer schools and seminar series. These meetings not only provide students with opportunities to practice making presentations to a less-than-familiar, but generally friendly, audience but also to network with other students who share their interest.

Research in soft matter, as in hard matter, can generally be divided into experiment, theory and simulation, and ideally students should be familiar with but not practitioners of all three. Theorists should know of different experimental techniques, from X-ray scattering to confocal microscopy to microrheology, and what information can be gained from them, just as experimentalist should have sufficient understanding of at least the rudiments of the theory underlying their experiments. Both theorists and experimentalists should have a sense of what information modern simulation can produce and where its limits are, just as simulators should be familiar enough with experiment and formal theory to know where their contributions will be the greatest. Regular meeting, preferably under the umbrella of the kind of centers discussed above, in which experiments, theorists, and simulators present their work to the others, is a good way to foster this kind of education.

Although soft matter is relatively young as a field, there are now a number of textbooks that can serve as bases for courses in soft matter and as research resources for the aspiring student. These include Soft Matter Physics by Richard Jones, Introduction to Soft Matter by Ian W. Hamley, Structured Fluids: Polymers, Colloids, Surfactants by Thomas Witten and Philip Pincus, and Soft Matter Physics: An Introduction, by Maurice Kleman and Oleg Lavrentovich. In addition, there are both classic and relatively new books on specialized soft matter subjects such as Scaling Concepts in Polymer Physics by P. G. de Gennes, Theory of Polymer Dynamics by M. Doi and P. G. de Gennes, Polymer Physics by M. Rubinstein and Ralph H. Colby, Statistical Thermodynamics of Surfaces, Interfaces, and Membranes by Samuel A. Safran, The Physics of Liquid Crystals by P.G. de Gennes and Jacques Prost, Liquid Crystals by S. Chandrasekhar, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, and Waves by P. G. de Gennes, Françoise Brochard-Wyart and David Quéré, and my own book with Paul Chaikin, Principles of Condensed Matter Physics. Finally, there are various series that are either completely devoted to or have volumes devoted to soft matter including the Wiley-VCH soft matter series edited by Gerhard Gompper and Michael Schick and the NATO Advanced Science Institute (ASI) and Scottish Universities Summer Schools in Physics (SUSSP), which have produced volumes entitled Soft and Fragile Matter and Soft Condensed Matter in Molecular and Cell Biology.

Thoughts for the future

There are clearly a variety of educational resources available for graduate work in soft matter science, but do they actually meet current needs and can they be increased and improved? For the most part, I think that one can argue that current needs are close to being met, but not in a regular and predictable manner. It would, for example, be preferable for every student doing thesis work in soft matter to take at least one course devoted to soft matter in addition to more standard courses like statistical mechanics, continuum mechanics and fluid mechanics, electromagnetism, and traditional solid state physics. This course would at minimum cover colloids, polymers, liquid crystals, and complex fluids. Although such a course could be squeezed into one semester, it would be better as a one-year course. In most departments, teaching resources are stretched to the limit, and there are many research groups pushing for their specialized courses. This makes adding a soft matter course to be taught on a yearly basis difficult. One solution to this problem would be to create soft matter departments or programs, in some institutions at least, analogous to current Departments of Polymer Science and Engineering at the University of Massachusetts, Amherst and the University of Akron, or the Program of Polymer Science and Technology at MIT, which admits students to participating departments but defines a specialized course of study for them. These programs could define a uniquely soft matter curriculum, which could draw on existing courses in different departments. The time might be right for creating new departments at some institutions, but I suspect that the MIT model is more realistic for most.

In developing soft matter courses for specific departments, it is important to try to make them interesting to students and faculty members who do not do research in soft matter, both to provide support for offering these courses and to educate the broader scientific community about soft matter. For example, in physics departments, a course presenting aspects of broken symmetry and its associated low-energy excitations, low-frequency modes, and topological defects and how fluctuations destroy order as spatial dimensionality is reduced should engage the interest of both particle physicists and “hard” condensed matter physicists. Examples taken from liquid crystals (e.g. hedgehog defects in nematics) or colloids (e.g. two-dimensional hexatic phases with 6-fold orientational but not periodic crystalline order) display these concepts with visual clarity. A course that includes the formation of quasicrystals in systems of hard tetrahedral² or dendrimeric molecules³ should engage the interest of students in many departments from chemistry to materials science.

For the future, it is important to continue offering summer school courses on soft matter and to maintain the vitality of both intra-university interdisciplinary groups and inter-university organizations that foster education in and communication about soft matter.

One aspect of graduate education in soft matter that I believe could be improved is in relation to industry. An important fraction of students trained in soft matter, particularly experimentalists, ultimately pursue careers in industry, often in new start-up companies. The current norm is for very few students and postdocs in an academic environment to have any contact with industry before they are actually on the job market. There are exceptions of course: students in institutions that have spun off start-ups often interact with those running them and there are a few real university–industry agreements, like that between RHODIA, the French CNRS and the MRSEC of the University of Pennsylvania, which provide students with some exposure to industry, but more could be done.

I believe that the future of soft matter science is bright, though it is subject to the same uncertainties of funding that besets most of science today. There are still challenging fundamental science questions to unravel, there are new technologies to develop, and there are new interfaces with other fields of science to establish. With care, graduate education in soft matter should flourish along with the field.

Tom C. Lubensky, University of Pennsylvania, USA

References

1. Demain la Physique, ed. É. Brézin, Odile Jacob, Paris, 2004.
2. A. Haji-Akbari, M. Engel, A. S. Keys, X. Y. Zheng, R. G. Petschek, P. Palffy-Muhoray and S. C. Glotzer, Disordered, quasicrystalline and crystalline phases of densely packed tetrahedra, Nature, 2009, 462, 773–U791.
3. X. B. Zeng, G. Ungar, Y. S. Liu, V. Percec, S. E. Dulcey and J. K. Hobbs, Supramolecular dendritic liquid quasicrystals, Nature, 2004, 428, 157–160.

---

Footnote

† This article is part of a collection of editorials on Soft Matter Education.

---