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Introducing GSOFT

24 April 2015
A new topical group within the American Physical Society is devoted to the science of soft condensed matter.

Toothpaste flows but keeps its shape on your toothbrush. Is it solid or liquid? Sand can be poured from one bucket to another. How can you stand on it? Do glasses flow very slowly or not at all? Why do rubber bands shrink on heating? Can fluids get thicker when you stir them? Thinner? How can simple, spherically symmetric interactions lead to exquisitely organized structures? How does entropy lead to order? Is temperature only an equilibrium concept?

Such are the questions that people who study soft matter try to answer.

Unlike its older brother, hard condensed matter, soft matter has no fundamental overarching theories. The challenge in soft matter is to pose the right questions, make the right measurements, and find patterns in the materials that nature has provided.

Optical microscopy and scanning electron microscopy of the wing scales of Callophrys rubi (A–C) and Teinopalpus imperialis (D–F). Scale bars are 100 μm (A,D), 2 μm (B,C,E,F), and 200 nm (inserts of C,F). The light micrographs (A,D) reveal a tessellated appearance that manifests the polycrystalline nature of the crystallites with different orientation of the gyroid structure (B,C,E,F) within the wing scales.  CREDIT: Gerd Schröder-Turk.  See Materials Today: Proceedings 1S, 193 (2014).

Optical microscopy and scanning electron microscopy of the wing scales of the butterflies Callophrys rubi (A–C) and Teinopalpus imperialis (D–F). Scale bars are 100 μm (A,D), 2 μm (B,C,E,F), and 200 nm (inserts of C,F). The light micrographs (A,D) reveal a tessellated appearance that manifests the polycrystalline nature of the crystallites with different orientation of the gyroid structure (B,C,E,F) within the wing scales. CREDIT: Courtesy of Gerd Schröder-Turk; see Materials Today: Proceedings 1S, 193 (2014)

The term “soft matter” comes from Pierre-Gilles de Gennes, Nobel laureate and founding father of our field. He referred to such systems as matière molle, “fragile matter.” Yet although it may be fragile, such matter is certainly robust. It can self-assemble, re-assemble, and repair itself spontaneously, or sometimes with just a little help.

Within the time scale of physics as a discipline, soft matter is a relatively young field, but it is one that is quickly expanding its scope, breadth, and impact. Soft matter is relevant not only because of the novel sets of questions it asks and systems it studies, but also because of its profoundly interdisciplinary nature. From climate change to the manipulation of light and sound to oil extraction from the Canadian tar sands to new developments in medicine, the field of soft matter provides a natural arena for the interaction of applied mathematicians, biologists, chemical engineers, chemists, materials scientists, mechanical engineers, pure mathematicians, and, of course, physicists.

Monolayer of 3:1 (mol:mol) dipalmitoylphosphatidylcholine:palmitic acid with 4 mol% cholesterol on water at a surface pressure of 10 mN/m.  The black domains nucleate as circular crystals of  1:1 DPPC:PA, then the arms branch off as DPPC-cholesterol crystals as the PA is depleted.  The cholesterol is a line-actant, which causes the DPPC crystals to grow as thinning, twisting arms, with the twist being determined by the chirality of the DPPC.  All the spirals rotate in the same direction because  DPPC is natural and has only the one type of chiral center.  Contrast in the image is supplied by a fluorescent lipid (Texas-Red DHPE) that partitions into the disordered phase.  DPPC, PA, and cholesterol are all components of human lung surfactant that is responsible for lowering the surface tension in the alveoli to make for easy breathing.  CREDIT: Pictures by Qiong Tang, courtesy of Joe Zasadzinksi

Monolayer of 3:1 (mol:mol) dipalmitoylphosphatidylcholine:palmitic acid with 4 mol% cholesterol on water at a surface pressure of 10 mN/m. The black domains nucleate as circular crystals of 1:1 DPPC:PA, then the arms branch off as DPPC-cholesterol crystals as the PA is depleted. The cholesterol is a line-actant, which causes the DPPC crystals to grow as thinning, twisting arms, with the twist being determined by the chirality of the DPPC. All the spirals rotate in the same direction because DPPC is natural and has only the one type of chiral center. Contrast in the image is supplied by a fluorescent lipid (Texas-Red DHPE) that partitions into the disordered phase. DPPC, PA, and cholesterol are all components of human lung surfactant that is responsible for lowering the surface tension in the alveoli to make for easy breathing. CREDIT: Pictures by Qiong Tang; courtesy of Joe Zasadzinksi

The number of problems that soft matter addresses continues to grow faster than the number of scholars devoted to them. Fortunately, in soft matter there is unusually strong interplay between theory and experiment. As a result, soft matter theorists are constantly challenged by the data of complex systems and by phenomena that cannot be described through the standard formulations of statistical mechanics, electromagnetism, and quantum mechanics.

At the same time, soft matter experimenters are challenged by the need to measure things on new scales of length and time, often requiring real-space methods through cloudy media, measurement of timescales over a huge dynamical range, and, very importantly, the ability to create novel, controlled materials. Consequently, progress in soft matter occurs along multiple fronts: pure, applied, fundamental, and technological.

Three-dimensional forces in a granular material obtained by optical tomography using refractive index matching. Force chains supporting the load are tracked as the grain assembly is stressed. CREDIT: Adapted from N. Brodu et al., Nat. Commun. 6, 6361 (2015); courtesy of Nicolas Brodu

Three-dimensional forces in a granular material obtained by optical tomography using refractive index matching. Force chains supporting the load are tracked as the grain assembly is stressed. CREDIT: Adapted from N. Brodu et al., Nat. Commun. 6, 6361 (2015); courtesy of Nicolas Brodu

Not united by an all-encompassing single problem or goal but, rather, by a broad, inclusive, and open-minded set of interests, tools, and techniques, many soft matter scientists feel that the breadth of our field resulted in its fragmentation within the American Physical Society (APS). Biological physics, chemical physics, condensed matter, computational physics, fluids, materials, polymers, and statistical mechanics all have their own units with ad hoc coordination between them from the point of view of membership and meetings.

The soft matter community within APS is therefore decentralized, its impact within the society diluted, and its visibility outside the society diminished. More importantly, many soft matter researchers outside of physics are members of the soft matter community but are not yet APS members in the absence of a dedicated unit.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform—that is, to be locally bouquet-like or “splayed.” However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, in this class of fluid polar smectic liquid crystals, local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.  CREDIT: Adapted from D. A. Coleman et al., Science 301, 1204 (2003); photo by Michi Nakata

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform—that is, to be locally bouquet-like or “splayed.” However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, in this class of fluid polar smectic liquid crystals, local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases. CREDIT: Adapted from D. A. Coleman et al., Science 301, 1204 (2003); photo by Michi Nakata

But in Spring of 2014, we founded GSOFT, the topical group on soft matter. The first official GSOFT meeting, held during the 2015 March meeting, generated a roughly 50% increase in the number of submitted soft matter abstracts over previous years. After existing for just one year, we are already at over 900 members. What’s more, beginning in July, a soft-matter theorist, Michael Cates, will become Cambridge University’s next Lucasian Professor, a position whose previous occupants include Isaac Newton, Paul Dirac, and Stephen Hawking.

Has soft matter matured into a bona fide subfield of physics? Yes! Join now!

Articles about soft matter in Physics Today

Randy Kamien is the Vicki and William Abrams Professor in the Natural Sciences at the University of Pennsylvania in Philadelphia. He also chairs GSOFT.

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