Soft Condensed Matter Physics

This article is a part of  that  published in "Solid State Communications (1997)" .

T. C. Lubensky

Professor in the Department of Physics and Astronomy, University of Pennsylvania

In recent years soft condensed matter physics, or simply soft physics, has emerged as an identifiable subfield of the broader field of condensed matter physics.
As its title implies, it is the study of matter that is ``soft," i.e., of materials that will not hurt your hand if you hit them.  This is in contrast to ``hard" materials such as aluminum or sodium chloride that are generally associated with the field of solid state physics.  Though the term soft physics has only recently gained acceptance, its purview is vast. It subsumes all of fluid physics, including both microscopic structure and macroscopic phenomena such as hydrodynamic flow and instabilities.  It includes liquid crystals and related materials with their vast variety of broken-symmetry states.  It includes colloids, emulsions, microemulsions, membranes, and a large fraction of biomaterials. It is a field that presents fundamental scientific challenges and one that has substantial economic impact.

The defining property of soft materials is the ease with which they respond to external forces.  This means not only that they distort and flow in response to modest shears but also that thermal fluctuations play an important if not dominant role in determining their properties.  They cannot be described simply in terms of harmonic excitations about a quantum ground state as most hard materials can.  There are soft materials that possess virtually every possible symmetry group, including three-dimensional crystalline symmetries normally associated with hard materials and many others not found at all in hard materials.  Ordered phases of soft materials can easily be distorted, making it possible to study and to control states far from equilibrium or riddled with defects. Thus, soft materials offer an ideal testing ground for fundamental concepts, involving the connection between symmetry, low-energy excitations, and topological defects, that are at the very heart of physics.
 

REFERENCE AND NOTES

1.  For a mored detailed and pedagogical account of many of the items presented here, see P.M. Chaikin and T.C. Lubensky, Principles of Condensed Matter Physics (Cambridge University Press, Cambridge, 1995).