Soft matter occupies a middle ground between the solid state and the fluid state. These materials have neither the crystalline symmetry of solids, nor the uniform disorder of fluids. For instance, a smectic liquid crystal is comprised of a one-dimensional solid-like periodic stack of two-dimensional fluid monolayers. Liquid crystals, polymers, and colloids are commonly cited examples, but soft matter also encompasses surfactants, foams, granular matter, and networks (e.g. glues, rubbers, gels, cytoskeletons), to name just a few. The interactions governing soft matter behavior are often weak and comparable in strength to thermal fluctuations, so these usually fragile forms of matter can respond much more strongly to mechanical stress, electric fields, or magnetic fields than what is possible in solid-state systems. Common themes in the behavior of soft matter include the propensity for self-organized structures (usually at length scales larger than molecular sizes), self-organized dynamics, and complex adaptive behavior, often in the form of large macroscopic changes triggered by small microscopic stimuli (which is ideal for applications like controlled drug release). This can be seen in the surprisingly wide range of recent examples: shape-memory polymers for ‘smart’, self-knotting surgical sutures [1], DNA in artificial gene delivery systems [2], colloidal crystals for templating photonic bandgap materials [3], cubic lipid matrices for crystallizing integral membrane proteins [4], and electronic liquid crystalline phases in Quantum Hall systems [5], an example that shows we have come full circle, where soft condensed matter physics meets hard condensed matter physics.

  1. A. Lendlein & R. Langer, Science 296, 1673-1676 (2002)
  2. See for example, Y. A. Vlasov, X. Z. Bo, J. Z. Sturn, & D. J. Norris, Nature 393, 550 (1998).
  3. See for example, J. O. Radler, I. Koltover, T. Salditt, & C. R. Safinya, Science 275, 810-814 (1997).
  4. E. Pebay-Peyroula, G. Rummel, J. P. Rosenbusch, & E. M. Landau, Science 277, 1676-1681 (1997).
  5. See for example, S. A. Kivelson, E. Fradkin, & V. J. Emery, Nature 393, 550 (1998).

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