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Ten examples of soft materials

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Soft Materials

Soft condensed matter science deals with the properties of condensed phases that appear soft to touch. Examples include colloids, polymers, liquid crystals, gels, emulsions, foams and the tissue that makes up most of the animal world.

This distinction of condensed phases of matter into soft and hard has greater utility than its subjective nature might at first suggest. In fact, it directly correlates with an important physical difference: the free energy of hard condensed matter phases is dominated by the internal energy, while that of soft condensed matter phases is dominated by the entropy. In other words, thermal fluctuations are of crucial importance in a soft matter system. To give a concrete example, the different metallic phases are obtained by minimising the energy, whereas the phases of a of hard-sphere system are obtained by maximising the entropy.

The importance of thermal fluctuations implies that a purely mechanical theory of soft materials cannot be complete: a statistical mechanical theory is needed. The concepts of coarse-grained order parameter fields, effective Hamiltonians, broken symmetry, topological defects and generalised hydrodynamics have been used with great success in understanding the statics and dynamics of soft materials. At a complementary level, when microscopic details are crucially relevant, molecular simulations are being used with increasing effectiveness to understand statics and dynamics at short wavelengths. The problem of bridging between the long wavelength hydrodynamic description, and the short wavelength molecular description is a central challenge in the science of soft materials. For molecular simulations, this means obtaining coarse-grained potentials; for the theorist, this means beginning with a more fine-grained description like kinetic theory, which nonetheless contains and subsumes the hydrodynamic description.

Two introductory papers by T. C. Lubensky and M. E. Cates provide a much more complete description from the theorist's perspective. The techniques of molecular simulation most useful in soft matter are described in the text by Daan Frenkel and Berend Smit

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