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Effective Interactions in Soft Materials

Soft materials, such as colloidal suspensions, polymer solutions, and biological systems, are typically multicomponent mixtures of macromolecules and simpler components (e.g., microions, monomers, solvent) that can assemble into complex structures spanning broad length and time scales. Many characteristic traits of soft matter, e.g., mechanical fragility, sensitivity to external influence, and tunable thermal and optical properties, emerge from a mingling of microscopic and mesoscopic constituents. Large asymmetries in size and charge between macromolecules and microscopic components often make impractical the explicit modeling of all degrees of freedom over physically significant length and time scales. The complexity often can be reduced, however, by pre-averaging (coarse-graining) the degrees of freedom of some microscopic components, thus mapping the real system onto a simpler model of fewer components, but governed by effective interparticle interactions. This chapter reviews the statistical mechanical foundations of effective interactions in soft materials and discusses illustrative applications to charged colloids, polyelectrolytes, and colloid-polymer mixtures