® ective potentials for polymers and colloids : beyond the van der Waals picture of ° uids ?

This paper brie®y reviews some recent work demonstrating the partial breakdown of the colloidal ®uid $ atomic ®uid analogy. The success of liquid state theory for atomic ®uids stems in part from the van der Waals picture, where steric interactions dominate the structure, and attractive interactions can be added as a perturbation. For complex ®uids described by e¬ective potentials, this picture may break down. In the ­ rst example discussed, depletion potentials in non-additive hard-sphere mixtures are shown to be surprisingly complex, leading to ®uid structure and ®uid{ solid transitions dominated by properties of the attractive potentials instead of by the hard-cores. Many colloidal suspensions, and possibly globular proteins, fall into this energetic ° uid category. In the second example, the coarse-graining of polymers leads to soft-core e¬ective potentials and associated mean̄eld ° uid behaviour distinguished by a breakdown of the virial expansion, an equation of state that is nevertheless nearly linear in density, and correlation functions well described by the random phase approximation.