Critical behavior of soft matter fluids in bulk and in random porous media: from Ising to random-field Ising universality

We discuss and review critical behavior in soft matter fluids in bulk and in quenched random porous media, focusing mostly on colloid–polymer demixing. Bulk critical behavior in these systems is of practical importance, and has attracted renewed attention from confocal and fluorescence experiments. The general picture that emerges is that bulk demixing belongs to the Ising universality class, consistent with theoretical expectations and computer simulations. The state-of-the-art in theoretical and simulational modeling of critical demixing is discussed, considering also recently discovered effects arising from the Yang–Yang anomaly. In the second part of this review, critical demixing in the presence of quenched porous media is discussed, with focus on the universality class. In view of a famous conjecture by de Gennes, random-field Ising universality is to be anticipated, but its confirmation in experiments remains elusive up to now. We review recent simulational efforts showing that random-field effects in soft matter fluids are, in fact, remarkably strong, possibly making these systems attractive for experimental studies in search of random-field effects. The subtleties concerning finite-size scaling in simulation data of fluids with quenched disorder are reviewed in detail. We also discuss symmetric mixtures with quenched disorder, and point out that in this case the random-field hypothesis does not apply.