Crossover critical phenomena in complex fluids

It is assumed that near-critical complex fluids, such as polymer and ionic solutions, belong to the same universality class of criticality as simple f luids. However, the range of universal critical behavior in complex fluids is usually so small that it may not be experimentally accessible. In practi ce, physical properties of complex fluids in the critical region often exhi bit some kind of crossover behavior rather than universal asymptotic critic al behavior. The character of the crossover reflects an interplay between u niversality caused by long-range fluctuations and a specific supramolecular structure characterized by an additional nanoscopic or mesoscopic length s cale. When the correlation length of the critical fluctuations becomes comp arable to this supramolecular length scale, a specific sharp and even nonmo notonic crossover from classical (van der Waals-like) behavior to universal asymptotic behavior is exhibited. In the region far away from the critical point, where the correlation length is still smaller than the characterist ic length scale, one can observe classical behavior. Ultimately, in the imm ediate vicinity of the critical point, the correlation length becomes domin ant and one should expect universal asymptotic behavior. Such a crossover h as been indeed observed in polymer and ionic solutions. (C) 1999 Elsevier S cience B.V. All rights reserved.