Crossover critical phenomena in simple and complex fluids.

摘要

The thermodynamic properties of fluids are affected by long-range critical fluctuations in a very large range of temperatures and densities around the critical point. To treat these effects, the universal scaling laws, which are valid in the immediate vicinity of the critical point, need to be extended to account for a crossover to classical thermodynamic behavior far away from the critical point. Several approaches for dealing with the crossover from singular asymptotic critical thermodynamic behavior to classical thermodynamic behavior have been proposed in the literature. However, in light of recent theoretical and experimental studies on the universal scaling amplitude ratios, the existing crossover formulations require revisiting and improvement.; We have developed an extended parametric scaling representation of the equation of state for the universality class of the three-dimensional Ising systems which include fluids and fluid mixtures. The crossover from Ising-like behavior asymptotically close to the critical point to classical (van-der-Waals-like) behavior far away from the critical point has been incorporated into the parametric equation and is governed by two crossover parameters: the range of interaction and a molecular-size "cutoff". Explicit crossover expressions for the correlation length and the thermodynamic properties have been obtained. In the asymptotic limit, the crossover equation reproduces the most recent theoretical estimates for the universal ratios of the leading critical amplitudes. The correction-to-scaling amplitude ratios are in good agreement with theoretical estimates as well. The equation has been tested on experimental data for 3 He and polymer systems and provides an excellent description of the data in an extended critical region. We have compared our new equation with other crossover equations of state. The equation also describes numerical simulation data for the crossover critical behavior of the susceptibility and for the order parameter for three-dimensional Ising lattices with various interaction ranges.