Jan Sengers is supported by a grant from the Theoretical and Computational Chemistry program to perform experimental studies of criticality and crossover phenomena in complex fluid mixtures. This research will elucidate how the properties of complex fluids are affected by the interplay of the properties of the fluid very near the critical point, where the long range interactions are the cause of the universal behavior, and the classical mean field (van der Waals-like) region where the local microscopic structure plays the dominant role. The cross-over static and dynamic phenomena will be studied using optical techniques. A test of crossover from universal scaling behavior asymptotically close to the critical point to mean-field behavior upon increase of the distance from the critical point is proposed for solutions of polymers in low-molecular weight solvents. The hypothesis to be tested is that the extent of the crossover is affected by a competition between the correlation length of the critical fluctuations, and the size of the polymer molecule. The investigation will try to probe experimentally this point by performing highly accurate light scattering measurements of polymers and blends (ethane+ n-heptane, and ethane+ n- dodecane, as well as others such as methylcyclohexane and polystyrene) of varying chain lengths. The influence of the chain length on the correlation length of the critical fluctuations will be studied.
A reliable understanding of phase-transition and critical behavior of complex fluids and fluid mixtures is needed for many innovative applications such as supercritical extraction, enhanced oil recovery, and supercritical pollution oxidation. Critical phenomena in fluids have been the subject of many experimental and theoretical studies, and the surprising result of these studies has been the discovery of phenomenon known as critical point universality: the observation that microscopic structure of fluids becomes unimportant in the vicinity of a critical point. The current study is directed at a detailed understanding of this phenomenon. This knowledge could have a substantial impact on a number of areas that rely on supercritical fluids for chemical processing.
