Experimental Study of Criticality & Crossover Phenomena in Complex Fluid Mixtures

Jan Sengers得到了理论和计算化学项目的资助，用于对复杂流体混合物中的临界性和交叉现象进行实验研究。这项研究将阐明流体性质在临界点附近的相互作用如何影响复杂流体的性质，在临界点附近，长程相互作用是普遍行为的原因，以及在经典平均场(类范德华)区域，局部微观结构起主导作用。将利用光学技术研究跨越的静态和动态现象。对于聚合物在低分子量溶剂中的溶液，提出了一种从渐近接近临界点的普适标度行为到距离临界点增加的平均场行为的交叉检验。要检验的假设是，交叉的程度受临界涨落的关联长度和聚合物分子大小之间的竞争影响。这项研究将试图通过对不同链长的聚合物和混合物(乙烷正庚烷、乙烷正十二烷以及其他如甲基环己烷和聚苯乙烯)进行高精度的光散射测量，从实验上探索这一点。研究链长对临界波动关联长度的影响。对于超临界萃取、提高石油采收率和超临界污染氧化等许多创新应用，需要对复杂流体和流体混合物的相变和临界行为有一个可靠的了解。流体中的临界现象一直是许多实验和理论研究的主题，这些研究的惊人结果是发现了被称为临界点普适性的现象：观察到流体的微观结构在临界点附近变得不重要。目前的研究旨在对这一现象有一个详细的了解。这一知识可能会对许多依赖超临界流体进行化学处理的领域产生重大影响。

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.