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1 Introduction

the thermodynamic properties of a two-dimensional Ising model, that had been frequently discussed. It was a great surprise to find the theory of Landau to fail completely in predict- ing the behavior close to the critical point. Fisher, played a leading role by his analysis of experimental data, combining theoretical analysis and numerical calculations, [9], [10], [11]. Important theoretical contributions have been made by Widom [12], Patashinskii and Pokrovski [13], and most important by Kadanoff, [14]. Kadanoff put forward a very important new and original idea which seemed to have a strong influence on the later de- velopment of the field. However, his theory did not allow to calculate the critical behavior. The problem was solved in a comprehensive and profound way by Wilson [15]. Wilson was the first to realize that critical phenomena are different from most other phenomena in physics in that close to the critical point one has to deal with fluctuations over widely different scales of length. Nevertheless, investigations which have been performed on critical system have indicated the description of their behavior is not sufficient for a wide temperature range. The range between the background and mean-field behavior could not be described satisfactorily by classic theories of critical phenomena. Consequently, a new formalism has been developed by Albright [68], Burstyn, Sengers, Bhattacharjee and Ferrell, [16], [17], to describe the range between the consolute point and background behavior. This formalism is named crossover theory.

The ideas of universality appear to be also applicable to phase transitions in complex fluids like polymers and polymer solutions, micro-emulsions and liquid crystals, fluids in porous media as well as phospholipid bilayer vesicle solutions. Besides dynamic light scattering an important measurement method to study the cooperative effects in complex fluids is the broad band ultrasonic spectroscopy. Being a non-destructive technique it al- lows for the measurement of chemical relaxations as well as critical fluctuations. During the past decades various theories have been developed to treat the critical slowing down of the critical ultrasonic attenuation in binary liquids within the framework of the dynamic scaling theory. The most prominent examples of the dynamic scaling theory have been presented by Bhattacharjee and Ferrell [18], [19], [20], [21], [22]. Folk and Moser [23], [24] developed a renormalization group theory of the mode-coupling model and Onuki [25], [26] derived a model from an intuitive description of the bulk viscosity near the critical point. The verification, with the aid of dynamic light scattering and shear vis- cosity measurements as well as with broad band ultrasonic measurements, of the validity of these dynamic scaling functions of various critical mixtures was one of the aims of the present thesis. Essential quantities are the characteristic relaxation time Γ, the fluc- tuations correlation length ξ, the mutual diffusion coefficient D, as well as the reduced half-attenuation frequency Ω1/2, the adiabatic coupling constant g, and the critical ampli- tude S of the ultrasonic attenuation spectra. Also considered was the coupling between the critical contributions and the noncritical relaxations due to chemical processes, as well as the predictions by Procaccia et al. [27], on the critical slowing of those chemical re-

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