1 Introduction

Most processes in nature are described by the laws of thermodynamics or statistical physics. Understanding and generalizing such processes, beginning with the formulation of the first postulates of phenomenological thermodynamics, has presented a major challenge to physics. Chaotic and nonlinear processes in particular are the subject of extensive research. This chapter introduces the objectives and investigations of the present thesis, which addresses an area of thermodynamics.

Many transitions in matter associated with external parameters such as temperature and pressure are accompanied by so-called critical phenomena, formally introduced in Chapter 2. Processes of this kind occur in various areas of physics, including superconductivity, magnetic phase transitions and changes in the state of aggregation. One objective is to examine these changes of state and investigate the laws governing them. The present thesis specifically studies a binary critical liquid mixture of nitromethane and pentanol.

The underlying regularities are reflected in the existence of critical exponents. These quantities are results of the so-called static scaling hypothesis, also discussed in Chapter 2. This successful theory provides a bridge between different critical systems in physics: the exponents exhibit universality and analytical relationships with one another.

Despite the success of static scaling, it does not encompass the dynamics of critical fluctuations, another phenomenon observed in critical systems. Near the critical point, the spatial extent of fluctuations increases while their temporal evolution slows. To describe these dynamic processes, the static scaling hypothesis was extended to the so-called dynamic scaling hypothesis.

One pioneer in this field was Fixman [8], who published a theory of ultrasound absorption in critical systems in 1964. This theory was subsequently extended and supplemented by other researchers, including Kawasaki [9]. It is now an established theory, offering, for example, a highly successful approach to interpreting light-scattering data and considerably broadening our understanding of critical systems.

Other theories addressing critical ultrasound attenuation in critical mixtures were developed by Folk and Moser and by Bhattacharjee and Ferrell. Although based on entirely different approaches, both aim to describe the critical attenuation of an ultrasound wave in terms of a scaling function.