The kinetic theory of fluids—an introduction

The main concern of kinetic theory over the last 35 years has been to understand the properties of dense gases and liquids in terms of the interactions and motions of the molecules. For dilute gases, the Boltzmann equation has provided a basis for a kinetic description of nonequilibrium properties, and although there are still many unresolved questions, there is little doubt that this equation is the correct starting point for an understanding of dilute gases. In spite of numerous efforts, no one has yet succeeded in deriving a comparable equation for dense gases or liquids. Several generalizations to higher densities of the Boltzmann equation have been found, and many interesting and unexpected results have been discovered, but something we could call a systematic and complete theory has so far eluded us. I want to emphasize that this article does not pretend in any way to be a survey of kinetic theory. I shall only try to sketch a few of the most striking developments in the kinetic theory of dense fluids to give an idea of what has been achieved so far. It will become clear then that a dense fluid behaves in many respects quite differently from what one would expect on the basis of the properties of dilute gases as known from the Boltzmann equation.