Diffusion in Gases and Porous Media

Textbooks usually treat diffusion on two levels: at the physicochemical or molecular level, making use of the kinetic theory of gases, or at the level of a transport phenomenon, a level geared toward applications. The influence of walls is usually disregarded or is treated very briefly in a way unconnected with previous studies. Only recently has a unifying theory, the dusty gas model, been developed that correctly takes account of the influence of walls and which thus clarifies many of the problems and much of the confusion that has beset this subject. The dusty gas model shows why, for example, the fluxes of the species in a binary system are not always equal and opposite, (and its predictions in this regard have been experimentally verified), it shows clearly that there is a different between systems “without walls” and systems “without wall effects,” and it clarifies the nature of the total diffusive flux of a system and the nature of the coupling between the diffusive and viscous fluxes. In this book we present the subject of diffusion from the point of view of the dusty gas model, pointing out where the confusion and errors have arisen – and where they can and still unfortunately do arise if care is not taken. We will be more concerned with the foundations and preliminary questions than with the applications.