Propagation in Systems Far from Equilibrium: Introduction and Overview

Abstract

At absolute thermodynamic equilibrium, a given macrosopic system rests in a structureless and time-independent state. It can be brought out of equilibrium in several ways. A first possibility is to prepare the system in a metastable state and to let the stable phase nucleate. This is the case of an undercooled liquid that solidifies as soon as a sufficiently large germ is present; at a given temperature the solid phase is more stable than the liquid phase and the fraction of the solid phase spontaneously increases, separated from the remaining liquid by a solidification front. In the same way, after ignition by a spark, a mixture of fresh gases releases its chemical potential energy by burning; a flame front propagates from burned towards unburned regions. Propagation of a front is the simplest way to restore uniformity at the expense of a metastable state. Questions of interest then relate to the propagation velocity and the morphology of the front, either unmodulated or modulated, and further, either cellular or irregular, i.e. e.g. dendritic (solidification) or turbulent (flame). It is important to realize that in such cases, the system can be assumed to be closed in the thermodynamic sense, but if it is actually closed, the propagation will stop after a while. In a conveniently designed open system, this kind of propagation of a front in a multi-phase medium can be maintained indefinitely. These topics form the basis of the first part of our workshop.