INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 15 (2003) S65–S73 PII: S0953-8984(03)54650-1 The fluid dynamics of foams Denis Weaire 1 , Stefan Hutzler, Simon Cox, Norbert Kern, Maria D Alonso and Wiebke Drenckhan Physics Department, Trinity College Dublin, Republic of Ireland E-mail: denis.weaire@tcd.ie Received 11 October 2002 Published 16 December 2002 Online at stacks.iop.org/JPhysCM/15/S65 Abstract Liquid foam is an example of soft matter (or a complex fluid) with a very well-defined structure, first clearly described by Joseph Plateau in the 19th century. Current research addresses many aspects of the fluid dynamics of this system. How is liquid transported through it in response to a pressure gradient or gravity? How does it respond to stress, particularly above the yield stress? What is the nature of the local fluid flow in the Plateau borders and their junctions? Simple first-order answers to many such questions exist but ongoing experiments continue to challenge our understanding. 1. Introduction The more or less stable equilibrium structure of a foam has been understood in general terms since the 19th century [1]. It remains a subject of intense interest [2], but increasing attention is now being given to the dynamic side of the subject, having to do with liquid flow within the foam, or the shearing motion of the foam as a whole. This is not an easy subject, but the well-defined structure of the foam (figure 1) gives us hope of a detailed analysis of flow properties in due course, and some real insight into the factors that control its behaviour. A lot remains to be done and though a substantial book exists on the subject [3], those unfamiliar with the field should not be misled into believing that the physics of foams is now well founded in its entirety. Only relatively dry, static foams are well described by current models. We begin our review of problems concerning the fluid dynamics of foam under drainage, that is, the transport of liquid through it. 2. Drainage The liquid within a foam flows, with consequent changes in the local liquid fraction,in response to the local pressure gradient and gravity. Equilibrium is achieved when these two forces are 1 Author to whom any correspondence should be addressed. 0953-8984/03/010065+09$30.00 © 2003 IOP Publishing Ltd Printed in the UK S65