Dissipation in flowing foams

The study of the rheology, or flow, of foams aims to understand how a foam moves when pushed or squeezed. The motivations for wanting to understand the properties of foam are widespread and diverse. Foams are common in oil extraction and industrial cleaning. They are also used in the process which separates metal ores, such as lead and zinc, from the rock in which they are found. Closer to home, an understanding of flowing foams helps to extinguish fires more efficiently, to generate the perfect pint of beer, and to get a chocolate mousse into its pot. Foams also have peculiar and remarkable properties: they fall half-way between the familiar extremes of liquid and solid. When only a small force is applied to it, a foam behaves as a solid, and bounces back to its original shape. If the force is larger, or applied more quickly, then a foam moves like a liquid. They therefore generate a rich range of behaviours. Apart from their industrial uses, where an understanding of foam rheology can help to make processes more efficient and cost-effective, anyone who has looked closely at a foam in their bath can tell you that it has a beautiful, easily visible, structure. This structure is very well-defined, and using the structure allows us to analyse the flow behaviour of a foam more easily than that of many other complex fluids. The investigator wants to improve the agreement between what is seen in real-life experiments and the mathematical models that researchers are designing, to show that the models can predict what a foam will do in any given set of circumstances. He also wants to train a graduate student in the techniques and theory of rheology, particularly applied to foams.The way that this will be carried out is as follows. Researchers have noted that the traditional quasi-static bubble-scale model of foam flow, in which the foam is always at equilibrium, is not always applicable. Instead, when a foam moves, it loses energy to friction, for example where it rubs against the sides of the container. So there are now a few conceptually simple models of how this dissipation occurs, but the agreement with experiment, although improved, is still not good enough to provide accurate predictions for all flows. This project will therefore identify and add further ingredients to one of these models, and use computer simulations to test the model against experiments.