Foam is not only important in today’s cafe-bar society, for making cappuccinos and giving beer an attractive head, but it is also vital to industry. Foams are used to separate mineral ore from crushed rock and for enhanced oil recovery to extract the final reserves from near-depleted oil wells. So anything that improves our use of foam can produce economic benefits.
‘Aqueous foams are complex fluids whose properties lie between the familiar extremes of liquid and solid,’ said Cox. ‘For small strains a foam behaves as an elastic solid while at large strains a foam moves like a liquid. So they generate a rich range of behaviours, but with a well-defined local structure.’
Foams are subject to a number of dynamic phenomena. The structure coarsens due to gas diffusion between bubbles, any liquid between the bubbles drains, primarily due to gravity, soap films burst, leading to coalescence and the foam may flow under an applied strain. It is the last of these processes that will form the main part of Cox’s project. ‘The idea is to use the precise, known structure of the foam to predict its rheological response,’ he said. ‘Only then can we begin to develop continuum models that accurately reflect this response in any given situation.’
A second strand of the project is to identify ways in which the computer software used for studying foam flow can be applied to developmental biology. The shells of sea urchins, for example, show a pattern of small plates very similar to a 2D foam. The ultimate goal of the simulations is a growth model predicting the full variation of urchin morphology. ‘Once this has been achieved, we will have a methodology that can be applied to other animals and plants exhibiting similarly close-packed structures,’ said Cox.