Foams & Surfactants

A surfactant is a dissolved chemical agent that (i) resides preferentially at a gas-liquid interface and (ii) when there reduces the local surface tension. Perhaps the most familiar surfactant is washing-up liquid, but they also occur widely in nature and in industry. The principal effects that surfactants may have on a liquid flow are the following

Surface tension reduction

Surface tension tends to cause closure of the airways in the lungs. THe body produces natural pulmonary surfactant to reduce the surface tension and thus reduce airway closure.

Enhanced spreading

A localised spot of surfactant can spread vary rapidly over a liquid surface. This property is used in drug delivery and in enhanced coatings (eg so-called super-spreaders)

Surface stiffening

Paradoxically, as well as causign rapid spreading, surfactant can also act to resist surface expansion. If a surface is loaded with surfactant, then local expansion of the surface leads to a decrease in surfactant concentration and, hence, an increase in the local surface tension that resists further expansion. This property explains the ability of surfactant to stabilise thin liquid films and thereby encourage the formation of bubbles and foams.

These properties of surfactants are increasingly being exploited in novel techniques and therapies. They may also be unwanted, for example by causing excessive foaming in the petrochemical and brewing industries.

Polymer-surfactant mixtures are being increasingly used in a wide range of domestic, industrial and technological applications. The mixtures are in general aqueous-based and polymers are added to the systems to control solubility, rheology and stability. Interactions within the mixtures are driven by hydrophobic, dipolar and electrostatic forces. The complex nature of these interactions lead to interesting surface tension profiles as the concentrations of polymer and surfactant are varied. We are especially interested in developing a model to explain the surface tension profiles of solutions containing ionic surfactants and oppositely charged ionic polymers.

We have strong links with experimental groups in the Department of Engineering and the Physical and Theoretical Chemistry Laboratory. Our aim is to gain a greater understanding of surfactant properties via interdependent experiments and mathematical modelling.

People working in this area within OCIAM are

For detailed information see

• Howell PD and Breward CJW, Mathematical modelling of the overflowing cylinder experiment. J. Fluid Mech. 474: 275-298, (2003).
• Breward CJW and Howell PD, The drainage of a foam lamella., J. Fluid Mech., 458: 379-406, (2002).
• Breward CJW, Darton RC, Howell PD and Ockendon JR, The effect of surfactants on expanding free surfaces. Chem. Eng. Sci., 56: 2867-2878, (2001).
• Breward CJW, Darton RC, Howell PD and Ockendon JR, Modelling foam drainage. IChemE Symposium Series. 142(2), 1009-1019, (1997).
• White AJP. The effect of ionic surfactants on expanding free surfaces. MSc Thesis. (2001)
• Breward CJW. The Mathematics of Foam. DPhil Thesis. (1999)