The Physics of Foams

What is a foam?  courtesy D.J. Durian Kirk-Othmer Encyclopedia of Chemical Technology

A foam consists of gas bubbles of different sizes densely packed within a liquid. It is formed, for example, when you agitate soapy water or spray the contents of a fire extinguisher. Since a foam is made up of a large quantity of gas with a little bit of liquid, you might think that its properties should be intermediate between those of a gas and a liquid. But foams have unusual mechanical properties: their response to external forces ranges from solid-like to liquid-like behavior, depending on the strength of the applied force. It is of great practical interest to know how to control this behavior - you would like fire fighting foam to spray out very fast, like a liquid, but then to retain its shape in order to be effective in decreasing the flames. To a physicist, the challenge is to understand this bulk behavior without building up from the molecular level. In fact, a number of very different foams share this behavior, independent of their exact chemical makeup. We shall focus instead on the bubble-level structure.

The existence of structure at an intermediate or mesoscopic scale is a property shared by complex fluids which are so named because their behavior is unlike simple liquids and gases.

Research projects available

These are simulations of a model two-dimensional foam. For the first, all that is needed is to understand and work with existing software. For the other, some programming will be needed and learning it will be part of the project. See me for further details about these projects.

1. What happens when foams melt? The ability of a driven foam to go from solid-like to liquid-like behavior is one of my interests. (See my earlier work on this). I would like to examine in greater detail how this happens closer to the "melting transition" when enough liquid is added to the foam that it loses all its solid-like elasticity.
   
   
2. How do foams age? Foams age via various mechanisms, one of which is coarsening: the diffusion of gas from small bubbles to larger ones. An open question is to explore what happens to the dynamics of driven foams when coarsening is present. During the year 2000-2001, a Physics senior, Tara Cubel, worked with me on modifying the simulation we use (see below) to include coarsening effects.

The foam simulation we use was written and developed by Dr. Steve Langer at the National Institute of Standards and Technology. Check out his web page here for a movie of the simulated foam being subjected to a steady shear.